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Probing Ultrahot Jupiters

Speaking of getting really, really close to a star, as we were yesterday in our discussion of the Parker Solar Probe, I couldn’t help but turn to new computer models of the ‘ultrahot Jupiter’ WASP-121b. I still find it delightful that the earliest exoplanet detections involved a category of planet that few scientists had imagined existed. These days we routinely discuss gas giants blisteringly close to their hosts, and even manage to extract information about their atmospheres through transmission spectroscopy, but few people expected such planets when we began to discover them.

In fact, Apollo 11’s Buzz Aldrin had a role to play in what may be considered to be the first prediction of the worlds we would start calling ‘hot Jupiters.’ Working with John Barnes on his novel Encounter with Tiber (Grand Central, 1996), Aldrin asked physicist Greg Matloff whether a hydrogen-helium atmosphere as found in a Jupiter-class world could survive in an inner stellar system. Here’s how Matloff recalls the discussion:

Although I was initially very skeptical since then-standard models of solar system formation seemed to rule out such a possibility, I searched through the literature and located the appropriate equation (Jastrow and Rasool, 1965)….To my amazement, Buzz was correct. The planet’s atmosphere is stable for billions of years. Since I was at the time working as a consultant and adjunct professor, I did not challenge the existing physical paradigm by submitting my results to a mainstream journal. Since “Hot Jupiters” were discovered shortly before the novel was published, I am now credited with predicting the existence of such worlds.

Indeed, for this was just at the time when 51 Pegasi b swam into our consciousness in 1995 (the book was finished but not yet published when the discovery was made). Now we knew that hot Jupiters were out there, and the radial velocity method of exoplanet discovery ensured that large planets close to their star would be the most likely to be detected in our earliest efforts.

The Realm of the ‘Ultrahot’

But hot Jupiters are but one variety of inner system gas giants. Today we can catalog a planet like WASP-121b as a member of a still more unique class of worlds, distinguishing between ‘hot Jupiters’ and those gas giants that come astoundingly close to their stars.

Dubbed ‘ultrahot Jupiters,’ these worlds reflect about as much light as charcoal. What distinguishes them is a temperature that on the dayside causes them to glow like an ember. Thus the image below, in which WASP-121b is simulated based on computer models that draw on observations of the planet conducted by the Spitzer and Hubble space instruments.

Image: These simulated views of the ultrahot Jupiter WASP-121b show what the planet might look like to the human eye from five different vantage points, illuminated to different degrees by its parent star. The images were created using a computer simulation being used to help scientists understand the atmospheres of these ultra-hot planets. Credit: NASA/JPL-Caltech/Aix-Marseille University (AMU).

The word ‘hellish,’ so often used to describe the surface of Venus, can with even more justice be applied to ultrahot worlds like this. They orbit closer to their host stars than Mercury does to the Sun, tidally locked and with temperatures that can range between 2,000 and 3,000 degrees Celsius. Even the nightside of such a world can reach 1,000 degrees Celsius, though as we’ll see, this makes enough of a difference to explain some anomalous observations.

For new work from Vivien Parmentier (Aix Marseille University, France) and colleagues goes into the question of why we find no water vapor in the atmospheres of these worlds. Hot Jupiters — gas giants in an inner system that experience dayside temperatures below 2,000 degrees Celsius — have been found with abundant water vapor in their atmospheres. But ultrahot Jupiters seem to lack it. One theory on why is that these planets formed with high levels of carbon instead of oxygen, but the new study points to the occasional traces of water that have been detected at the dayside-nightside boundary as a refutation of the idea.

Parmentier’s team, applying a model of brown dwarf atmospheres developed by co-author Mark Marley (NASA Ames), went to work on ultrahot Jupiter atmospheres as if they were the atmospheres of stars. After all, says Parmentier, “The daysides of these worlds are furnaces that look more like a stellar atmosphere than a planetary atmosphere. In this way, ultrahot Jupiters stretch out what we think planets should look like.”

The team used Spitzer observations of WASP-121b at infrared wavelengths to probe carbon monoxide levels in its atmosphere. CO atoms have a bond strong enough to withstand the dayside heat. The result: The planet’s atmosphere reveals a strong temperature gradient, burning hotter higher up than further down. A uniform atmosphere could have masked the signature of water molecules, providing one explanation for the apparent lack of water.

