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A 20th Anniversary Review of Ward and Brownlee’s ‘Rare Earth’

Ramses Ramirez, whose work on what he calls the Complex Life Habitable Zone was the subject of a recent Alex Tolley essay (see Are Classic Habitable Zones Too Wide for Complex Life?), joins us today with a look back at Rare Earth on the occasion of the book’s 20th anniversary. Written by Peter Ward and Donald Brownlee, Rare Earth examined a wide range of factors that argued against the ubiquity of complex life in the cosmos. I remember well when it came out, as I was in the midst of writing my Centauri Dreams book for Copernicus, Ward and Brownlee’s publisher, and my editor (the brilliant Paul Farrell) and I had to wrestle with the question of whether Rare Earth rendered the search for intelligent life elsewhere irrelevant. Fortunately, we plunged ahead anyway. As Dr. Ramirez shows this morning, many of the factors put forward by Ward and Brownlee can be re-examined with new data as work on exoplanets continues. Ramses is a research scientist at the Earth-Life Science Institute (ELSI) in Tokyo specializing in habitability issues, and a member of the Martian Moons eXploration (MMX) science team. His research at ELSI is augmented by collaborations with both JAXA, the Japanese space agency, and the National Astronomical Observatory of Japan.

by Ramses M. Ramirez

One of the most profound questions that mankind has ever asked is: Are we alone in the universe? This age-old question has motivated many philosophers and scientists alike to question how common life, particularly intelligent life, is in the universe.

In 2000, Peter Ward and Donald Brownlee (University of Washington) wrote the influential book, Rare Earth: Why Complex Life Is Uncommon in the Universe, to address this very question based on our (then) understanding of various scientific fields, including astronomy, geology, atmospheric science, planetary science, and biology. Summarizing this evidence, these authors had concluded that simple unicellular lifeforms may be common throughout the universe, although complex life, including multicellular plants and animals, may be very rare.

Although this sounds like bad news for many science fiction aficionados, this is acceptable for the field of astrobiology, which is dedicated to searching for alien lifeforms of any sort, including simpler microbial life. Hence, the conclusions inferred by Ward and Brownlee most directly impact SETI, or the Search for Extraterrestrial Life, a scientific endeavor dedicated to the search for intelligent life on other planets.

Shortly after the publication of Rare Earth, my Ph.D. advisor and colleague, Evan Pugh Professor James F. Kasting (Penn State University), had written a comprehensive and overall positive review of Rare Earth (title: Peter Ward and Donald Brownlee’s “Rare Earth”). Interestingly his assessment of the same scientific evidence led to a different conclusion, however: That Ward and Brownlee may have been overly pessimistic regarding the prospects of extraterrestrial complex life. Professor Kasting instead argued that many of the points raised in Rare Earth could be used to support the premise that complex life may be common.

However, the exponential increase in data and observations over the past 20 years warrants a re-examination of the scientific arguments made in this book. These are truly exciting times and I am honored that Centauri Dreams has invited me to review Rare Earth on its 20th year anniversary. In here, I revisit these and other arguments using the most updated science.

I decided to write my critique using a similar outline that Professor Kasting implemented in his original review, which I hope proves useful both for comparing the arguments and for highlighting how the field has evolved over time. I have also added a couple of additional scientific questions that have become important since the publication of Rare Earth.

Jupiter’s Importance as a Shield Against Large Impacts

Previous work had shown that Jupiter helps deflect comets and asteroids away from the inner solar system [1]. This is because Jupiter’s large mass (approximately 300x Earth’s) reduces the frequency of such impacts by approximately a factor of 1,000 to 10,000. From this, the authors of Rare Earth reasoned that the existence of a Jovian in the outer solar system may be a key ingredient for the emergence of life (and, eventually, complex life) on potentially habitable planets. However, during the time in which Rare Earth was published, there were very few of these gas giants discovered orbiting other stars. The few that were found were “hot Jupiters”, which are located in the inner stellar system and could potentially destabilize the orbits of neighboring terrestrial planets.

It is then reasonable to wonder how advances over the past 20 years have updated our understanding of exoplanetary distributions. By most measures, this has been a great success story. Approximately 4,000 exoplanets have been confirmed to date. Out of the nearly 1,300 discovered Jovian planets, over 800 are hot Jupiters (Figure 1). Although a seemingly impressive number, ~ ½ of 1% of M-dwarf stars host a hot Jupiter at all, whereas this number is closer to 1% for F – K stars [2], which is consistent with the lower inferred disk masses for M-dwarf systems [3]. Indeed, studies calculate that impact frequencies and velocities should be higher for M-dwarf systems [4,5], which suggests that such environments may not be ideal for supporting complex life. Recent work finds that the relatively lower Jovian frequency around M-dwarfs may facilitate water delivery to terrestrial planets [6]. Such models predict worlds with water masses many times that of Earth. However, the interior pressures of these water worlds would also be too high to support volcanism and plate tectonics, likely inhibiting the emergence of complex life [7,8]. If such models are correct, they suggest that complex life may be rare on M-dwarf planets, even if simpler life cannot be ruled out [8].

We have some information for larger stars too. Limited data from ground-based methods, including radial velocity surveys, indicate a much larger frequency of “cold Jovians” orbiting more massive stars [3]. Even so, it is not clear that such Jovians would always shield terrestrial planets from cometary and meteoritic impacts. Alternatively, Jupiter could have helped trigger instabilities that delivered large influxes of cometary and asteroidal material into the inner solar system, possibly delaying the emergence of life. Thus, the importance of Jupiter-like planets to complex life remains highly uncertain.

Figure 1. Confirmed exoplanets classified as a function of stellar proximity and mass as of July 2, 2018. RE = Earth radii, ME = Earth masses. (Credit: Abel Mendez, PHL @UPR Arecibo).

The Effect of the Moon in Stabilizing Earth’s Obliquity

The importance of the Moon in maintaining the habitability of Earth was a key issue raised in Rare Earth. Earlier modeling simulations [9] predicted that Earth would have exhibited much larger obliquity swings (from 0 to ~85°) than it does today (~22.1 to 24.5°) in the absence of a large Moon and assuming a present day orbital spin rate. As we know, a non-zero obliquity is necessary for the existence of seasons. At an obliquity (tilt) of zero degrees, our planet would cease to have them. Nevertheless, the relatively small ~2 degree change in tilt occurs over a period of about 41,000 years, in accordance with the predicted Milankovitch Cycles.

According to Ward and Brownlee, without the stabilizing presence of the Moon, the resultant obliquity cycles and climate variations would be so drastic that conditions would prohibit the emergence of complex life. This argument is likely still valid because even though more complex models find slightly smaller obliquity variations than originally predicted [9] (~50 degrees [10]), the resultant temperature variations are still large enough to be catastrophic for the survival of complex life.

One way to diminish these tilt-dependent temperature variations is by greatly increasing the atmospheric pressure, which enhances the transport of heat and moisture across the planet. Although this may be accomplished by having a multi-bar CO2 atmosphere [11], such a dense CO2 atmosphere is likely unbreathable by animals or other complex life, at least as-we-know-it to exist on our planet [12].

Another way to minimize climatic variations on a Moon-less Earth is by reducing its spin rate [11]. This can be achieved if, for example, the impactor event that led to the formation of the Moon never happened. However, the problem here is that much of the water delivered to the Earth was brought in during the late stages of accretion, possibly by the Moon-forming impact [13]. Thus, without that catastrophic event, Earth may have been a much drier, possibly lifeless, planet, one without plate tectonics (more on this next).

The Importance and Uniqueness of Plate Tectonics

Ward and Brownlee argue that plate tectonics is rare because it would require a serendipitous Moon-forming impact. In their book, they stress that plate tectonics is key to continental formation, which potentially promotes biodiversity and are key habitats for animal (complex) life. It is also thought that liquid water is necessary for the initiation of plate tectonics, for at least two reasons. First, water lubricates the plates, which permits sliding motions. Secondly, water cools the seafloor as it spreads, which helps maintain plate rigidity.

If the Moon-forming impact (among any other late impactors) was what delivered much of our planet’s water, then Ward and Brownlee could be right about what initiated plate tectonics. However, we still do not know when plate tectonics on our planet actually started. If plate tectonics was triggered by ample water delivery, then we would expect plate tectonics to have initiated early on, possibly soon after the Earth’s surface became habitable and capable of supporting liquid water ~4.3 – 4.4 Gyr [14]. The problem is that a wide range of start dates for plate tectonics has been proposed, between ~4.2 – 0.85 Gyr ago [15]. Thus, if plate tectonics started later, the implication is that water may only be a necessary – but insufficient – condition for plate tectonics.

In recent years, there has been a debate over whether plate tectonics is possible on terrestrial planets that are larger than Earth. One school of thought says that increased gravitational resistance on super-Earths reduces subduction tendency, dampening the potential for plate tectonics [16,17]. However, others argue that increased shear stresses help overcome resistance to plate motions [18]. Clearly, how interior processes scale to more massive planets is poorly understood.

The Difference Between the Habitable Zone and the Complex Life Habitable Zone

Ward and Brownlee define the “animal habitable zone” (AHZ) as the region around a star where a planet could support a mean surface temperature between 0 and 50 degrees Celsius. They correctly argue that that the AHZ should be significantly narrower than the classical habitable zone (HZ) (~0.95 – 1.67 AU) originally formulated by Kasting et al. [19]. This is because the classical HZ posits that multi-bar CO2 atmospheres could exist near the outer edge, which would be deleterious to life as-we-know-it on Earth. Thus, this classical HZ definition should not be used to search for complex life.

Recent studies have shown that a HZ for complex life would be approximately half as wide as the classical HZ [12, 20]. However, such work shows that a complex life habitable zone (CLHZ) is not just dependent on temperature ranges, but on respiration limits determined by the atmospheric composition and pressure. The Meyer-Overton correlation [21], which relates respiration to the solubility of gases in lipids, was used to show that many land animals on Earth could acclimate to breathing in air with CO2 and N2 pressures no higher than ~0.1 bar and 2 bar, respectively [12]. Experimental data also support these theoretical predictions [12]. Above such limits, narcotic effects, similar to intoxication, could become potentially lethal. I have used these theoretical and experimental results to compute a solar system CLHZ outer edge at ~1.31 AU [12] (Figure 2).

At the inner edge, 1-D models had predicted that a moist greenhouse is triggered above a mean surface temperature of ~340 K, leading to atmospheric escape rates to space that can deplete an Earth-like surface water inventory within in ~4.5 Gyr (the age of the solar system). Other more advanced models find that a full runaway greenhouse is triggered at lower mean surface temperatures (~330 K) before a moist runaway greenhouse can even occur [22, 23]. Nevertheless, the CLHZ width for our solar system is approximately half that of the classical HZ, assuming alien life would exhibit similar respiratory limits as does life on Earth. This latter assumption may or may not be the case, however, and future experiments and studies would need to shed light on this question.

Figure 2. The complex life habitable zone [12], most applicable to animal life, is significantly narrower than the classical habitable zone [19]. Adapted from work in ref:12.

The Pre-Main-Sequence Habitable Zone

The temporal evolution of the HZ is central to our ability to find potentially habitable exoplanets for follow-up atmospheric characterization. Although the classical HZ is concerned with the main-sequence evolution of the host star, it is the pre-main-sequence evolution that largely determines whether a planet can acquire and retain volatiles, especially those orbiting M-dwarf stars [24-26]. This impacts the likelihood that a planet that is later located in the main-sequence HZ is habitable or not.

The reason for this is straightforward. At the beginning of the pre-main-sequence stage, a young star is at its brightest and becomes dimmer as it slowly contracts until it achieves gravitational equilibrium at the start of the main-sequence phase. Unlike other stars, which are only slightly brighter during the pre-main-sequence phase, M-stars can be orders of magnitude brighter during their pre-main-sequence phase as compared to when they appear on the main-sequence (Figure 3). As a result, this pre-main-sequence HZ is much farther out during this time, sweeping inward as the star dims. During this stage of stellar contraction, conditions within the inner stellar system are very hot and runaway greenhouse conditions are triggered, possibly desiccating any planets before the star reaches the main sequence.

Figure 3. Pre-main-sequence stellar luminosity starts high and decreases with time until the start of the main-sequence (red point). Figure reproduced from ref:58 and based on work from ref: 25.

Binary Star Systems do have Planets

Ward and Brownlee had argued that binary star systems, which comprise about half of all stellar systems [27], may not have planets. Indeed, when Rare Earth was written, planets had not been detected around binary systems. Since then, however, we have certainly detected many such systems [28]. At this point, the debate is no longer whether such planets exist (they do), but whether they can be habitable. Some of these planets have been found in the HZ of their stars [29], which suggests the potential to host life. However, the fluxes that planets in these systems receive can vary wildly throughout the orbital cycle, impacting their habitability. Some may be repeatedly entering and exiting their stars’ HZ. Dynamic effects also impact the orbital stability of these multiple star systems.

The Galactic Habitable Zone

The galactic habitable zone (GHZ) is the galactic equivalent of the traditional stellar HZ and is the region within the Milky Way where liquid water could be stable on a planetary surface and animal life can be supported [30]. Subsequent work further quantified the GHZ based on stellar age, metallicity, and galocentric distance [31] (Figure 4). For instance, too close to the galactic center, and deadly supernovae and gamma ray bursts are much more abundant. Likewise, if the star is either too far away or too old, the system is unlikely to possess the heavier elements that are necessary to produce rocky planets.

Ward and Brownlee had applied the GHZ to argue that our solar system happens to be located in a very special region where these factors are just right for animals and beings like us to emerge. Even if this was true, however, one should note that our Milky Way galaxy spans ~100,000 light years and contains a few hundred billion stars. This would still mean that there are many millions of stars located within just a few hundred light years from our Sun, providing ample opportunity for life, even complex life, to be found within our solar neighborhood. Moreover, recent telescopes, like Kepler, are designed to observe planets that are no more than a couple thousand light years from our solar system, which would be well within GHZ regions that are most favorable to life.

Figure 4: The galactic habitable zone [30]. Our Sun is located within the galactic region that is most suitable for life. Figure reproduced from ref: 31.

The “Anomalously” High Metallicity and Lower Activity Level of Our Sun

Ward and Brownlee cite Gonzalez [32], who indicated that our Sun is more metal-rich than the average star, including those of its same spectral type (G-star). On this basis, it was concluded that our Sun has an anomalously high metallicity. However, there is some confusion on this topic because Gonzalez [32] did not actually measure the metallicity per se, but the ratio of iron to hydrogen. To further confuse a planetary scientist (like myself), astronomers tend to refer to all elements heavier than H or He as “metals”. This Fe/H ratio is often taken as a proxy for metallicity. Nevertheless, Gonzalez [32] found that ~10% of the stars had an even higher Fe/H, suggesting an above average, but not anomalously high metallicity for our Sun.

However, subsequent studies have done more extensive stellar mineralogy that go well beyond the Fe/H story. These investigations find that refractory element (non-volatile) abundances are somewhat higher whereas those for volatiles are somewhat lower for solar analogues than for the Sun [33]. The cause for this pattern is unclear, but it may be attributed either to loss of disc material during planet formation or ingestion of planetary material by the host stars themselves [34-36]. More recent work has found that stellar magnetic activity and galactic chemical evolution can also affect measured stellar abundances, making diagnostic determinations difficult [37,38].

Overall, the inferred mineralogic trends and associated mechanisms to explain them continue to be a subject of debate. However, there is no a priori reason to assume that our Sun is a unique outlier in this regard. We even have two neighboring stars – Alpha Centauri A and B – that are quite similar to our Sun in many respects. Indeed, the mean surface temperature of Alpha Centauri A is almost identical to that of our Sun and Fe/H is somewhat higher [39].

Even though solar metallicity may not be that unusual, the Sun exhibits lower levels of stellar activity than those measured for nearly any other Sun-like star [40]. This could suggest that our Sun is a rare star. Alternatively, perhaps the Sun is in a temporary state of low activity and will become much more active in the future. If so, then complex life is only possible during a very narrow window of time in a Sun-like star’s history.

Most Stars don’t have Planets

Ward and Brownlee argued that only 5 – 6% of detected stars have planets, and that most of these are hot Jupiters, which are uninhabitable. Of course, planetary data were relatively scarce when this inference was made. Thanks to the Kepler mission, however, we now believe that nearly all stars have planets [41]. What’s more is that a wide variety of planet types, including Earths, super-Earths, mini-Neptunes, Neptunes, and Jupiters (both hot and cold), populate the cosmos. Indeed, the earlier reported high frequency of hot Jupiters can now be attributed to observational biases in the transit and radial velocity methods, techniques that are particularly sensitive to larger planets traveling in smaller orbits. What’s more is that two classes of relatively common exoplanets, super-Earths and mini-Neptunes, have no equivalents in our solar system. We live in exciting times. Indeed, the apparent ubiquity of exoplanets is an argument some use to suggest that our Earth may not be so rare after all.

It even seems quite common to find terrestrial planets located within their star’s HZ. Recent work has estimated ηearth, which is the fraction of stars that host at least one terrestrial planet within their star’s HZ. This ηearth value is important because missions use it to estimate the number of habitable zone planets that can be detected by upcoming missions, directly influencing the size of the telescope that is eventually designed. This ηearth value was found to be between ~15 – 45% for M-stars [42]. Extended to F-K stars, ηearth approaches 9% [43], assuming a conservative definition of the classical CO2-H2O HZ [44]. Other studies over the years have found similar results. The point here is that rocky planets are common, including those located within the HZ. That said, ηearth has not been estimated for the CLHZ [12], which is more appropriate for animal life. However, given that the CLHZ is approximately half as wide as the classical HZ, the above numbers would probably not drop by much more than a factor of 2, suggesting a CLHZ ηearth approaching 5 – 10% or so.

Massive Stars cannot evolve complex life

Main-sequence stars that are heavier than our Sun are hotter and burn their stellar fuel (converting hydrogen to helium) more quickly. This is why massive stars have shorter lifetimes. Rare Earth then suggests that the main-sequence lifetimes of stars with masses exceeding ~1.5 times that of our Sun (~ a F5) are too short for animals to evolve. This comes from assuming that it would take an Earth-like 3.5 – 4 Gyr for animals to emerge on potentially habitable exoplanets. However, there is no a priori reason to believe that complex life elsewhere would take the same amount of time to arise that it took here. I think it is more reasonable to expect that animals can arise either more slowly or quickly on other planets, depending on the specific circumstances.

The authors further argue that the high UV radiation on planets orbiting more massive stars could be harmful to surface life. We know this from everyday experience, which is why we should wear sunblock on particularly sunny days. However, enhanced UV radiation from massive stars is offset by ozone absorption in an oxygen-rich atmosphere [45]. Moreover, UV-C radiation (between 100 and 280 nm) may actually be beneficial to prebiotic chemistry [46].

Ward and Brownlee also argue that UV fluxes would be high enough from massive stars to drive atmospheric escape (EUV and XUV fluxes are energetic enough). Although such fluxes may be 2 or 3 times higher for massive stars than they are for the Sun [47], HZ planets orbiting these brighter stars are also on longer orbits, which is partially offsetting because stellar radiation decreases rapidly with distance.

Earth has Just the Right Amount of Water

Another argument made in Rare Earth is that Earth has just the right amount of water present on its surface. Too much, and there are no continents. Too little, and it may not be enough for life to exist. As Ward and Brownlee point out, water-rich impactors (e.g. carbonaceous meteorites) would have supplied the young forming Earth. During this late stage of accretion, a steam atmosphere would have likely arrived in an unsteady equilibrium with the water dissolved within the hot silicate melt that characterizes the surface and much of the interior.

There has been significant progress on this topic within the last several years. In the case of some M-stars, the interior may need to exceed ~5% wt. H2 to produce a steam atmosphere at all [48]. This problem is further compounded by early atmospheric escape processes associated with high stellar radiation fluxes. Thus, whether a planet can eventually retain any surface water at all depends on a complex interplay between these competing escape and delivery processes.

Furthermore, decay of radioactive isotopes, like Al-26, within the interior can also desiccate planets [49]. The half-life of Al-26 is very short (only about 700,000 years) so this mechanism can potentially desiccate small planets with very short formation timescales, like (possibly) a young Mars [50]. Radiogenic heating can also desiccate very young planetesimals in the early stages of accretion before they combine to form planets [49].

However, Earth took a long time to form, approximately ~50 million years. If Earth had acquired most of its water during the late stages of accretion, possibly in the Moon-forming impact [13], among any others, early desiccation via radioactive decay of Al-26 would not have occurred. Thus, given all of the various ways that things could go wrong during water delivery, the possibility that Earth might be rare in this aspect deserves further attention.

Earth has just the right amount of Carbon Dioxide

The authors assert that if Earth had more CO2, it could have ended up as hot as Venus (~ 735 K mean surface temperature). However, this exact scenario has been assessed by myself and others and we find that predicted mean surface temperatures are not much higher than ~500 K, even if 100 bars of CO2 were abruptly added to the atmosphere [51,52]. This is because the atmospheric water vapor pressure is outstripped by that of CO2. The lower atmospheric water vapor concentration then produces a weakened overall CO2-H2O greenhouse effect. Nevertheless, such temperatures are still much too high for the existence of life as-we-know-it, being more than 100 K hotter than what is needed to break atomic bonds [53]. Again, such CO2 pressures are also much too high for complex life, which requires significantly lower CO2 pressures [12].

In a more realistic case, CO2 levels would increase slowly and modestly above current CO2 levels. In that scenario, some of that atmospheric CO2 would be slowly drawn down through precipitation and silicate weathering, in a process called the carbonate-silicate cycle, which operates over million-year time scales. Over these relatively long timescales, Earth would likely re-equilibrate to a slightly higher temperature to compensate for the gradually brightening Sun. In any case, CO2 levels would be low enough to support complex life.

The Importance of Earth’s Magnetic Field

As Ward and Brownlee noted, various indicators suggest that Earth’s magnetic field contributed to its habitability. Earth’s magnetic field ensures that harmful solar particles don’t gradually erode the atmosphere. To make some sense of this, it is worth looking at the other classical HZ planet in our solar system, Mars. Although the Red Planet does not possess an intrinsic magnetic field today, it did have one at least 3.5 Gyr ago, which would have granted some protection to its early atmosphere, possibly keeping it thick long enough to help explain the geologic evidence indicating a once warmer and wetter planet. Unfortunately, Mars was too small to retain its interior heat for as long as Earth has, which led to a shutdown in its magnetic field. This would have rendered its atmosphere exceedingly vulnerable to sputtering and other loss processes, as MAVEN has shown are still occurring today [54].

It is also helpful to evaluate the other two terrestrial planets. Although Mercury does not possess an atmosphere, it does have a magnetic field, albeit weaker than Earth’s. One may use this to argue against the importance of a magnetic field except that Mercury is located too close to the Sun and far away from the HZ. Therefore, the planet would have been unable to sustain water on its surface with or without a magnetic field.

Likewise, one may be tempted to argue that Venus is able to retain a thick atmosphere without a magnetic field. On face value, this argument may make sense but there are a couple of key caveats. First, like Mercury, Venus is not located in the HZ, and its close proximity to the star ensures that water cannot be supported on its surface today. Secondly, its thick atmosphere likely results from a cessation in plate tectonics, possibly caused by an early runaway greenhouse that desiccated the planet [55]. This would have shut down atmospheric CO2 removal processes (like rainfall), resulting in unmitigated volcanism and an excessive buildup of atmospheric CO2. Such dense CO2 atmospheres exhibit high cooling rates, making them far less susceptible to losses from solar wind stripping. This is how a planet with no magnetic field, like Venus, can still retain a thick atmosphere. However, such a planet is clearly not habitable, so this is a moot point. Magnetic fields do not necessarily make planets habitable, but they can sustain habitable conditions on planets that already are. For the two known cases of terrestrial HZ planets, one has a thick magnetic field (Earth) whereas the other doesn’t (Mars). So, Ward and Brownlee may be on the right track by suggesting that potent magnetic fields may be common among habitable planets capable of producing complex life (Figure 5).

Figure 5: Strong magnetic fields appear to help maintain the habitability of habitable planets Figure reproduced from ref: 59. Image Credit: Tulsa Voralia

The Cambrian Explosion Was Triggered by a Special Event

We still do not know what triggered the Cambrian explosion on our planet nor how it happened. This “explosion” refers to the sudden appearance of most of the major body plans for complex lifeforms, in an event that occurred at the beginning of the Cambrian period (~540 million years ago). Ward and Brownlee note 4 proposed hypotheses: 1) a rise in atmospheric O2; 2) increased availability of nutrients; 3) an increase in global temperatures following a global snowball event; or 4) rapid true polar wander and continental reconfiguration. However, in isolation, each of these explanations is problematic. For instance, the rise in O2 occurred some 50 Myr or more before the Cambrian explosion, suggesting the former may not have been the immediate trigger for the latter. Also, molecular evidence indicates that simple metazoans may have existed more than a hundred million years before the major rise of O2 [56]. An increase in global temperatures also cannot explain why this increase in biodiversity had not occurred when temperatures were presumably higher in other times during Earth’s past. As mentioned earlier, atmospheric CO2 levels exceeding ~0.1 bar CO2 (~250 times PAL) may be deleterious to complex life as we know it on Earth. So, it would not matter much if rapid true polar wander occurred while CO2 levels remained too high for the emergence of complex life. Indeed, CO2 levels were gradually decreasing throughout Earth’s geologic history [57], decreasing below ~0.01 bar by the Cambrian explosion. Altogether, it is probable that there was not one single trigger, but a series of overlapping conditions that had to be met before complex life finally arose. Such a special scenario could be seen as supporting Ward and Brownlee’s core argument.

Conclusions

An impartial evaluation of the evidence supports the premise that complex life, including animal life, might be rare throughout the cosmos. At the same time, there remains sufficient uncertainty to question that conclusion. The Fermi Paradox, which attempts to explain the apparent contradiction between the perceived high probability of extraterrestrial intelligence with the lack of actual contact, still cannot be answered at this time. If the Rare Earth hypothesis is correct, a SETI search for complex (i.e., intelligent) life faces unbelievably low chances of success.

On the other hand, if animal and (ultimately) intelligent alien life are relatively common, we could obtain positive proof of their existence within the next few decades. As exciting as this latter prospect is, it could also be rather frightening. After all, some of these civilizations may be more advanced than we are, requiring the formulation of careful protocols to ensure safe contact. However, if intelligent life elsewhere really is common, we have to ask why nobody has visited us yet. This scenario could suggest that the technology to visit such alien systems is extremely difficult to develop, if not impossible, unless we are among the most advanced civilizations in the universe.

Nevertheless, we wish to maximize our chances of finding life elsewhere by assuming that E.T. is rare and difficult to find. Under that assumption, we will not find complex life on other planets with our current technology. This news should not discourage us, but instead embolden our leaders to invest more heavily in space science and engineering. Such initiatives would help produce improved telescopes and techniques that would enable us to search for extraterrestrial intelligence and have a reasonable chance of finding it.

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{ 104 comments… add one }
  • Henry Cordova June 26, 2020, 13:11

    Although I too share the conviction that complex life forms are rare in the cosmos and that it is highly unlikely that we will ever stumble on to any, I still find the reasoning of the “Rare Earth” type very suspicious. I get the impression it really boils down to something like this: Earth has a civilization, therefore whatever Earth has is a necessary, if not sufficient, condition for civilizations. No. Since what Earth has is not common, therefore complex life is not common either. No, no, no.

    Sure, we have a big moon. So it is easy to “discover ” reasons why big moons are useful. How do we know this, from one example? Maybe life arose here in spite of our moon, not because of it. Sure, big gas giant planets protect us from asteroidal and cometary impacts, but we also know now that big gas giants occasionally go walkabout in inner solar systems, wreaking havoc among Earthlike worlds.. So which is it?