But the carbon monoxide work showed that the answer lies elsewhere. According to the study, hydrogen and oxygen atoms are indeed found on ultrahot Jupiters, but the strong irradiation on the dayside simply tears the water molecules apart. The researchers have placed WASP-121b in the context of recently published studies authored by Parmentier, co-author Michael Line (ASU) and others on fellow ultrahot Jupiters WASP-103b, WASP-18b and HAT-P-7b.

They have concluded that the fierce stellar winds of the dayside blow the broken water molecules onto the nightside, where they can recombine and condense into clouds before, inevitably, drifting back onto the dayside to undergo the destructive process again.

The paper sees the transmission spectrum of WASP-121b as being consistent with a solar composition atmosphere having partial cloud coverage. Within its dayside atmosphere, molecules are being continually sundered. And here is why we see no water:

Ultra hot Jupiters with dayside temperatures larger than 2200K are good targets for thermal emission measurements. However, the majority of the observed planets have weaker-than-expected spectral features in the 1−2µm range. Using the example of WASP-121b, we interpret this lack of strong features as being due to a combination of a vertical gradient in molecular abundances due to thermal dissociation, and to the presence of H [the hydrogen anion, or negative ion of hydrogen] absorption at wavelengths shorter than 1.4µm.

Thermal dissociation affects all spectrally important molecules in the atmospheres of ultra hot Jupiters except CO. It creates a large vertical gradient in the molecular abundances. We show analytically that the presence of such a molecular gradient weakens the features in emission spectra. This is a qualitatively different effect than a global depletion of the abundances.

Image: Jupiter-like exoplanets are 99 percent molecular hydrogen and helium with smaller amounts of water and other molecules. But what their spectra show depends strongly on temperature. Warm-to-hot planets form clouds of minerals, while hotter planets make starlight-absorbing molecules of titanium oxide. Yet to understand ultrahot Jupiter spectra, the research team had to turn to processes more commonly found in stars. Credit: Michael Line/ASU.

What a place an ultrahot Jupiter must be. The idea of this kind of circulation in the atmosphere is given force by the previous detection by Hubble of clouds at the boundary between night and day. The same process may affect titanium oxide as well as aluminum oxide. Because the latter is the basis for the gemstone ruby, this JPL news release speculates that there could be clouds producing rains of liquid metals and fluidic rubies in the atmospheres of ultrahot Jupiters. That would make the ultrahot Jupiter about as exotic an environment as we can conceive.

The paper is Parmentier et al., “From thermal dissociation to condensation in the atmospheres of ultra hot Jupiters: WASP-121b in context.” submitted to Astronomy & Astrophysics (preprint).

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Musings on the Parker Solar Probe

The first thing I did when I heard about the Parker Solar Probe’s successful launch (0731 UTC Sunday) was to double-check the spacecraft’s projected velocity when it makes its closest approach to the Sun. I always think in terms of high speed when contemplating operations close to our star, the legacy of the two Helios missions, which at present hold the record as fastest man-made objects. Placed in highly elliptical orbits after their launches in 1974 and 1976, the Helios spacecraft managed a sizzling 70 kilometers per second.

The Helios missions were a joint venture between what was then West Germany’s space agency and NASA, the craft themselves built by German aerospace firm Messerschmitt-Bölkow-Blohm. Helios 2 flew closer to the Sun by about 3 million kilometers, closing to 0.29 AU (43 million kilometers), which took it inside the orbit of Mercury. The Parker Solar Probe ups the ante considerably, with an eventual closest approach of just 6.1 million kilometers.

The spacecraft at that point will be moving at roughly 192 kilometers per second, easily eclipsing the Helios record. Now imagine if we could put a spacecraft at these speeds on a course for Alpha Centauri. Context is everything, and what is truly a blistering pace in comparison to our previous records turns out to be a good deal less than 1 tenth of one percent of lightspeed when pondered in interstellar terms. It gets us to Centauri A/B in 6000 years.