    We are told there is a galactic metallicity gradient and we are in the middle of its sweet spot, so only stars of Sol’s age and location have rocky planets suitable for our kind of life. But there is no reason “metals” may not be common even among the earliest generation of stars.

    Consider the early galaxy, soon after the Big Bang. It is mostly Hydrogen and Helium. But there is also a burst of star formation, and stars forming from the early interstellar medium are not too likely to have metals in their parent nebulae which can condense into a retinue of planets. But wait, some of those new stars that form in the galaxy’s early history are massive blue supergiants; they evolve quickly and manufacture metals in their cores through nucleogenesis. These metals are then quickly returned to the interstellar medium through supernovae and planetary nebulae and available to later generations of stars. Sure, the early galaxy may be relatively metal-free, but there will be pockets that have been metal-enriched. The same can be said for the globular clusters. Sure, they formed early in the galaxy’s history and their surviving stars show little spectroscopic evidence of surface metals, but they have been cooking and storing them in their cores. The short lived massive stars that formed alongside them are gone now, and have returned their metals to the surrounding nebula so that some of those globular cluster stars may have formed in a metal rich local environment. Of course, these clusters are very old, and the nebulae and molecular clouds have long since vanished, but there could be planets there. Even civilizations.

    Yes, there are cosmic catastrophes, but we obviously escaped all of them. Perhaps others did too.

    There are good reasons why life can’t arise around the most common and long-lived, stable type of star, red dwarfs, (tidally locked rotation, stellar flares, narrow habitable zones, etc). But they are very common, and we’ve found a lot of planets inside red dwarf HZs already! But life need not evolve on a hostile surface. It can arise underground, or under an ocean, or under an icecap.

    The same goes for double stars. Binaries may be the cause of complex orbits in nearby planets, and one member of the binary may evolve much faster than the other, causing erratic conditions hostile to biological evolution. But it is possible for a planet to orbit a third, stable star far from the close components of the system. Proxima is a long way from Alpha Centauri A and B. It has at least one planet.

    In short, no matter where you look for life, or intelligence, there are lots of good reasons why it shouldn’t exist. We need to think about them to keep us grounded. But the universe is very old, and it is extravagantly large, and we know rocky worlds, microbial life, multicellular creatures and a technical civilization arose in the one place we’ve actually looked, so we can’t really rule it out anywhere else. The Fermi Paradox proves nothing except that the speed of light is finite. The logic behind Rare Earth is very useful because it tells us civilizations may very well be rare in the galaxy, but we knew that already, didn’t we?

    I’m a big fan of Rare Earth, and I’ve got it on the stereo right now. Check it out. “Get Ready”, (1970).

    https://www.youtube.com/watch?v=Yan9WilVmEg

  • Harold Shaw June 26, 2020, 13:15

    Thank you, I have been looking forward to this article.

    I am confused by this quote: “Another way to minimize climatic variations on a Moon-less Earth is by reducing its spin rate.” Wouldn’t a slower spin rate increase the likelihood that a planet’s tilt would vary over time?

    • Ramses Ramirez June 26, 2020, 16:19

      Hi Harold,

      I can see why that statement may be confusing! This part is different from tilt. What I meant was that equator-pole temperature gradients would be smaller on a more slowly-rotating planet (assuming a relatively dense atmosphere). This is another way to reduce climatic variations.

  • Thomas Mazanec June 26, 2020, 13:41

    Can we tell superEarths apart from miniNeptunes yet?

    • Adam Crowl July 1, 2020, 17:38

      Hi Thomas
      If we have both the radius and the mass, then it’s fairly easy to tell them apart. The H/He atmosphere of a mini-Neptune inflates the radius significantly for the same mass.

  • Alex Tolley June 26, 2020, 14:59

    Need for a Moon

    the resultant temperature variations are still large enough to be catastrophic for the survival of complex life.

    Oceanic temperatures are far more stable than continental temperatures. The deep ocean is a very stable 4C from pole to pole.
    For obliquity changes to be a problem these need to be both relatively rapid, and that animals and plants can only speciate in shallow waters more subject to temperature changes. That would be a “great filter”. Once evolved, however, animals, in particular, can move great distances, whether via ocean currents (e.f. zooplankton, jellyfish), or migrate (fish). Given these possibilities, I am far less inclined to believe the Moon is critical. The Moon is important for species that have evolved to use the effects of the Moon, whether tidal ranges or bright full moons to coordinate spawning.

    <Need for Plate Tectonics

    In their book, they stress that plate tectonics is key to continental formation, which potentially promotes biodiversity and are key habitats for animal (complex) life. It is also thought that liquid water is necessary for the initiation of plate tectonics, for at least two reasons. First, water lubricates the plates, which permits sliding motions. Secondly, water cools the seafloor as it spreads, which helps maintain plate rigidity.

    Continental land masses imply a landform bias in thinking. If the ultimate issue is whether technological species can evolve, then dry land is important. But apart from humans, every other extinct and extant complex life form on Earth is not technological and has been happily evolving away for hundreds of millions of years. Is plate tectonics the only way to create land masses? No. Vulcanism can do so, from islands upwards in size. Large islands can even move around if the planet’s obliquity is not very stable too, just as plate tectonics moves continental landmasses around.

    But again, if we accept that complex life in the oceans can evolve, then the need for plate tectonics largely disappears.

    A more important issue is if abiogenesis requires ocean vents. Does the absence of plate tectonics make the appearance of such vents far less common, or is magma convecting beneath the ocean sufficient> The other issue is whether plate tectonics is necessary for the geologic carbon cycle to operate fast enough, or whether vulcanism is sufficient to create the landmasses needed for this cycle.

    All speculation aside, as Ramses notes, we don’t know what triggered the emergence of multicellular life. I am not convinced that the distinction between unicellular and multicellular life is that binary. Unicellular life does have colonial forms, like the alga Volvox. They are not complex, but arguably a transitional form. Then there is the social amoeba Dictyostelium sp. whose individual cells will differentiate into spore structures which is closer to true multicellular, complex life. Many invertebrates have different life stages that go through various metamorphoses. It is quite possible that this mechanism could be harnessed to create life forms with differentiated components. At some point, natural selection saw the emergence of true complex life forms, where cells remained together, but differentiated differently, but in a coordinated fashion based on adapted chemical signaling. All we can infer is that some condition favored that selection, and thenceforth evolution picked up the pace.

    • Harold Shaw June 26, 2020, 19:59

      If we define life as first a thermodynamic phenomenon, then hydrothermal vents and plate tectonics in general may be crucial sources of free energy, moving electrons, and lipid capsule mangling.

      Endosymbiosis may be the key to complex life and it occurs with some frequency. There is a broad spectrum of endosymbiotic relationships that include mitochondria and chloroplasts and possibly the eukaryotic nucleus.

      https://www.sciencedirect.com/science/article/pii/S0960982212006550

    • Joe H. June 29, 2020, 13:14

      So Alex, if the moon is not critical, what do you think of the theory that tides were important in the evolution of amphibians and thus other land based animals?

      • Alex Tolley June 29, 2020, 21:51

        Unimportant. Without the Moon, the sun provides tides too, just lower tidal ranges. But even without tides life continually expands into new niches. I have no doubt that life would have evolved terrestrial forms without tides at all.

  • Mike Serfas June 26, 2020, 15:08

    Your statement that “the Sun exhibits lower levels of stellar activity than those measured for any other Sun-like star [40]” roused my interest, since this would be a very good clue regarding what makes Earth special. I looked up the preprint ( https://arxiv.org/pdf/2005.01401.pdf ) and I’m not so sure. This isn’t my field, but when I look at Figure 3, it seems like there are stars in the histogram with less variability than the Sun usually exhibits. More to the point, if I mentally subtract the “periodic sample” from the “composite sample” (I think that’s valid to do…) then what’s left seems to (roughly) match the Sun’s histogram for variability, especially given that the upper and lower limits for that were artificially imposed, according to the legend. The Sun’s history is limited to 140 years but the non-periodic sample in a sense is not, since many stars were surveyed. So the only thing seeming out of place here is that the Sun is considered periodic rather than non-periodic, and is that only because we’ve looked at it so carefully? Could the non-periodic stars also be given periods, with more effort?

    • Ramses Ramirez June 26, 2020, 18:48

      Mike, yes you are right. It is more accurate to state that the Sun has lower stellar activity than most sun-like stars, but not all of them. I am not a stellar astrophysicist, but there will need to be more observations of periodic and non-periodic stars.

  • John walker June 26, 2020, 15:38

    Thank you, Dr. Ramirez. Your review reads indeed quite neutral. The basic tenet of Ward’s and Brownlee’s book offended me initially. Yes, I admit it was a purely visceral reaction. Even before I had read it. Haha. So powerful can our desire for something be that we become hostile to the very thought that reality may in fact not be as we wish. Optimism daren’t cloud our ability to analyze. Their thesis rings very plausible for me today. Given the many papers I’ve subsequently read from a range of exoplanetary related fields.

    But, as always we need more data.

    Best of success with MMX, a very exciting mission.

    • Ramses Ramirez June 26, 2020, 18:51

      Thanks John! I’m glad you liked my review. COVID-19 has slowed things down, but we are certainly excited about MMX. Should be a great mission!

  • Ramses Ramirez June 26, 2020, 17:15

    Hi Alex,

    My understanding is that complex (or any) life is very hard to evolve on ocean worlds with no continental landmasses. This is because it is difficult for essential nutrients (e.g. P) to concentrate. This is why we had argued (in Ramirez and Levi 2018) that such nutrients could potentially concentrate on the sea ice of landless planets, potentially exposed to freeze-thaw cycles favorable to the origin of life. Even then, one may produce microbial life in such circumstances, but not complex life (the atmospheres we predict for ocean worlds are much too dense in CO2.. the ocean would also be CO2-rich). Also, too much water on the surfaces of such planets causes volcanism + plate tectonics to shut down. Note that for plate tectonics, volcanism is part of it (e.g. carbonate-silicate cycle). They work together.

    I agree that other mechanisms, aside from plate tectonics, *might* be possible (I discuss this some in my 2018 review paper). However, these are so speculative and seemingly less effective than plate tectonics (e.g. stagnant lid planets), that I don’t think we can make a convincing argument for these other mechanisms supporting complex life, let alone intelligence.

    Also, SETI is currently unable to detect ocean life. So, in my response, there is a slight bias towards what we might be able to observe.

    • Alex Tolley June 26, 2020, 20:15

      Your point about the need for phosphorus is well taken. The argument about plate tectonics {PT) is that it will shut down on water worlds whose deep oceans will prevent any surface landmasses and hence any phosphorus will remain very dilute in the ocean. Is this the only circumstance where PT does not occur (theoretically), yet water is still present? What about drier worlds where there is still much surface water, but no PT? Venus has no PT (no water) but it does have a lot of volcanoes. Is there no condition that allows for lots of volcanoes and shallow seas to exist, but no plate tectonics, ensuring the adequacy of P for life? The scenario does not need to be optimum, just sufficient to remove the obstacle of lack of PT.

      Detecting oceanic complex life is definitely going to be harder than life on land. That is due to the dense chlorophyll of forests on the land surface. The oceans will have both unicellular and macroalgae which will be hard to tell apart. The “red edge” will be there, but we won’t know the organisms bearing the chlorophyll (assuming it is very similar to terrestrial chlorophylls).

      • Ramses Ramirez June 26, 2020, 22:48

        Alex, this starts to get into speculative territory, but it is possible to envision a planet that is drier than Earth, but somewhat wetter than Dune (perhaps), with volcanism and perhaps some shallow seas. In that case, it isn’t clear if plate tectonics would start or not. If it did, one can imagine it being more sluggish than on Earth, assuming that the bit of water available does not all get completely incorporated into the sediments (e.g. clays). There are certainly some origin of life theories that favor smaller seas and ponds for concentrating higher amounts of essential elements.

      • Siyu Lai June 27, 2020, 0:16

        The factors behind the generation of PT are speculative. Whether it depends on planetary mass is also unknown, perhaps ab initio modelling can answer this question soon. The work of Van Den Berg is promising in this area.

        A world without PT but formed with perfect amount of water covering only a fraction of surface? This condition is probably quite, if not very, rare. Earth after magma ocean did not have landmasses. Large emerged continents only begin at Archean-Proterozoic transition, accompanied by increased continental weathering flux (Phosphorus) and Great Oxidation Event. See recent work by Bindeman et (2018) and Johnson et (2020)
        If Earth had formed with more than 4 oceans on surface, very likely Earth might still be submerged and unoxygenated. Even PT subduction cannot remove enough surface water.

        Without subduction as a major sink of weathered carbonate and hydrous lithosphere, the planet would most likely soon become uninhabitable on billion-year timescale.

        Venus, although still erupting magma, is quite dead geologically, for probably 2.5 billion years already. 40-argon, released by cooling magma, is an indicator of mantle degassing degree. It does not sink or escape so its atmospheric abundance is directly proportional to the amount of mantle degassing. The low abundance of 40-argon in Venusian atmosphere strongly constrained that active magmatism did not last more than 2 billion years on Venus. See work by Kaula (1999) and O’Rourke et (2015)
        Catastrophic resurfacing on Venus was only episodic.

        Although we have proposed many methods based chemical disequilibrium or planetary reflectance to detect life, the only biosphere we are most confident in being able to detect is still the Phanerozoic biosphere.

      • Nicky June 27, 2020, 1:23

        PT is central to removing overabundance water (likely after magma ocean solidification) and emerging continents, to drive orogeny and increased weathering production: phosphorus and potassium flux. PT continuously supply new basaltic and granitic crust with abundant nutrients.

        All these mechanisms are tied to PT’s efficient crustal recycling rate. Stagnant-lid would be limited to only mantle plumes, which might be comparable in the first billion years or so, but shuts off early like Venus did.

        • Michael Fidler June 27, 2020, 11:48

          What would happen on planets with a faster rate of movement for PT?

          Interesting article on PT recycling crust, but what about the water that is being recycled?

          Geochemists solve mystery of Earth’s vanishing crust.

          https://phys.org/news/2020-06-geochemists-mystery-earth-crust.html

          • Michael Fidler June 27, 2020, 18:13

            Oceans Deep Within Earth Trigger Earthquakes, Tsunamis and Volcanoes Near The Surface.

            https://strangesounds.org/2020/06/oceans-in-earth-trigger-earthquakes-tsunamis-volcanoes-surface.html

            Variable water input controls evolution of the Lesser Antilles volcanic arc.

            https://www.nature.com/articles/s41586-020-2407-5

            How water in the deep Earth triggers earthquakes and tsunamis.

            https://www.sciencedaily.com/releases/2020/06/200624120450.htm

            What’s funny is Kanlaon Volcano in Negros, Philippines became very active just as and the day after the June 21, 2020 Annular Solar eclipse just north o the Philippines.

            White steam, 136 quakes observed in Kanlaon Volcano in Negros.

            https://news.abs-cbn.com/news/06/22/20/white-steam-136-quakes-observed-in-kanlaon-volcano-in-negros

            The TAAL VOLCANO in the Philippines exploded 17 days after the December 26, 2020 Annular Solar eclipse past just south of the Philippines.

            2020 Taal Volcano eruption.

            https://en.m.wikipedia.org/wiki/2020_Taal_Volcano_eruption

            • Michael Fidler June 27, 2020, 22:03

              Correction: December 26, 2019 Annular Solar eclipse

          • Nicky June 27, 2020, 21:19

            A portion of ingassed water (subduction) is released back to surface through volcanic arc and mid-ocean ridge degassing, and the rest is trapped in the mantle resulting a deep hydrated mantle reservoir (such as transition zone). Earth has maintained higher water ingassing rate than degassing rate for 3-4 billion years, resulting a storage of 1 ocean or more in the mantle.

            Same with CO2, without efficient crustal recycling, the crust will almost immediately become saturated of carbonates and unable to remove additional atmospheric CO2 from volcanic degassing. Some recent modellings argued for long-term habitability on stagnant-lid planets, but they have all assumed very optimistic conditions such as 100% carbonation of erupted rocks.

            • Alex Tolley June 28, 2020, 17:03

              Earth has maintained higher water ingassing rate than degassing rate for 3-4 billion years, resulting a storage of 1 ocean or more in the mantle.

              The mantle contains at least 3 oceans of water, believed to be primarily primordial. If ocean water has been reduced by storage in the mantle, how would we know? (If the source was primordial too, then the D/H ratio would be the same). To separate the ocean water from primordial water would require the ocean water to be sourced differently, to know the various D/H ratios for each source, and then infer any loss to the mantle. Do you have a reference for such a study? Other proxies might work, although I am not familiar with other lines of reasoning.

              • Nicky June 28, 2020, 18:49

                As the early magma ocean solidifies, volatiles originally dissolved in melt become incompatible and exsolve. During this process, most volatiles (>70%) are outgassing into the atmosphere, leaving a relatively dry mantle and a surface ocean with much larger volume than today. From modelling view, much of mantle water should be recycled, instead of was preserved in mantle from the start. It is completely possible that models are wrong, and magma ocean simply doesn’t outgas that much.
                See Elkins-Tanton (2008)

                At least a portion of lower mantle (LLSVPs) has retained some water from solar nebula ingassing during magma ocean stage. But this portion is rather unrelated to surface ocean, which was sourced from carbonaceous chondrites.
                See Williams & Mukhopadhyay (2018)

                Better constraints are from studies that looked at the relative heights of sea-level and continental freeboard and mantle thermal evolution. They found that positive net ingassing (4×10^14 kg/yr) must have been the dominate process since at least Mesoarchean, equivalent to subduction of an ocean.
                See Korenaga et (2017)

                Regardless of where mantle water came from, it has reached a consensus that before Mesoarchean, surface was submerged and large landmasses only appear at A-P transition.
                See Johnson & Wing (2020)

    • AlexaTru June 28, 2020, 3:02

      … SETI is currently unable to detect ocean life
      Sorry, but SETI is able to detect only some limited electromagnetic signatures of technology, technology that is result of civilization that has higher technological level than our civilization. It is not fact that this technological signatures will be signs of life, it can be also post-results of dead civilization…
      SETI is not able to detect even civilization that had technological level equal to first half of XX century…
      So connection between extraterrestrial life search and SETI is wrong… and misleading.

    • Alex Tolley June 28, 2020, 17:20

      Also, SETI is currently unable to detect ocean life. So, in my response, there is a slight bias towards what we might be able to observe.

      Perhaps we should be using the acronym SETCL instead, as we are definitely not discussing intelligence here, just complex life. ;)

  • Gareth Wilson June 26, 2020, 19:25

    “Although this sounds like bad news for many science fiction aficionados, this is acceptable for the field of astrobiology, which is dedicated to searching for alien lifeforms of any sort, including simpler microbial life.”
    One consolation for science fiction aficionados is that the Rare Earth scenario doesn’t rule out human-habitable planets. You can easily imagine a planet where there’s the equivalent of cyanobacteria in the oceans to produce enough oxygen to breathe, and whatever stopped complex life evolving won’t bother a colony for a few thousand years. You’d end up with something like the “terraformed” planets in science fiction, except the colonists don’t actually have to add oxygen to the atmosphere.

    • Abelard Lindsey June 30, 2020, 22:55

      This is actually an ideal situation if we ever get FTL. Lots of habitable planets. Just none with complex life. I think its actually likely based on the hydrogen hypothesis of endosymbiosis (the emergence of the Eukaryote) as described by Bill Marten and Nick Lane.

      • Ramses Ramirez July 1, 2020, 17:14

        Yes! In no way does the Rare Earth hypothesis invalidate space exploration and humanity’s destiny to colonize the cosmos. On the contrary, even if Rare Earth were correct, that may be even better in some ways.. In that case, we would have a near-infinite number of potentially habitable planets (within their stars’ habitable zones) just waiting for us to engineer them to our liking.
        The universe would then become our literal playground and there would be virtually no limitations.

    • Dave Abbey July 2, 2020, 12:43

      Even lifeless worlds could become habitable. Mars sounds like a likely candidate, if it is currently lifeless. Per=fluorocarbons sound plausible for warming, lava tubes for early availability of radiation shielding, and eventual importation of volatiles from the outer solar system, if necessary.

      Long before anyone can travel to another star system we will have a pretty good knowledge of where to go and what we will find when we arrive.

  • Harold Shaw June 26, 2020, 19:46

    I’ve always understood the danger of broadly swinging obliquities to be the orientations where an atmosphere can freeze out or bake off. The moon prevents the Earth from finding those orientations. Could a planet without a moon avoid those orientations by spinning fast enough or by being in a quiescent system? Even if a moon is necessary, a planet could remain habitable at a wider obliquity range than Earth’s. We don’t need to duplicate the Earth and Moon system or its perhaps rare impact conditions.

  • Geoffrey Hillend June 26, 2020, 20:08

    I never heard the idea that Earth got it’s oceans from Theia. From what I recall reading is that Theia’s iron core went into the Earth, so as a result, Earth has a larger Iron Core which gives it a strong magnetic field. The angular momentum of the collision caused a fast rotation of the Earth which has slowed down to give us a twenty four hour day. The rotation causes charged particles in the liquid iron in Earth core to move in circles which gives Earth a magnetic field. Water could have still come from inside the Earth from the mantle and was released by the giant impact.

    Venus has lost 99.9 percent of it’s water, a whole ocean through solar wind stripping due to a lack of a magnetic field which is shown by Venus higher DH2O heavy water ratio than Earth as shown by the Pioneer Venus. UV splits apart the H2O and DH2O and the lighter hydrogen escapes easier through solar wind stripping and heavier deuterium remains behind. Venus still has a thick atmosphere due to the temperature, so all the volatiles must remain a gas including water. Also without an ocean, there is no plate tectonics as already written in this article. It is thought that Venus had an ocean in its early history and it was lost a little over a billion years ago. as the Sun’s brightness increases 7 percent every billion years.

    The only double stars we know have exoplanets orbit close to each other within several AU’s apart. There are not any double stars that are more than five AU apart and less 40 AU’s that we know have exoplanets. I assume that they don’t which is an educated guess or axiom based on astrophysics. In other words, I don’t think we should assume that all double stars have planets which has yet to be proven. I would say that the possible lack of planets around double stars given the parameters I mentioned is an argument in favor of the rare Earth hypothesis, but I don’t agree with the rare Earth hypothesis that there is only one planet with intelligent life in our galaxy which is our Earth. The physical conditions and astrophysical contingencies for life may be rare, but they are not unique. There is a probability that these initial conditions can be repeated more than once even if we include all of the known contingencies. I will admit, that the more restrictions there are, the rarer or fewer the potential ETI worlds in our galaxy.

    • Henry Cordova June 26, 2020, 22:55

      Geoffrey-

      I am puzzled by your remark;

      “The only double stars we know have exoplanets orbit close to each other within several AU’s apart. There are not any double stars that are more than five AU apart and less 40 AU’s that we know have exoplanets. ”

      According to the Wikipedia article on Proxima, it is gravitationally
      bound to the Alpha Centauri system in a highly elliptical orbit whose distance varies from 4300 to 13000 AU to the barycenter of the A/B binary pair. Proxima may not have formed with A and B (it might have been captured later) but we do have an example here of an exoplanet orbiting a member of a triple star system, safely protected by distance from any gravitational perturbations or evolutionary outbursts that might originate from the close pair. The separation of A and B, which varies between 11 and 36 AU, will have little effect on Proxima’s planet.

      There is another factor that may be at play here, Proxima is a very nearby, very small star, a massive nearby planet would be easily detected by our planet-finding techniques. If these exoplanets seem common, it may be more of a selection effect rather than any intrinsic property or characteristic of this type system.

  • Nicky June 27, 2020, 3:44

    I think I will write some comments based on recent advancements in astronomy, biology and geology. Specifically, I will focus on the last four.

    1) Right amount of water is probably rare.
    If continents and plate tectonics are necessary, on Earth, the minimum ocean depth is around 1-km to ensure largely submerged mid-ocean ridge, and the maximum ocean depth is around 4-km to ensure largely subaerial continents. This put the lower limit at 0.3 ocean mass. Plate tectonics could subduct 1-2 oceans into mantle in 4 billion years, setting the upper limit at 3 oceans, setting a range of 0.002-0.2 wt% Earth mass.
    This range on super-earth is even narrower because ocean depth increases much more rapidly with mass compared to radius.
    Forming such little amount of water is difficult, because the disk needs to avoid large-scale solid drift, including inward pebble drifting or migrating embryo.
    The abundant, or even dominant, short period (<100 days) super-earths and mini-neptunes (SPSE) show how common solid drift happens. From planetary formation point of view, SPSE and HZ rocky planets are mutually exclusive. See recent work by Scora et (2020), Bitsch et (2019)
    Similarly, many formation models have found that HZ rocky planets often always accrete too much water, tens of oceans, even without giant planets disturbance. See recent work by Sato et (2016), Zain et (2018), Quintana & Lissauer (2014)
    Just right "0.002-0.2 wt%" is rather uncommon from planetary formation point of view. But of course, formation models must be tested against observations.

    2) Right amount of carbon dioxide could possibly be rare.
    A perfect C-S cycle might only exist in imagination. C-S would work perfectly only if temperature-dependence is the strongest factor. Walker et (1981)'s weathering model has been modified significantly by incorporating cation concentration limit, which is a function of pCO2, soil age, rock composition, land fraction.
    In this model, C-S becomes weakly dependent on temperature. Planets with slightly different tectonics and initial pCO2 can end up in a very different climate regime, more like bifurcation dynamics.
    See recent work by Winnick & Maher (2018), Graham & Pierrehumbert (2020)

    3) Magnetic field probably does not matter.
    It's funny that Venus Express measured O+ escape rate on Venus is 1-5×10^24/s, and Mars has similar escape rate measured by MAVEN. But on Earth, this so-called magnetic field protected planet, has O+ loss rate 50×10^24/s, or at least 10 times greater.
    This is probably because without magnetic field, solar wind energy is uniformly distributed across atmosphere, but in the case of Earth, magnetic field lines direct all the energy to polar region where the extreme energy density drives even faster atmospheric escape rate.
    See recent review by Brain et (2013), Gunell et (2018), Jakosky et (2018), Masunaga et (2019), Gronoff et (2020)

    4) Oxygen delayed origin of animals?
    The molecular work you cited [56] is no longer supported, and this view of early animal origin lasted for only a decade or so.
    Fossil and more accurate molecular clock strongly favor a recent origin at 650 Ma, coinciding with Neoproterozoic Oxygenation Event (NOE). See comprehensive review by Cunningham et (2017).
    Further evidence of oxygen and complex life connection comes from Francevillan biota. Before NOE, if there was ever a time when oxygen was high enough to allow complex life, it was Lomagundi Event.
    Strangely, Francevillian biota was a fossil record of a life form which the complexity had far surpassed the life in its time. It was deposited during Lomagundi Event and immediately went extinct after, and such complexity was never seen again until Ediacaran. PNAS had called it "record of comparatively complex life forms that existed more than a billion years before animals". They were not animals, but breathed, digged, moved like animal.
    I would argue that evolution had done preparing everything for the emergence of complex life, but Earth environment (specifically oxygen level) did not allow it.
    See recent review by El Albani et (2014), El Albani et (2019), Cole et (2020)
    Francevillian biota existed during a time with pCO2 between 0.01-0.1 bar.

    We have made great progress towards answering the questions proposed by Ward and Brownlee in the past two decades.

    • Ramses Ramirez June 27, 2020, 21:09

      Hi Nicky,

      I appreciate your excellent and thought-provoking comments! I have no issues regarding the H2O and CO2 bullet points. They are also consistent with the points I made in the essay, if through a slightly different perspective (which is most appreciated). However, I do have some additional comments regarding magnetic fields and the Cambrian explosion…

      On magnetic fields..