Image: The United Launch Alliance Delta IV Heavy rocket launches NASA’s Parker Solar Probe to touch the Sun, Sunday, Aug. 12, 2018, from Launch Complex 37 at Cape Canaveral Air Force Station, Florida. Parker Solar Probe is humanity’s first-ever mission into a part of the Sun’s atmosphere called the corona. Here it will directly explore solar processes that are key to understanding and forecasting space weather events that can impact life on Earth. Credit: NASA/Bill Ingalls.

We’ll track the Parker Solar Probe with great interest over the course of its seven year mission, which gets interesting quickly as the craft heads toward Venus for the first of seven flybys of that planet, using Venus’ gravity to tighten up its solar orbit. By November, the Parker Solar Probe will be positioned to pass through the Sun’s corona with the first of its projected 24 total passes by the Sun. Bear in mind that the corona is more than 300 times hotter than the Sun’s surface.

We have an 11-centimeter thick carbon-carbon composite shield to thank for making operations in this environment possible, one whose front surface is capable of withstanding temperatures beyond 1300 degrees Celsius. This advanced thermal protection will keep four suites of instruments alive to study plasma and energetic particles, magnetic fields and the solar wind.

That last point has great relevance to our discussions on Centauri Dreams, namely the methods we may one day use for fast transportation around the Solar System by way of building the infrastructure we’ll need for interstellar flight. The more we learn about the solar wind, which can hit 800 kilometers per second, the more we’ll understand the variables that may help us harness it through variously designed magnetic sails. That assumes, of course, that this highly mutable and unpredictable flow is manageable enough to navigate with such craft.

Image: Artist’s impression of the Parker Solar Probe spacecraft leaving Earth, after separating from its launch vehicle and booster rocket, bound for the inner solar system and an unprecedented study of the Sun. Credit: JHU/APL.

I’ve commented before in these pages that there are also interesting implications for future ‘sundiver’ missions in the Parker Solar Probe. In these concepts, a solar sail, perhaps furled behind an occulter such as a small asteroid, would be taken as close as possible to the Sun before being unfurled at perihelion to achieve the highest possible acceleration. If a mission like that is ever to happen, we’ll need the kind of data the Parker Solar Probe delivers as we learn how to operate in an environment as extreme as any spacecraft has ever encountered.

The Parker Solar Probe’s solar arrays have already deployed. Immediately ahead for the spacecraft is deployment of its high-gain antenna and magnetometer boom, as well as the first of a two-part deployment of its electric field antennas. The instrument testing period begins in early September and continues for four weeks, after which science operations will begin.

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Detecting Life On Other Worlds

Now that we’re getting closer to analyzing the atmospheres of terrestrial-size exoplanets, it’s worth remembering how difficult the call on the existence of life is going to be. Long-time Centauri Dreams contributor Alex Tolley takes on the issue in his essay for today, pointing out along the way just how easy it is to see what we want to see in our data. While we can learn much from terrestrial biology, new approaches looking at ‘pathway complexity’ may offer useful indications of biology and a set of markers not constrained by our own unique sample of life on Earth. A lecturer in biology at the University of California, Alex brings us up to speed with extending our methods of life detection in ways that are ‘biology agnostic.’ Expect controversy ahead — will we know life when we see it, and how can we be sure?

by Alex Tolley

Manuel Werner, CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=633977

Life: [noun]​ The condition that distinguishes animals and plants from inorganic matter, including the capacity for growth, reproduction, functional activity, and continual change preceding death. – Oxford Living Dictionary [6]

Life, like pornography, is notoriously hard to define, but we mostly recognize it when we see it. Life, as we know it, is identified by a set of features, which individually, may be shown by non-living systems. A classic example is “fire”, that can exhibit a simple metabolism (combustion), growth (size and spread), and even “reproduction” (sparks ignite new fires). Fire, however, fails the test of life, as all terrestrial life has the cell as a basic unit, which is not a feature of fire, nor can fire evolve.

Fossils are clearly not living, yet they show the order that life exhibits which indicates that they are a remnant of an organism that was living. For example, the fossilized skull of a dinosaur shows considerable order with features that indicate it was from a living animal and very similar to other fossil skulls of its type. Fossil bone fragments are far harder to identify and experts can detect these when a layperson would see only a piece of rock. Microfossils are even harder, as the controversial objects in the meteorite ALH84001 indicate [7]. Are they natural formations or organisms?

When we consider how to recognize extraterrestrial life, we are largely constrained by the single sample we have. That should not stop us looking for Earth-type life as the low hanging fruit, as Earth-type life is an existence proof and well worth searching for signs of, whether with telescopes or probes.

Recent focus has shifted to spectroscopic analysis of exoplanet atmospheres. The logic is largely that of James Lovelock’s Gaia hypothesis, where the production of certain gases is a proxy for their generation by life. For a terrestrial-like world in the habitable zone (HZ), the existence of both oxygen (O2) and methane (CH4) implies life as these are primarily produced by life on Earth in the ratios required to prevent equilibrium. For a world more like the Archaean Era, an atmosphere rich in methane but excluding other gases like carbon monoxide (CO) indicates bacterial methanogens, as the geological serpentinization of ultramafic rocks like olivine is insufficient to maintain the CH4 levels.

It is this geological reaction that makes the presence of CH4 in the Martian atmosphere so ambiguous, as the masses are small enough to be produced by geology as well as subsurface pockets of life.

Where we have extraterrestrial samples, such as carbonaceous meteorites, asteroids and the recent confirmation of organic material on Mars, there is a need to differentiate abiotic from biotic processes. The classic examples of biotic processes based on our Earthly sample include the chirality of amino acids and sugars, the isotopic changes of elements due to favored selection in biological processes, such as the reduced carbon-13/carbon-12 ratios, and the odd number of carbon atoms in many lipids.

As our planetary probes increase in sophistication, and the idea that subsurface icy moons might be hospitable for life, there is a need to include the instruments to test for possible biosignatures to try to reduce ambiguity.

Biology as a System

Returning to the question of recognizing life, a key point is that it exhibits a number of features that need to be present so that we can distinguish it from inanimate objects. For terrestrial life, the basic unit is the cell, which encapsulates all the components needed to exhibit the features we identify with life, maintaining order and fighting entropy, by interacting with the external world. With the evolution of photosynthesis, that order is maintained by the capture of a tiny amount of the energy emitted by our sun. This is now the dominant source of energy for the terrestrial biosphere. Even the simplest unicellular organisms require hundred of genes, and therefore unique functional protein molecules to maintain themselves. Higher organisms require tens of thousands of genes, producing hundreds of thousands of unique proteins to maintain their more complex structures and life cycles.

We can again see the problem of detecting life from limited features with the three Viking experiments that proved ambiguous. Had there been a microscope to view a culture, the presence of cells, their growth and reproduction over time would have clinched the presence of life.

While this approach can work for samples in our solar system, for exoplanets, we must rely on proxies that are primarily measurable using spectrographic techniques. Conceivably, a telescope could image a world, detecting seasonal changes in photosynthetic organisms, providing direct evidence. For worlds with life still only in its prokaryotic state, remote direct imaging of life may prove impossible.

For samples in our solar system, we can expect a search based on terrestrial life analogs, so the usual suspects will be searched – proteins with chiral amino acids, DNA, lipids with odd-numbered carbon atoms, as well as more subtle signs such as carbon-13/carbon-12 ratios. But we should also look for evidence that is terrestrial biology agnostic, especially if we are hoping to discover very different life forms from unique geneses.

Sara Seager: Going Beyond the Presence of a Molecule

Sara Seager’s team has been at the forefront of considering biosignatures beyond the usual proxies of atmospheric gas mixing ratios. Her paper [8] (see also CD post Ambiguity in Life Detection​, October 31, 2017) collated the range of small organic molecules that exist and their source whether biotic or abiotic or both. At the 2018 Breakthrough Discuss conference, she noted that biology does have some apparent constraints and explained the paucity of biotic molecules with nitrogen-sulfur (N-S) bonds, even though these compounds abound in industrial chemistry because of their usefulness. Terrestrial biology is rich with thiol reactions and has evolved replication and metabolisms that generally eschew molecules with such N-S bonds. This phenomenon constitutes a possible biosignature. While this is one specific example, there are likely many others. However, constraining our ideas to terrestrial biology may result in us missing non-terrestrial biologies that are different, providing false negatives. What is needed is a more general approach that is biology agnostic.