      I am not sure I agree with you that a magnetic field is not important for habitable planets. As discussed in my essay, it is apples-oranges to compare the escape rates of planets within the “habitable zone” against those of planets that are not, especially a planet like Venus with an overly-thick and compressed 90-bar CO2 atmosphere. So, right off the bat, we know that this atmosphere cannot support complex life as-we-know-it. Such compressed and dense CO2 atmospheres would also be relatively cool towards the top, and would exhibit relatively low escape rates as compared to the O2-rich low CO2 atmosphere of Earth. This really, to first order, has less to do with magnetic fields and more to do with the distinct atmospheric differences between the 2 planets (and, in my view, the dense CO2 atmosphere on Venus is probably a direct result of the breakdown in its C-S cycle, so certainly not habitable). The other way to think about it is this.. In spite of the low escape rates on Venus, it is not a habitable planet.

      The apples-apples comparison, in my opinion, is that of 2 potentially habitable planets (or at least 2 planets in the habitable zone), like Earth versus Mars. Earth has a strong magnetic field, Mars does not. When Mars did have a magnetic field in the past, it was able to support a thicker atmosphere, one that ended up producing abundant surface fluvial geology. Not long after that field went away, much of that activity proceeded to die off.

      I had spoken to Dr. Brain (which you cite) about this topic and it seems that magnetic fields can be good or bad for planets with weaker magnetic fields than the Earth (certainly, as you mention, they can funnel energy to the poles and produce high loss rates). However, once such fields get close to Earth-level strength (most of the theoretical work has been on modeling planets with weaker magnetic fields), my understanding is that magnetic fields are mostly a good thing. The argument is not that potent magnetic fields make planets habitable, but that they help sustain habitability on planets that are.

      On the Cambrian explosion…

      Thanks also for the Cunningham et al. (2017) review article and associated references. There is definitely some debate over the molecular clock and when the emergence of the earliest animals fits in with the rise of O2 and the Cambrian explosion. However, even if we assume that some early animals (e.g. sponges) exactly coincided with the rise of O2 (which is highly debatable because inferred biomarkers , not actual trace fossils, are what are being claimed before ~550 – 560 My ago), the point is that they don’t coincide with the Cambrian explosion (when virtually all of the major body plans arose), which occurred well after the rise of O2 or any potential early metazoans. As a result, I don’t think that the rise of O2 or any other single event was the direct trigger for the Cambrian Explosion, even if it may have contributed to the emergence of any early sponges and cnidarians. There is a very clear difference between the awesome diversification of life that occurred at the Cambrian Explosion with the inferred biomarkers (and the biomarker evidence is actually not very strong) some 50 – 100+ Myr earlier. Also, how does a simple rise of O2 cause all of the rapid GENETIC diversification necessary for the appearance of the body plans? In my view, the conditions supporting the simplest forms of complex life were building up for some time before the Explosion (which includes the rise of O2), but the remaining conditions for the sudden appearance of nearly all of our animal complexity were not met until ~550 My ago.

      • Nicky June 28, 2020, 20:59

        The interplay of oxygen and complex life surely deserves more high-resolution studies. We do see some variability in oxygenation process, given that it was stepwise, correlated with the different stages in Cambrian radiation. More recent integrated fossil record is even arguing that CR was only a phase in a long process of radiation started back in Ediacaran, questioning the notion of “explosion”.
        See Wood et (2019), Chen et (2015)
        To make it more complicated, oxygenation was also strongly limited by phosphorus runoff. But the relative importance of oxygen in the evolution of complex life is surely arguable.

        In regard of magnetic field, Brain’s study does show highly magnetized planets are more protected and weakly magnetized planets are the most vulnerable. Gunell et (2018) shows similar trend, except that the transition in their model occurs at magnetic strength several magnitudes higher than what rocky planets can possibly have. I read an article on O+ escape flux directly measured on Mars. I’m not an expert, but escape rate is indeed lower directly above the crustal magnetic field (Fan et 2019). If the atmosphere is protected by the field, this observation will invalidate both Brain’s model and Gunell’s model.

        Additionally, on a tectonically active planet, mantle degassing is more than enough to replenish ion escape, if the host-star is not a M-dwarf. Volatiles sink and recycling are magnitudes greater than ion escape. With or without magnetic field should not be a huge problem.

        I’m not so sure about the causal link between MF and the cessation of Mars fluvial erosion. Mars’ major crustal production also stopped at Noachian-Hesperian boundary, followed by episodic volcanism. MAVEN’s measured atmospheric loss, when extrapolated to Noachian, is only 0.8 bar CO2 (Jakosky et 2018), which can be replenished on million-year timescale if there was active tectonics. The lifespan of active mantle degassing is probably much more relevant to the retention of atmosphere, compared to ion escape rate.
        I don’t think active tectonics guarantees the existence of MF, because the later can be prevented with a stratitified core.

        • Ramses Ramirez June 29, 2020, 3:26

          Nicky, what all of these magnetic field papers show (e.g., Brain et al. 2013; Gunell et al. 2018, Atri et al., Fan et al. 2019..etc), which are often contradictory, is how poorly-understood the modeling of magnetic fields is at the current time (IMHO). So, in light of all of this, I think observational analyses and experimental/field data may be useful.

          Earth’s magnetic field has obviously not been too bad for us, or else we wouldn’t be here. Indeed, it was probably around since almost the beginning (Tarduno et al. 2020). Life didn’t take too long to emerge here either, even with the high impact flux. The observational analyses by Fan et al. for Mars also support theses inferences for Earth, including the interesting idea that crustal magnetic fields may also influence atmospheric escape rates. I’d argue that the observational (e.g. MAVEN) and experimental results (e.g zircon records) tend to argue that an intrinsically strong magnetic field is important for long-term habitability.

          I agree that we can’t necessarily correlate the demise of the martian dynamo to the end of valley network formation, but the magnetic field may have been a factor influencing how long “habitable” conditions on the Red Planet lasted. In any case, we need even more observations and better models to further address this question.

          • Nicky June 29, 2020, 19:14

            Thank you for your reply!

            The Royal Society just dedicated an entire issue to explore the most recent progress around the origin of animals. Definitely worth a read.
            https://royalsocietypublishing.org/toc/rsfs/2020/10/4

            One article is extremely interesting. Despite of increased impact flux and constant frequency of mantle pulses and glaciations in since the end of Paleozoic, the occurrence of mass extinctions and extinction rate of complex life have been decreasing. The authors argue that the diversification and evolution of animals in Mesozoic and Cenozoic have strengthened the negative feedbacks of weathering, as if Gaia hypothesis would have predicted.

            I know you are a proponent of “constantly warm Noachian Mars” (but arid), and there is the equally likely scenario “episodically warm Noachian Mars”. Both hypothesis seem to produce similar amount of erosion and morphology. Is there an observational test to validate either one of them without lander? It looks like Mars 2020 is exploring Hesperian hydrology.

            • Alex Tolley June 29, 2020, 22:46

              I had just discovered that RS issue over the weekend. The last 3 papers look particularly interesting. I also note that none of the papers mention the FB in the abstracts, with most starting with the Ediacaran biota as the earliest examples of complex life.

            • Ramses Ramirez June 30, 2020, 2:35

              Nicky, thanks for the heads up on the “origin and rise of complex life” issue.

              It depends on what you mean by an “episodically warm Mars” .. Nearly all iterations of that idea assume an icy baseline climate with no major source of water. However, that view is simply not supported by the known geologic evidence (too many references to cite), nor does it produce sufficient erosion to explain valley formation or the overall surface erosion we see.

              However, if by “episodic”, you mean a seasonally warm climate with an appropriately large water source (e.g. ocean) with enough power to carve the valleys, I believe that the observations may be consistent with that interpretation. We’ve recently demonstrated the viability of this scenario:
              https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019JE006160

              As already shown at Gale Crater (e.g. Grotzinger et al. 2015), where they found no evidence of icy or glacial/periglacial features in older valley terrains, the same sorts of tests can be performed with Mars 2020 (Jezero crater, which is pretty old) and other missions.

              I had recently written a short article (on p. 16 – 17) in The Planetary Report (magazine of the Planetary Society) discussing the latter in greater detail.
              https://planetary.s3.amazonaws.com/assets/tpr/pdf/tpr-2020-v40n1.pdf

              • Nicky June 30, 2020, 21:24

                Yes I have actually read your paper!

                From my understanding, “warm” in this context is allowing at least seasonal fluvial activity at >270K, as you found in your paper.

                Erosion regime in Early- and Mid-Noachian was dominated by crater obliteration and in Late-Noachian was valley networks. The amount of erosion only allows a small portion of Noachian (duration ~300Myr) to be sustaining fluvial activity. So I see two hypothesis, one by you and one by Kite.

                1) Warm Noachian: for the most of Noachian, Mars has seasonal erosion activity at >270K

                2) Episodically Warm: for the most of Noachian, Mars is cold (~250K?) with minimal erosion, but episodically Mars reaches warm regime (seasonal fluvial activity at >270K) for a period 10^3-10^5 years, allowing greater seasonal fluvial erosion.

                Considering the similarities, I believe these two scenarios are very difficult to be tested. The lack of glacial feature at Gale is strange to me, because Gale hydrology extends into Mid- and Early-Hesperian and the duration of individual lakes was only 10^4 yrs or so. There is no way Gale never had glaciation. I think we need to qualify the geological effects of glaciation with respect to surface water volume. A northern ocean would be hard to reconcile with the lack of glacial erosion in the northern plain, as a large retreating glaciated ocean hardly leaves no trace?

                • Ramses Ramirez July 2, 2020, 1:25

                  Hi Nicky. The problem that I have with Kite’s scenarios (in my view) is that they lack a major water source (like an ocean) and don’t produce the required water necessary to form the valleys and other features. As we showed in Ramirez et al. (2020) (see also Kamada et al. 2020), without a major water source, there is no appreciable fluvial erosion, even when the climate becomes warm. The existence of the valley networks (plus their morphology), and that of the overall landscape erosion, mandate the existence of a large source of water.

                  Plus, as we also discuss in our 2020 paper, the ocean on Mars would not have been that deep. So, the resultant ice following the warm period may not have been thick enough to form “wet-based” glacial erosion features, especially if the geothermal heat flux and near-surface air temperatures were very low. These conditions would have reduced the capacity to melt the base of the glacier and form these glacial features.

                  If early Mars was “icy”, I think that we would have found at least some dry-based periglacial features (e.g. frost wedges) at Gale Crater and other places in the ancient terrains. That we don’t is very revealing. Indeed, we see very little evidence that Noachian valley terrains were icy. If the climate was relatively warm and arid to semi-arid, why would you find much evidence for glacial features? I predict that Mars 2020 will uncover a similar result as the other rover and observational studies.

          • FrankH June 30, 2020, 15:49

            The low mass of Mars is a major, if not primary reason for its atmospheric loss – just from thermal/Jeans escape.
            Not having a magnetic field accelerated the loss immensely – but I doubt that having a magnetic field would have given us a Mars with a substantial atmosphere. Further away, maybe.
            Also missing from the Mars story is the effect of a sustained period of large impacts that would have heated the surface locally (releasing water/gases) and heating the atmosphere – at least locally – accelerating the loss.

        • EericSECT June 30, 2020, 9:09

          Great article and excellent feedback comments.

          Looks like the weight of the evidence favors the requirement for a strong magnetic field to help maintain a stable environment, i.e: an intact atmosphere, for the 4-ish billion years it probably takes for life to become complex enough to allow the emergence of intelligence on an Earth sized rocky planet.

          And to get and maintain a strong magnetic field a planet must have a largish core (NOT Mars) and revolve fast enough to stir things up (NOT Venus) and have enough mass/radioactive decay happening to keep it molten (NOT Mars).

          The Big Splat led to Earth’s large, molten core and fast rotation, the Theia impact. Stabilized our axial tilt, so….. no excessive polar wander, which is also beneficial to complex life. Likely started plate tectonics, which ALSO assists in climate stabilization.

          This grazing impact has got to be exceedingly rare.

    • Alex Tolley June 28, 2020, 17:14

      Further evidence of oxygen and complex life connection comes from Francevillan biota.

      I think I have previously indicated this biota is controversial. It is not definitively multicellular life.

      Song-Can Chen et al make an interesting claim [The Great Oxidation Event expanded the genetic
      repertoire of arsenic metabolism and cycling (2020)] that the diversification of life after the GOE required the evolution of genes to detoxify arsenic. It is based on their timing of the sequential origin of 13 genes involved in arsenic handling that preceded the GOE. This would be a coincident factor that would support the general idea of the GOE as facilitating the evolution of complex life.

      • Nicky June 28, 2020, 21:27

        Whether it it multicellular is controversial indeed, but its biogenicity isn’t. It cannot be excluded as some sorts of cellular aggregation. It might not even be eukaryotes because FB is at least 300 million years earlier.

        But I wasn’t even focusing on its taxonomy at all. Doesn’t matter what it was, its complexity was neither seen in predecessors nor in successors, without a doubt.

        Those early advanced eukaryotes in Changzhougou Formation from Paleoproterozoic are even not comparable to FB. The trace fossils are as wide as 5mm, if in Phanerozoic it would be seen as animal trace. It could be some cell aggregates exhibiting extremely complex behaviors. FB easily falls into the realm of early complex life.

        • Alex Tolley June 28, 2020, 23:24

          It seems we are using very flexible meanings for “complex life”. You seem to want to go in teh direction that eukaryotes constitute complex life (which they are because of their discrete cell organelles). However, unicellular eukaryotes can be anaerobes and this makes the GOE irrelevant to the evolution of complexity by this definition.

          At the other end of the scale, Ramses paper on the Complex Life HZ was using constraints on air-breathing organisms, which require multicellularity and physical structures. If just eukaryotes (even pre-eukaryotes) are considered, this is an irrelevant constraint.

          I have in mind complex life as metazoa, multicellular plants (e.g. macroalgae) and fungi. I would exclude organisms that have independent cells that can cooperate closely, e.g. slime molds. The complexity goes beyond cell organelles and requires distinct cell types in a multicellular organism.

          My sense is that the Ediacaran biota represents the best base for life with these characteristics, with Cambrian life well established as the base phyla for all living metazoa.

          As these are all marine organisms, it is unclear if they are all constrained by Ramses atmospheric constraint of 2 bar N2, 0.1 bar CO2.

          As for the Brownlee and Ward thesis, having not read their book, The Wikipedia page indicates that they consider complex life:

          sexually reproducing, multicellular organisms

          . Beyond that, they leave the definition rather fuzzy. There does not have to be some binary cutoff point, but clearly the expectations are that there will be a wide range of multicellular species both aquatic and terrestrial.

          If the GOE has any relevance in terms of energetics of motile organisms as predators, then size, and hence multicellularity, is a requirement.

          • Ramses Ramirez June 29, 2020, 18:15

            Alex, the animal habitable zone of Brownlee and Ward is binary. They had defined it as the range of distances where planetary surface temperature can support liquid water (0 C) and be no higher than 50 degrees C. Beyond that it had no additional constraints as regards to atmospheric composition, atmospheric toxicity..etc.

            Although we don’t know what the respiration limits of plants + fungi are (the data are unfortunately unavailable), all of these lifeforms emerged in atmospheres with CO2 and N2 pressures well below my calculated limits. Plants and fungi simply did not exist on Earth when CO2 pressures were higher than 0.1 bar. You’d have to go far back, beyond ~ 2 Gyr ago to get to CO2 pressures that high (e.g. Kanzaki and Murakami 2015). And N2 levels may not have ever been higher than 2 bar N2. In fact, N2 may have been lower in the past than it is today (e.g. Som et al. 2012). None of this is diagnostic, but I think they are big clues.

            Regarding deep marine life.. unless it can leave a detectable atmospheric biosignature, it is not really relevant for astrobiological searches.

            • Alex Tolley June 29, 2020, 22:40

              Regarding deep marine life.. unless it can leave a detectable atmospheric biosignature, it is not really relevant for astrobiological searches.

              My point is not that marine creatures might be counterfactuals to the atmosphere composition constraint, but that surface life might similarly evolve to such resistance to the atmospheric composition. You acknowledge that in your own paper. I think this removes one objection to complex life (the more advanced kind) on planets dissimilar to Earth. IMO, it also means that this is not going to be an observational constraint for astrobiologists looking for complex life biosignatures.

              To take another example. It used to be believed that bacteria were killed by temperatures exceeding around 60C and that nothing could survive boiling temperatures of 100C. Now we have discovered thermophiles than can live beyond 100C, typically under pressure to prevent their water habitat from boiling. To my mind, this means that if Earth had a surface temperature of 100C with high atmospheric pressure, the surface would be a suitable habitat for these thermophiles and that other forms of life would evolve with that same ability to live in temperatures unsuited to the vast majority of current Earth life.

              IOW, if there is an example of life that can live in conditions outside of the assumed constraints, then it will likely be possible that more complex organisms can evolve retaining the same ability. In the immortal words of Ian Malcolm: “Life will find a way”.

              • Ramses Ramirez June 30, 2020, 8:11

                “IOW, if there is an example of life that can live in conditions outside of the assumed constraints, then it will likely be possible that more complex organisms can evolve retaining the same ability. ”

                And that is the billion dollar question. How far can complex life elsewhere deviate from the Earth example? We have no idea. Optimistically, perhaps life (including complex life) on other planets can evolve to exist in many different ways than how it got manifested on Earth. That is one possibility. However, it is also possible that for life to exist on another world, conditions must be very similar to those on our planet, and only the slightest of slight deviations are permissible at all. I don’t know which of these views is correct (or an intermediate one), which is why I wrote my paper the way I did… but I bet E.T would be far more common in the former scenario than in the latter..

                • Alex Tolley June 30, 2020, 12:47

                  How far can complex life elsewhere deviate from the Earth example? We have no idea.

                  If niche habitats on Earth were common, would that be enough to say that this alt-earth deviated sufficiently from our Earth and with it the organisms present? The thermophile example I gave is one possible deviation. If the bathypelagic fish data is real (and relevant), and if those partial pressure conditions existed on the surface of this alt-Earth, would that be sufficient?

                  It is a pity that the Sealab habitats were scrapped, as they would have provided a good place to test the effects of high partial pressures of N2-CO2 atmospheres on plants, fungi, and various model animals.

                  Tardigrades have recently become a focus of interest as they survive through various extreme conditions. AFAIK, what has not been attempted is to adapt populations to live in more extreme conditions by selective breeding (what would be the purpose?). [The high radiation tolerance of some tardigrade species might provide a useful example for complex life living on planets subjected to high radiation fluxes, such as around M-dwarfs.]

                  What I think biology shows us is that there is hardly an environment anywhere on Earth that has not been colonized by some form of life. Complex life is slightly more constrained but has proved very adaptable. How far those ranges can be extended to impact the complex life habitable zone, we don’t know. It would be easy to do some experiments, but I suspect they would have to be funded largely on one’s own dime unless they can be pitched to answer some interesting and potentially useful, questions.[ Growing plant seedlings, or cultivating fruit fly populations, in pressurized containers is a home lab experiment that could test the max_bar(pN2) = 2 thesis in just a few weeks.] Not testable is whether complex life can evolve under very different conditions, or even the same conditions with just one necessary variable missing. IMO, it will be the detection of biosignatures on a sampling of worlds that will be required to test the possible limits of [complex] life across the HZ. If chlorophylls are similar to Earth’s, then a good “red edge” signature for land plants, coupled with modeling of the needed atmospheric composition and density for the planet’s surface conditions, might be one “easier” test of complex life HZ range. (assuming that we find any exoplanet biosignatures at all!)

                  • Ramses Ramirez July 1, 2020, 7:09

                    Alex, in your thermophile example (a planet with mean surface temperature of 100 C), if my model predictions and those of others are correct, then such a planet would have already suffered a planet-desiccating runaway greenhouse at ~55 – 60 C (Leconte et al. 2013; Ramirez 2020b), potentially making it uninhabitable. I guess that another planet might be able to bypass this fate if it has a significantly smaller water inventory than the Earth (e.g. Dune) because then the water vapor greenhouse effect would be significantly weaker. However, would such a planet really be habitable? Nevertheless, the point here is that the devil is in the details. Seemingly small alterations often lead to a cascade of big changes across the entire system (and perhaps not in the desired direction).

                    Life on Earth does appear to have limits and gradations (as cool as tardigrades are!). We see it, for instance, in the Atacama desert, especially in regards to humidity… Although life can live there in surprising rough conditions, below a certain level it is is hard to sustain. Biodiversity on our planet seems to decrease in drier environments whereas it is higher in warmer and humid ones. Above a certain temperature (~120 – 130 C, assuming enough pressure), H-bonds break. So, physical/chemical constraints ultimately limit what is possible. For complex life, I suspect such constraint would be even more severe.

                    I agree that exoplanetary biosignatures (e.g. red edge) would be the most definitive way to test the CLHZ. The big challenge here is in getting the technology up to par to do so!

                • Alex Tolley June 30, 2020, 14:45

                  A cheap way to test pressures on organisms.
                  Modify a pressure paint sprayer to retain pressure (up to 3.5 bar). I would add appropriate LED lighting with a battery power pack needed for the experiment duration. Place organisms in the vessel. Pump up the pressure with a compressor. This will use compressed air maintaining the N2/O2 ratios. One could put the compressor in a tank filled with the required gas mixtures.

                  The total cost of the equipment is around $200. Crude, but would allow some quick and dirty experiments to test hyperbaric N2 tolerance and also higher pCO2 tolerance if the pCO2 can be regulated in the gas mixture. If the different organisms are placed in tubes that retain them but are exposed to the compressed gas mixture, one could test a variety of small plants and animal over different periods. A control setup in a steel can to replicate light and moisture conditions could also be used.

                  https://www.youtube.com/watch?v=_nEKaDaHfMI

                  While very crude, this could be an inexpensive way to do some simple experiments that appear not to have been done, yet would offer some insight into the pN2 issue, as well as the higher pCO2 tolerances of plants and fungi.

                  • Ramses Ramirez July 1, 2020, 7:12

                    Thanks for these experimental setup ideas for testing CO2 and N2 respiratory limits for various organisms. This gives me quite a bit to chew on!

              • Ron S. June 30, 2020, 10:09

                I suspect that the range of environments conducive to abiogenesis is narrower than those to which organisms can adapt. That is, abiogenesis at 100 C may be impossible but an organism could evolve to live there.

                • Alex Tolley June 30, 2020, 12:49

                  You may be correct. However, it it turned out that life evolved in the deep hot vents, then this argument would be false. But we just don’t know yet.

          • Nicky June 29, 2020, 18:25

            To fulfill the definition, perhaps I can take a step back on insisting it is complex life.

            We can still talk about its complexity. With all the fossil evidence, FB apparently represents a sudden rising of complexity during Lomagundi and is followed by a drop of complexity in Orosirian period, creating a complexity gap in between Rhyacian period and Ediacaran period.

            I agree that Ediacaran animals are truly something different, and FB does not perfectly fit the strict definitions of complex life. But FB does exhibit complexity which its successors fail to replicate for a billion years.

            • Alex Tolley June 29, 2020, 22:03

              But FB does exhibit complexity which its successors fail to replicate for a billion years.

              If that proves true, then fine. But at this point, I think you are hand waving away the controversial nature of the fossils. I think there is an analogy with the “Martian fossil life” in the ALH 84001 meteorite.

              • Nicky June 30, 2020, 21:30

                Not necessarily as controversial as ALH 84001, because it has only been investigated by only one research team (led by El Albani). To provide a counterargument requires another independent team to study the fossil in situ and reach a different conclusion, which has not been done yet. But I would like to see another team investigate this matter for sure. The confirmation of FB complex behaviors equally needs an independent team as well.

  • Al Jackson June 27, 2020, 6:37

    What an excellent article!
    There is a very good to-the-point article about Rare Earth at Wikipedia.
    https://en.wikipedia.org/wiki/Rare_Earth_hypothesis

    I can’t say for sure but always seemed there was a bit of a hidden agenda in this argument. At least the authors did not wear on their sleeves like John D. Barrow and Frank J. Tipler.

    • ljk June 30, 2020, 11:42

      Yes I agree about the hidden agenda. It’s called most humans do not want any competition or to lose their self-made status as the Special Creation of the Cosmos. Advanced aliens, especially those that could come here, have always been a concern to those who prefer to keep their terrestrial status.

      • Al Jackson June 30, 2020, 18:13

        When Frank J. Tipler was a postdoc at the University of Texas, I had a long argument with him , once, about SETI . It became clear to me his mind was closed because of his , shall we say, personal philosophy.

        • ljk July 1, 2020, 11:51

          I once tried to have a discussion with Tipler as well. Let us just say it ended with him having his views and no other. The last time I examined his most current ideas on SETI and especially the Omega Point, it was clear that the difference between them and religion were thin at best. They also included some concepts I found to be more like fulfilling some personal fantasies of his rather than any kind of science.

          Jesuit Pierre Teilhard de Chardin had his own version of the Omega Point, but at least he did not try to pretend to have another agenda in mind.

      • Abelard Lindsey June 30, 2020, 22:57

        I’m a “rare Earth rare complex life sort of guy. But that does not make us special. It only means we are very lucky. Its just blind luck, nothing more.

        • Michael Fidler July 1, 2020, 13:26

          Well we are so rare that we are the only planet in the solar system that has 50/50 percent total to annular solar eclipses. But don’t tell anybody that there is a elephant in the room. We are also the densest planet in the solar system, which means earth had the highest in radioactive elements on its surface. Which means life could not evolve beyond the simple forms until the gamma levels died down. Which means low density planets will develop life early and become complex life faster. Which means low density volatile organic planets around red dwarfs will have the highest percent of intelligent life.

          All that from a total solar eclipse that should not be there. Rare it is around a big sun as the Pi into Phi creates from the center of all, another bubble universe,

          • Alex Tolley July 1, 2020, 14:01

            What significance does a total solar eclipse have on life?

            Even if Earth has/had a higher gamma radiation flux from the crust, relatively few meters of seawater would block that radiation.

            • Michael Fidler July 1, 2020, 20:33

              Well I for one, do not gamble, it’s the odds.

            • Michael Fidler July 3, 2020, 2:41

              Take a look at this;
              1. PT on early earth with the spreading oceanic ridge spewing high levels of radioactive elements and hot radioactive water from the still hot lithosphere/mantel. Higher level of impacts on thin ocean crust churn more material from deeper interior of earth. Life might develop on oceans surfaces at this time but black smokers and and deep ocean still radioactive.

              Nucleosynthesis_periodic_table.
              https://upload.wikimedia.org/wikipedia/commons/3/31/Nucleosynthesis_periodic_table.svg

              Did a Supernova Give Birth to Our Solar System?
              https://www.space.com/35151-supernova-trigger-solar-system-formation.html

              Large impacts even up to current time would cause large amounts of radioactive element to surface. The ancient oceans are gone and have been recycled but some material may still exist along continental boundaries and recent events may be found in the fossil record of ocean spreading from the ridge. (Think tree rings.)

              The Earth’s Heat.
              https://www.radioactivity.eu.com/site/pages/Earth_Heat.htm
              Radioactive decay is key ingredient behind Earth’s heat.
              https://phys.org/news/2011-08-radioactive-key-ingredient-earth.html
              Radioactive decay accounts for half of Earth’s heat.
              https://physicsworld.com/a/radioactive-decay-accounts-for-half-of-earths-heat/

              2. Solar system passes thru our galaxies arms where recent and often super novas have spewed large amount of radioactive elements. Earth impacts and rain of radioactive elements increase on continents and oceans.