Lee Cronin: A Generalized Approach

Lee Cronin’s group has been formulating a more biology agnostic approach, one that is based on living organisms being homeostatic systems [4]. His approach is to assume molecules can be constructed by assembling sub-units and that this confers a minimal construction pathway, which he calls “pathway complexity”. One can consider this as a tree of all possible molecules composed of the building blocks with the number of construction steps needed to build the molecule. This is an indication of the non-randomness of the molecule. If a molecule in a sample is highly enriched compared to the possible random set of molecules that could be constructed at random, this is indicative of a construction pathway that in turn is indicative of life.

Figure 1 below shows the concept of construction of a specific molecule from building blocks.

Figure 1. Illustration of a complexity pathway in blocks, with the target shown by the yellow box. A combinatorial explosion in structures is illustrated by the other faded structures shown, which are just a small set of the many alternative structures that could be constructed. (Online version in color.) [4]

Figure 2. An illustrative graph of complexity against size of the state space. Orange regions are impossible as they are above or below the bounds of the measure. The green region is where living systems may be most probable, where structures are neither too simple to be definitively biological, nor too complex to exist at all. [4]

Cronin states​ :

“We can extend the basic complexity measure above to cope with assessing the complexity of a group of objects that contain identical connection motifs (figure 5). In this case, we examine a population of objects and abstract out a common graph based on connected subunits that share features. For example, if examining a set of cups or mugs, then we can create a common graph of ‘handle connected to body’, regardless of potential variations in size/colour etc. If examining a set of human beings, then we could create a common graph of bone connectivity, ignoring variations in size/shape of individual bones, or any material in the body other than bones.”

He concludes:

“It is clear that biological and biologically derived systems have an ability to create complex structures, whether proteins or iPhones, that is not found elsewhere in nature. Assessing the complexity of such artefacts will be instrumental in searching for undiscovered biospheres, either on Earth [29] or elsewhere in the Solar System, and would make no assumptions about the details of the biology found. We propose Pathway Complexity as the natural measure of complexity for the production of artefacts. In this context, we argue that there is a critical value of Pathway Complexity above which all artefacts must be biologically derived. This approach provides a probabilistic context to extending the physical basis for life detection proposed by Lovelock [30]. In further work, we will show how this applies to a range of other systems, and propose a series of experimental approaches to the detection of objects and data that could be investigated as a possible biosignature. In the laboratory, we are interested in using this approach to develop a system that can explore the threshold between a non-living and living system. Pathway Complexity may also allow us to develop a new theory for biology. This might inform anew way to search for life in the laboratory in terms of the complex products a system produces and if they could have arisen in any abundance by chance, rather than trying to measure the intrinsic complexity of the living system itself.”

Kauffman: Self Organization Theory of Life

In 1993, Stuart Kauffman published The Origins of Order: Self Organization and Selection in Evolution [2]. I consider it a tour de force​ in theoretical biology. Of relevance to this post are chapters 7 and 8 on the concept of autocatalytic sets, and the crystallization of metabolisms.

Autocatalytic sets are best thought of as a linked set of components, e,g, catalytic RNA, that can build each component from others in the set. The effect of which is to rapidly increase the RNA species included in the set. From an origin of life perspective, Kauffman showed that the probability of autocatalytic sets arising increases to unity as the number of RNA species increases. Metabolisms similarly crystalize when there are enough reactants so that a complete, self-contained metabolism can be sustained. Again, the probability of such a complete metabolism will increase as the number of reactants increase.

As Kaufmann states:

“Thus we arrive at a new point of view. The emergence of a connected metabolism as a supracritical web requires a sufficient complexity of organic molecules and a sufficient complexity of potential catalysts. At that point, such a connected web is an inevitable emergent collective property of the chemical system.”​ [2] p348.