              Supernovae showered Earth with radioactive debris.
              https://phys.org/news/2016-04-supernovae-showered-earth-radioactive-debris.html

              3. Most red dwarfs are ancient population II stars in halo orbits around galaxy and pass thru galaxy arms infrequently and very quickly.

              Stellar Populations in the Galaxy.
              https://courses.lumenlearning.com/suny-astronomy/chapter/stellar-populations-in-the-galaxy/

              4. This depends on red dwarfs rate of production of neutrinos but may effect the amount of radioactivity in both early and later life of M dwarf stars since the planets are much closer to the star.

              The strange case of solar flares and radioactive elements.
              https://phys.org/news/2010-08-strange-case-solar-flares-radioactive.html
              New system could predict solar flares, give advance warning.
              https://www.purdue.edu/newsroom/releases/2012/Q3/new-system-could-predict-solar-flares,-give-advance-warning.html
              Earth is closest to the sun in January.
              https://s3-ca-central-1.amazonaws.com/quincy-network/wp-content/uploads/sites/6/2019/01/14.jpg

              [Submitted on 6 Feb 2020]
              Solar Flare Detection Method using Rn-222 Radioactive Source.
              “Solar neutrino detection is known to be a very challenging task, due to the minuscule absorption cross-section and mass of the neutrino. One research showed that relative large solar-flares affected the decay-rates of Mn-54 in December 2006. Since most the radiation emitted during a solar flare are blocked before reaching the earth surface, it should be assumed that such decay-rate changes could be due to neutrino flux increase from the sun, in which only neutrinos can penetrate the radionuclide. This study employs the Rn-222 radioactive source for the task of solar flare detection, based on the prediction that it will provide a stable gamma ray counting rate. In order to ascertain counting stability, three counting systems were constructed to track the count-rate changes. The signal processing approach was applied in the raw data analysis. The Rn-222 count-rate measurements showed several radiation counting dips, indicating that the radioactive nuclide can be affected by order of magnitude neutrino flux change from the sun. We conclude that using the cooled Radon source obtained the clearest responses, and therefore this is the preferable system for detecting neutrino emissions from a controlled source.”
              https://arxiv.org/abs/2002.02787

              Advance life did not develop on earth until last billion years mostly because of an overall higher radiation levels and the particularly long Potassium-40 half-life.

              To find intelligent life look for Population II M dwarfs halo stars that are passing near the solar system. I believe there are quite a few of them nearby. ;-}

        • Hamilton1 July 1, 2020, 15:50

          On the contrary; if we could prove that we are the only intelligence in the Universe, it would be the strongest possible evidence that a statistically inexplicable event has occurred. That life was deliberately put here by an intelligent agent in other words. Personally I don’t find the reasoning in the Rare Earth book overly compelling however.

          • Dave Abbey July 2, 2020, 11:41

            “…if we could prove that we are the only intelligence in the Universe, it would be the strongest possible evidence…’ ‘That life was deliberately put here by an intelligent agent…”

            I don’t think I quite follow the logic, unless you propose an intelligent agent from outside the Universe, which raises the question to a whole new level. What created it?

  • Alex Tolley June 27, 2020, 13:37

    I just wanted to say that the references cited by the commenters have really raised the quality of the discussion of this topic for me. The constraints on the planetary conditions for the emergence of complex life seem to be getting better understood. However, like physics, we have more theory than experimental results. I hope experimental data will come in due course to confirm or disprove those theories.

    It will be so frustrating to [astro]biologists if/when evidence of complex life arrives, but with no means to examine those worlds and organisms in detail without FTL flight (or in the deep future with sublight flight. Science missions isolated from the rest of humanity.)

  • Gary Wilson June 27, 2020, 14:45

    An extremely well written, well referenced article Dr. Ramirez. Thank you very much. Now we have to start looking at the things we know nothing or next to nothing about that are essential if we are to really know how rare Earth is as far as complex life is concerned. In no particular order: we know nothing about the prevalence of moons around rocky planets in the HZ or CLHZ; we don’t really know how big the Galactic Habitable Zone is (how much or how little metalicity is really necessare for complex life to arise) and we don’t know what the real cutoff from the galactic centre is as far as super novae and gamma ray bursts are concerned; we don’t know the range of water is that will allow complex life to arise; we don’t know if there are many routes by which abiogenesis can occur; and we don’t know that there even is a Fermi paradox as many explanations have already been provided that would suggest why we haven’t been visited by aliens (at least since the Industrial Age began) yet. Or have we? If aliens arrived 200,000 years ago and inconveniently left no permanent artifacts how would we know? How about 50,000 years ago? Ten thousand years ago? It’s just an impossible question to answer. I think we have a very long way to go to even begin to answer the question of whether Earth is rare. Has that ever been true before? We used to thing we were the centre of the Universe, and then we were the centre of the solar system and so on and so on. I think a huge amount of humility is required to realize we know next to nothing about what is going on, even in our own galaxy. I love the desire to test our limits though. Thank you again for your articles on here.

    • Ramses Ramirez June 28, 2020, 8:52

      Thanks for the comments Gary! You just outlined what makes my work so exciting and why studying planetary habitability and origins of life remain crucial to improving how we conduct the search for extraterrestrial life.
      Science is slow, but we are making steady progress.

  • Gary Wilson June 27, 2020, 19:05

    Just out of curiosity, do we have even a rough idea of how much volume of space and how many stars the Galactic Habitable Zone encompasses? That is assuming this descriptor really is useful or in any way accurate I suppose. I’m definitely not convinced by it.

  • AlexTru June 28, 2020, 3:39

    Summary conclusion from this article: our knowledge about “ what life is ?” extremely limited.
    This is equation that has lot of variables , where every variable is unknown, we even do not know how much variables should we use in simplified form of this “life” equation to get even very approximate result.
    If Quantum mechanics invented to use wave function ( probability ) to solve somehow problem related to uncertainty of human knowledge, when we are talking about “secret of life“, modern biology cannot even propose some analog of “wave function” for life existence.
    What we have? Only if, if, if … most of ifs have very limited scientific ground.

    • Al Jackson June 28, 2020, 9:10

      There are equations to address this, what irks me is how seldom I see the uncertainties of the variables stated. Carl Sagan pointed this out a long time ago. The ‘outer’ terms in the Drake equation (as and example) have huge error bars, such that any number derived from it has to be taken with an extreme degree of caution.

      • AlexTru June 29, 2020, 3:03

        I meant more common question : “What is life?” .
        Did not planed to write about Drake’s equation, despite my comment is fully applicable to Drake’s equation too , because equation has deal with advanced ,intelligent, forms of life, the any type LIFE s only one variable in this equation.
        I know that SETI every time try to convince public that they are searching ET life, when in reality SETI’s ability to find life is limited by small subset of huge “life’s” set.
        By the way, technological signature detection – does not mean automatically there is life, this techno signature can be artifact , left of dead civilization (beacon of so), but there is no life anymore…

  • ole burde June 28, 2020, 12:18

    IF single-celled life can evolve around hydrothermal vents on a water covered world, and can survive for a long period , then it might get a chance to evolve further in a later period : the solar wind wil blow away H2 at a much higher rate than O2 , creating an oxygen atmossphere …..and possibly in an even later stage losing enough water to expose a continent. Oxygen pressure could become too high eventualy , but could be buffered for a very long time by the oxygenation of rocks….variations of this scenario might play out on many tidally locked exoplanets with waterdominated atmosphere , the main difference being in how long TIME it will take to reach optimal conditions……but time is something that Red Dwarfs have

  • DCM June 28, 2020, 13:13

    Could the Cambrian Explosion have been the result of life developing some particular characteristic such as direct exchange of DNA between individuals or some superior metabolic process that made cells more efficient?
    Not an external influence at all but perhaps even a chance development internal to life as it was….?

    • Ramses Ramirez June 28, 2020, 22:33

      Hi DCM,

      This is a very good question. To be frank, I am not sure we really have a clue. The sudden increase in animal diversity at the Cambrian probably cannot be explained by just rewiring existing genes or an increase in O2 (and for the latter, what is the mechanism for the great increase in genetic novelty and why the time lag?).

      The 2018 article I reference below (for instance) illustrates the issue. There was a very large number of new genes required for the sudden emergence of animals, perhaps akin to some sort of super rapid evolution (e.g. punctuated equilibrium).

      Paps and Holland (2018)
      https://www.nature.com/articles/s41467-018-04136-5.pdf

      • DCM June 29, 2020, 12:05

        It’s likely one of those things we’ll never really get a handle on.

    • Dave Abbey July 2, 2020, 13:22

      Trying to remember, when did the last snowball Earth end?

  • Gary Wilson June 28, 2020, 15:56

    I should have said I’m not convinced we have any real idea yet of the boundaries of the Galactic Habitable Zone and therefore how many stars are contained within it. And the term rare is such a difficult one. If complex life arises every time unicellular life emerges and a planet remains habitable over billions of years then the number of planets with complex life will be fairly large surely. On the other hand if the transition from unicellular to multicellular life is rare, even over billions of years then we may indeed be a rare world. I’m assuming single celled life emerges whenever conditions are appropriate and we don’t even know that with certainly. Abiogenesis may remain the most difficult of all processes to determine. Even modelling with ever more powerful supercomputers may not be enough.

    • Ramses Ramirez June 28, 2020, 21:39

      Hi Gary,

      In my view, the Galactic Habitable Zone (GHZ) is not really important for SETI (or astrobiology) because all of our searches are occurring in a smaller area than its inferred outer limits. An accurate GHZ definition may be more important for our species’ eventual efforts to colonize the cosmos in deciding where to go.

      I am not sure that I would be that pessimistic about our knowledge on the limits of life. We do have quite a few constraints. If intelligence was extremely common, then we would have seen something by now, either aliens dropping over the White House lawn, some sort of detected radio/optical transmissions from our neighboring stars Proxima and/or Alpha Centauri, if not other technological evidence from these and other nearby stars (like “waste heat”, Dyson spheres..etc). Also, we know that this is the only planet in our solar system with intelligent life (although some may debate even that ). If intelligence (or even animal life) was extremely common, then it would have also arisen on at least Mercury, Mars, Venus, Titan, Enceladus, Europa, Ganymede, Callisto, Pluto (etc) among other rocky places in our solar system. It has not. Life also does not seem to like gas or icy giants in our experience.

      We also know how hard it is to recreate life in the lab, which has never been done (as you mentioned)… nor are we close to doing it. Life also seems to like water and carbon, at least the way we understand it.

      Thus, we know quite a lot already (and there are many other points I can raise), just based on our observations and experiments. So what is it that we know from all of this? Well, we know that (at least) animal life/intelligence is not exceedingly common, nor is it everywhere. The best we can hope for is that it is “somewhat common”, but it could turn out that complex life may be rare too. But we do not have to know how rare life is. For space missions, we should be preparing for the worst possibility (life is rare) and design the best technology we can muster.

  • Geoffrey Hillend June 28, 2020, 17:31

    Quote by Ramses Ramirez: ” Such compressed and dense CO2 atmospheres would also be relatively cool towards the top, and would exhibit relatively low escape rates as compared to the O2-rich low CO2 atmosphere of Earth.” This is not correct. Venus has a faster atmospheric escape rate due to hydrodynamic escape or thermal escape and photodissociation. Hydrodynamic escape pulls heaver atoms with it into space. Source Google and Wikipedia. https://en.wikipedia.org/wiki/Atmospheric_escape

    Venus looses much more atmosphere than Earth everyday through to solar wind stripping, a process that does not occur with Earth’s atmosphere since the solar wind is deflected by the magnetic field. This is supported by the larger DH2O ratio of Venus compared to Earth which is 2:1 so Venus has twice as much DH2O than Earth DH2O ratio tells how much water a planetary atmosphere has lost into space It also tells how much oxygen has been lost since the lighter hydrogen drags oxygen atoms with it into space.

    The primary atmospheric escape on Venus is hydrodynamic escape or thermal escape and photodissociation, but not Jeans escape. Ibid. A cooler, stable thermosphere of Venus does not apply to Jeans escape which occurs higher in the atmosphere at the exobase and depends on the mass, gravity strength and escape velocity of the planet and solar radiation received by the planet and it’s distance from the Sun. Venus has a lower escape velocity than Earth.
    https://geosci.uchicago.edu/~kite/doc/Catling2009.pdf
    https://www.annualreviews.org/doi/full/10.1146/annurev-earth-060313-054834

    • Ramses Ramirez June 28, 2020, 20:30

      Hi Geoffrey,

      Thanks for your comment. One should properly distinguish between thermal (e.g., hydrodynamic, Jean’s escape) and non-thermal escape (e.g., sputtering, photochemical..) processes.

      Although hydrodynamic escape was very potent on early Venus, early Mars, and early Earth (this is what the D/H ratios tell us), hydrodynamic escape is not a major atmospheric loss process occurring on those planets today. This is because of the much lower EUV fluxes today as compared to in earlier times.

      Escape on Venus today is mostly dominated by non-thermal escape processes (including solar-wind stripping). For Earth, the magnetic field provides additional protection against some of those sorts of losses ( although charge exchange and polar winds still induce losses.. thermal jean’s escape is also important). I agree with you, which is one reason I say that magnetic fields help sustain habitability on habitable planets.

      That said, what I meant in my comment is that atmospheres dense in CO2 (like Venus) tend to have cooler thermospheres than otherwise (all else equal) if they were not rich in CO2. This contributes to relatively lower escape rates on CO2-rich planets like Venus than what would otherwise be the case. This is likely why Venus’ dense CO2 atmosphere was not lost with time. In contrast, Earth’s thermosphere is warmer and less stable against solar EUV/XUV (specifically) because CO2 is a minor constituent. The relevant paragraph in the review paper you cite is the following:

      “The thermospheres of Venus and Mars are much colder than that of Earth, at ∼200–300 K, and are much more stable in response to variations in solar XUV radiation because CO2 is the dominant atmospheric gas. In contrast, the hydrogen- and helium-dominated thermospheres of Jupiter and Saturn are on the order of 1,000 K even though these planets are far from the Sun, because hydrogen and helium have relatively weak radiative cooling capabilities. Thus, a general rule for the thermosphere temperature is that CO2 atmospheres have cooler thermospheres and other atmospheres have hotter thermospheres.”

  • Thomas W. Hair June 28, 2020, 18:41

    Hi Ramses, very well written and reasoned article. I just have one flea to pick from your trousers:

    “As exciting as this latter prospect is, it could also be rather frightening. After all, some of these civilizations may be more advanced than we are, requiring the formulation of careful protocols to ensure safe contact.”

    They will all be far more advanced than us (it’s almost a mathematical certainty), and they will almost certainly be non-biological because any intelligence would want to live as long as possible. They also know we are here because they can directly image our planet to a degree that allows them to know something special is going on even if their view is delayed by thousands of light years. The METI debate is a dead idea.

  • Gregory Benford June 28, 2020, 20:16

    Excellent work, RR!
    Happy to see this subject expanding.

    • Ramses Ramirez June 29, 2020, 18:17

      Thanks Gregory. We definitely know more than we did 20 years ago and we will know even more in the next 20!

  • James Benford June 28, 2020, 20:33

    Dr. Ramirez says “If the Rare Earth hypothesis is correct, a SETI search for complex (i.e., intelligent) life faces unbelievably low chances of success.”
    One could ask: If Breakthrough Listen doesn’t find signals in its 10-year effort, will the SETI proposition be practically falsified? No, only that there is no intelligent life broadcasting signals toward Earth at the time we’ve listened, passing through at the speed of light.
    But the ultimate “technosignature” is an alien artifact. Stars come very close to our solar system frequently, ~ 200/Myr come within a 10 light years of the solar system. An extraterrestrial civilization that passes nearby can see there is an ecosystem here, due to the out-of-equilibrium atmosphere. They could send interstellar probes to investigate. The best locations from which to observe are near Earth: on the Moon’s surface, the Earth Trojans and Earth co-orbitals.
    The great virtue of searching for artifacts is their lingering endurance in space, long after they go dead. Another virtue of this Search for Extraterrestrial Artifacts (SETA) is that it can be falsified: if we investigate these near-Earth objects and don’t find artifacts, the concept is disproven for this near-Earth region.

    • Alex Tolley June 29, 2020, 14:58

      I endorse the need to search, although within reason. What I am not sure about is the endpoint concerning falsifiability. If the object is well hidden, or perhaps very small, it will be hard to find. Even on our planet, archaeological objects the size of cities remain hidden for 1000s of years despite humans walking over them and deliberately searching. Just look for example at the Roman finds just now turning up.

      SETI might have assumed that we could listen in on a “galactic club” using narrowband radio frequencies. This clearly was not found, with the search increasing in em frequencies, and scope both in space and with regards to types of evidence.

      Because not finding something is not proof of its non-existence, one is caught in the “black swan” problem. For objects in the solar system, even just nearby, lack of evidence can always be interpreted as “we just didn’t look hard enough”, and the search continues. You can falsify “There are large alien artifacts easily detected in near-Earth space using X method”, but not “There are alien objects in near-earth space”.

      In sci-fi, alien objects just present themselves. In 2001: A Space Odyssey, it was the magnetic anomaly that allowed finding the monolith. In the precursor short story, The Sentinel, it was the glint of sunlight off the prismatic tetrahedron that caught the eye of the moon explorer. Had either artifact have been intended to stay hidden, there would have been no discovery for millennia, possible forever.

  • Dave Abbey June 29, 2020, 11:01

    If I recall correctly, Luna has a much lower density than Earth, perhaps because the Thera collision left a significant amount of it’s core at the Earths core. At the same time, a significant amount of Earth’s forming crust may have been knocked into orbit. Presumably some of the material was knocked beyond orbit around Earth, some of which may have eventually escaped Earth orbit entirely.

    My perception is that none of our terrestrial planets have plate tectonics, although Mars may have started down that path before freezing-up.

    I speculate that in addition to other much discussed possible merits of having a large moon, perhaps reducing the amount of crustal material on the Earth may have been a significant factor in making the Earth conducive to the inception of life and continuously habitable to this time.

    I avidly read ‘Rare Earths’ when it was first published, and really enjoyed reading your article.

    Regards, Dave Abbey

  • Michael June 29, 2020, 14:49

    Wonderful article Paul,

    Would Stellar Elementology be more fitting than Stellar Mineralogy as there are no minerals present in stars but maybe on the coolest ones.

  • Geoffrey Hillend June 30, 2020, 0:01

    Hydrodynamic escape can use heaver elements like oxygen, etc. but I forgot although it is a thermal process, the atmosphere has to behave like a fluid. I agree that Venus atmospheric escape is dominated by suprathermal mechanisms that is photochemical energy into kinetic energy and charge exchange, etc. I stand corrected.

    Quote by Ramses Ramerez: ‘That said, what I meant in my comment is that atmospheres dense in CO2 (like Venus) tend to have cooler thermospheres than otherwise (all else equal) if they were not rich in CO2. This contributes to relatively lower escape rates on CO2-rich planets like Venus than what would otherwise be the case. This is likely why Venus’ dense CO2 atmosphere was not lost with time. In contrast, Earth’s thermosphere is warmer and less stable against solar EUV/XUV (specifically) because CO2 is a minor constituent. ”

    Venus mostly carbon dioxide atmosphere was not lost through time because Venus does not have any plate tectonics.

    Without any plate tectonics, Venus cannot have a carbon cycle like Earth, so the carbon dioxide can’t be removed from the air, so it has built up over a long time. In the carbon cycle, the rain takes the carbon dioxide out of the atmosphere through the Urey Reaction with the land and rivers which transport the calcium carbonate to the sea. It is transported to the sea as calcium carbonate and it sinks to the bottom of the ocean where it builds up as limestone. The limestone is subducted into the mantle where it is melted and the carbon dioxide is expelled through volcanoes or returned to the atmosphere. Venus does have some volcanism which has added more carbon dioxide to the atmosphere. Carbon dioxide is also a heavier molecule, so it is harder to escape based on thermal escape. The escape rates are not necessarily lower on planets rich in carbon dioxide. The escape rate on Earth is still lower than on Venus since Earth has a larger escape velocity than Venus and it receives much less solar radiation than Earth. The speed of the escape rate always depends on a lot of factors Thermal escape includes the temperature, the escape velocity based on the mass of the planet, a planet with much less mass has a lower escape velocity than one with a larger mass. Also the kinetic energy of the atom or molecule which is determined by the atomic weight. The light elements H, and He escape more easily and the heavier element remain since they take more energy to escape. The lighter elements atoms will move faster than the heavier atoms at the same temperature which is why the lighter elements escape easier which is called thermal escape or hydrostatic escape, planetary science 101.

    We agree that Venus can’t be used as an example because it has not been in the life belt long enough. It had a runaway greenhouse effect which began almost a billion years ago due to the increasing brightness of the Sun which increases in brightness by seven percent every billion years. Venus is an example of what our Earth will look like a billion years in the future.

  • Geoffrey Hillend June 30, 2020, 0:28

    Excuse me, I was supposed to write that Earth’s atmospheric escape rate should be lower for a particular gas because it receives less solar radiation than Venus based on thermal escape. Earth also has a higher escape velocity.

  • Athena Andreadis June 30, 2020, 16:12

    My review of Rare Earth, still relevant. http://www.setileague.org/reviews/rarearth.htm

  • Geoffrey Hillend June 30, 2020, 20:43

    I agree with Ramses Ramirez that the non thermal escape processes like solar wind stripping, sputtering etc. Looking at the layers of an atmosphere like Venus more carefully, I see why Jeans escape and thermal escape is not dominant. It’s how thermal escape is defined which confused me and of course CO2 is a heavier molecule. The exobase is where the gas molecules escape into space. The exobase on Venus at only 160 kilometers, but it is 500 km on Earth. The ionopause on Venus is at only 100 km, the ionopause is the place where the solar wind and ionospheric pressures are balanced. The exosphere and exobase is lower on Venus that on Earth since Venus has no magnetic field and bow shock to block the solar wind. The bow shock on Venus is from the solar wind shearing at the ionosphere and exosphere. Also there is solar wind blows around the ionopause and exobase.

    Quote by Henry Cordova: “The only double stars we know have exoplanets orbit close to each other within several AU’s apart. There are not any double stars that are more than five AU apart and less 40 AU’s that we know have exoplanets.” Yes, that was my point. I am making the hypothesis or educated guess that the of the double stars of a certain distance apart never form an accretion disk, so they can’t make any planets. The reason is that when two stars form a certain distance apart they form a ring of gas and dust with the two stars at opposite ends of the ring. The ring is not very wide like a hoop with no gas and dust in the center so no accretion disk can form. There is no gas in the center space because the two stars steal all the angular momentum that any gas, and dust would need to form planets. “A small portion of the original mass of the solar system (the planets) grabbed almost all of the angular momentum of itself and the larger portion of mass free to form a central body, the Sun.” P. 231. One Hundred Billion Sun’s, Kippenhahn, 1983. I think I recall it saying that half stars in our galaxy are double stars which might not have planets as a result. I don’t know how he go half or the recent estimate is only one third of the stars are double stars, and I don’t what percentage of the double stars fit the parameters of 5 and 40 AU apart which are my parameters, an educated guess based on astrophysics and Kippenhahn’s ring idea, The generally accepted idea of 1983 was that all double stars didn’t have any planets, but that is not the case today as the close binaries have been found to have planets. I am making the assumption that they don’t based on his model, so only if that were fact it would limit the number of stars in our galaxy, so not every star can have planets. I am assuming the Alpha Centauri system formed by the ring hypothesis, so it does not have any planets.

  • Abelard Lindsey June 30, 2020, 22:48

    I believe plate tectonics are essential for making a habitable planet. Plate tectonics allows for all of that heat and energy inside the Earth to be released in a controlled manner in the form of vulcanism and earth quakes. Venus does not have plate tectonics, which is why it has crustal overturn events every 500 million years or so. It is these crustal overturn events that makes Venus such a hot hell. The question is: Is a giant moon-forming impact necessary to make plate tectonics? More specifically, is the crust-thining that resulted from the Giant Impact a necessary pre-condition for plate tectonics? How often are these kind of impacts early in a planet’s history?

  • Jim Benford July 1, 2020, 6:56

    Alex Tolly:

    Objects on the moon will not be buried as they are on Earth. The accumulation of dust, dirt, and the effects of water at a very rapid rate due to the many processes on the surface bury objects on earth, such as ancient cities. In contrast, on the moon objects deteriorate or are covered in a far far slower pace. The lifetime of objects on the moon has been quantified; NASA did a study of how long Apollo sites would survive. For spacecraft it’s estimated to be of order 1 billion years. The median survival time for centimeter-to-meter scale rocky material on the surface of the Moon is estimated to be between 40 to 80 Myr, with some surviving up to 300 Myr, depending on the material. Spacecraft hardware will surely be harder than rocks. Therefore objects left there hundreds of millions of years ago will still be observable now.

    Although the advocates of nanotechnology think that we could be observed by tiny objects, that doesn’t account for the reality that you can’t look at the Earth from far through a tiny aperture and get any information. They likely will also be large, as aperture size needed to study the Earth will determine the scale of a spacecraft. Nanorobots are not going to be able to observe from a distance. So we cannot and should not look for them.

    • Alex Tolley July 1, 2020, 14:15

      Jim, you seem to assume that such probes would not hide beneath the surface. You may well be right, as they would be no need unless they are deployed specifically to monitor planets where species are becoming technological and able to leave the planet.

      However, the Lunar Reconnaissance Orbiter camera has a maximum resolution of 50cm/pixel, which suggests to me that if there is a probe sitting on the lunar surface, it is probably less than a meter in diameter to remain relatively well hidden from detection. If it isn’t and was the size of one of our spy satellites, it will be detected at any moment now.

      If it isn’t detected in the near future does that falsify the sub hypothesis that there is a probe sitting on the Moon? Or does it mean the search will require increasing resolution or even subsurface imaging, which was my original point?

      • Alex Tolley July 1, 2020, 19:45

        Seth Shostak makes the argument for doing the search in this piece on why SETI? Why Look for Extraterrestrial Life?

        In the text he says:

        The hypothesis of a universe laced by biology can’t be falsified.
        So, by the conventions of science, you could say that our experiments to find other life – whether microbes in the Solar System or extraterrestrials on a planet hundreds of light-years away – are not really experiments: they’re searches. They’re exploration.

        I suggest the same applies when hunting for probes/lurkers. And there is nothing wrong with exploration.

    • Abelard Lindsey July 3, 2020, 10:50

      It seems to me that if alien intelligence is or was out there at some point in the past, we should be looking for evidence that our solar system was visited at sometime in the past, say 500 million years ago. If aliens visited Earth then, they would most certainly have visited the Moon as well. We should look for artifacts there since, as you pointed out, they should remain visible and of recognizable artificial origin for up to a billion years.

      • Alex Tolley July 3, 2020, 13:42

        If they are easy to spot on the surface. But if they are camouflaged or under the surface? The LRO camera has not turned up anything unusual on the surface with a 50cm/pixel resolution. So “easy to spot” seems ruled out except for areas not yet photographed in hi-res.

        We could identify/track such objects in Earth orbit unless they are in stealth mode, so that also seems to no longer be an option. NEAs are next, or in free space at Lagrange points. Of course, “space is big, really big…”, but we are steadily detecting objects further and further out. Maybe planetary defense or future military radar in orbit will increase our knowledge enormously, and something turns up. However, I don’t expect anything like the objects in SciFi movies.

  • DCM July 2, 2020, 4:35

    Overall I’m reminded of using Earth as proof of divine creation because life wouldn’t be possible if conditions weren’t as they are. In reality such “proofs” simply assert that if things weren’t as they are they’d be otherwise.
    Alas, it’ll likely be centuries before we have enough actual concrete information…..

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