The relevance to Cronin’s work should be clear. Once a self-sustaining set of components appears, the components in that set will increase rapidly compared to others in the vast space of possible components. Cronin’s metrics, such as “pathway complexity” naturally emerge when considering the number of components compared to the possible components due to random reactions.

While Kauffman’s work is theoretical, Cronin has shown that lab experiments [5] support this basic concept. In terms of biology, they are agnostic in origin, therefore freeing us from focusing on terrestrial biology as our single sample of life, and informing us of possible biosignatures.

Biosignature Search

Sampling the compound space is not something that is likely to be possible anytime soon, if ever, using spectral analysis of [exo]planet atmospheres. Even Seager’s list of possible biosignature compounds are effectively trace compounds, and there is no way to determine whether her N-S bonds hypothesis works for an exoplanet from telescopic observations.

However, the solar system is another matter, and targets such as Mars, the plumes of Enceladus, the organics on the [sub]surface of comets, Ceres and the Europan sub-surface ocean are ripe for this sort of systems analysis using mass spectrometers and IR spectroscopy on probes to determine the mix of compounds in a physical sample.

While future telescopic observations that can image worlds directly may show up life as lush, boreal zones on exoplanets, nearer to home we may be able to sample the biological detritus of such worlds through wanderers like ‘Oumuamua that may have captured bacterial life from living worlds. If exoplanet life is largely bacterial, then probes sampling the upper atmosphere or even the surface can use this technique to determine if life exists without the difficulty of trying to cultivate bacterial colonies and observing the results. While interstellar probes that could sample such worlds are a relatively distant prospect, they are possible in the centuries to come using propulsion technologies that do not require new physics.

Conclusion

Confirmation bias involves seeing the data supporting what you are expecting. The lack of artificial objects in the heavens that is the context for the Fermi Question elicits polar views of “we are the only life” to “technological life is there, we don’t recognize it”. Similarly, the lack of unambiguous signals found by SETI results in a similar dichotomy. As we noted, the ambiguous objects in the ALH84001 meteorite that came from Mars have proponents for either proposition — life and non-life. Early searches for “missing links” in the evolution of humans that found a few bones and partial skulls also resulted in polar views of whether modern humans had evolved from an apelike ancestor or been created in his present form. We can be sure that any spectroscopic evidence of proxies for life – biosignatures – will be similarly interpreted.

So far, chemical analysis of samples in our solar system have been teasers, hinting at possible life, but no more. While a video of a living animal in a sample tube would be unambiguous (although there will no doubt be claims of “it is a hoax”), the most compelling approaches would be confirmation of DNA or proteins, preferably with no known terrestrial copies. This, however, assumes life is very similar to terrestrial life, and techniques used to find such molecules will miss life that may be very different from terrestrial life. Starting from the model that living systems are complex systems, yet not so complex as to be random, chemical analyses within the scope of that with existing analyzers may well be able to indicate life with far less ambiguity than the focus upon a few proxy molecules. In this regard, the theoretical bases described by Kaufmann and Cronin, and confirmed with experiments on terrestrial living organisms, offers perhaps the best approach for sampling probes that we can envisage in the near future, although the mass penalty of a microscope would be very much appreciated.

References

1. Petkowski J et al “Natural Products Containing a Nitrogen−Sulfur Bond” J. Nat. Prod. 2018, 81, 423−446

2. Kauffman S. “The Origin of a Connected Metabolism” ch 10, p343 in The Origins of Order, 1993

3. Domagal-Goldman S et al “Life Beyond the Solar System: Remotely Detectable Biosignatures” 2018, arXiv:1801.06714 [astro-ph.EP]

4. Cronin Lee “A probabilistic framework for identifying biosignatures using Pathway Complexity” 2017, Philos Trans A Math Phys Eng Sci. 2017 Dec 28;375(2109). pii: 20160342. doi: 10.1098/rsta.2016.0342.

5. Doran D et al “A recursive microfluidic platform to explore
the emergence of chemical evolution” 2017, ​ Beilstein J Org Chem.​ 2017 Aug 17;13:1702-1709. doi: 10.3762/bjoc.13.164. eCollection 2017.

6. http://en.oxforddictionaries.com/definition/life

7. http://en.wikipedia.org/wiki/Allan_Hills_84001

8. Seager, Bains and Petkowski, “Toward a List of Molecules as Potential Biosignature Gases for the Search for Life on Exoplanets and Applications to Terrestrial Biochemistry,” ​ Astrobiology 16(6) (June 201), 465-485

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A ‘Rogue’ Object’s Strong Magnetic Field

Given the spectacular interactions between Io and Jupiter — the moon plays a major role in shaping the planet’s magnetic field and contributes a cloud of particles originally produced by its volcanic activity — it’s all but inevitable that a recently discovered ‘rogue’ object would be compared to the duo. The rogue in question is SIMP J01365663+0933473, a planetary mass object of perhaps 12 Jupiter masses that is at the boundary between brown dwarf and planet. Between 12 and 13 Jupiter masses is considered to be the deuterium burning limit; i.e., above this, we would expect a gaseous object to be a deuterium-burning brown dwarf.

What an intriguing situation we find here. Originally found in data collected by the Very Large Array, SIMP J01365663+0933473 (which I’ll now mercifully shorten, as per the paper, to SIMP0136) has a magnetic field some 200 times stronger than Jupiter’s. The discovery marked the first radio detection of a possibly planetary mass object beyond our Solar System, as well as the first measurement of the magnetic field of such an object. SIMP0136 is presented as part of a broader study examining five known radio-emitting brown dwarfs observed at the VLA.

Some brown dwarfs have previously been found to have auroras, not dissimilar to those we’ve found in our own Solar System’s gas giant planets. But how they are produced here is an interesting question. The spectacular auroral displays that can enliven winter skies on Earth are the result of our planet’s magnetic field interacting with the solar wind. SIMP0136, 20 light years out, is unaccompanied by any star. Thus the Io comparison — we may be looking at an orbiting planet (or moon, depending on what the primary object is) interacting with its magnetic field.

Image: Artist’s conception of SIMP J01365663+0933473, an object with 12.7 times the mass of Jupiter, but a magnetic field 200 times more powerful than Jupiter’s. This object is 20 light-years from Earth. Credit: Caltech/Chuck Carter; NRAO/AUI/NSF.

The lead author of today’s paper on this object is graduate student Melodie Kao, recently at Caltech, and now a postdoc at Arizona State University. Kao points out in this NRAO news release that SIMP0136 can help us understand magnetic processes on both stars and planets. Radio flaring was first observed in a brown dwarf in 2001, providing evidence for strong magnetic activity.

In fact, the original detection of SIMP0136 was as part of a study into brown dwarf magnetic fields and radio emission in cool objects, but SIMP0136 itself was then thought to be considerably older than it turns out to be. Last year an independent team headed by Jonathan Gagné discovered that it was part of the young Carina-Near moving group, and determined that its mass was 12.7 times that of Jupiter, with a radius 1.22 times that of the gas giant. All the brown dwarfs surveyed here turn out to be rapid rotators, with rotation periods between ∼1.44 and 2.88 hours. The authors suggest a rotation period of 2.3895±0.0005 for SIMP0136.

At 200 million years old (the projected age of the Carina-Near moving group), with a surface temperature of about 825 degrees Celsius (well below our Sun’s surface temperature of 5,500 degrees C), SIMP0136 looked more and more like a free-floating planet. It should be useful as we examine the production of magnetic fields in both brown dwarfs and exoplanets, along with the auroral activity they can generate. And here’s an interesting follow-up. Are we looking at a new way to detect a particular kind of exoplanet? From the paper:

…SIMP0136…was recently found to be a member of a nearby ∼200 Myr moving group. This new age constraint reduces its estimated mass to a mere 12.7 ± 1.0 MJ, possibly making SIMP0136 the first known planetary mass object detected in the radio. If SIMP0136 is indeed a field exoplanet, its detection demonstrates that auroral radio emission can open a new avenue to detecting exoplanets, including elusive rogue planets.

The paper is Kao et al., “The Strongest Magnetic Fields on the Coolest Brown Dwarfs,” Astrophysical Journal Vol. 237, No. 2 (31 July 2018). Abstract / preprint. Gagné and team’s work is Gagné et al., “SIMP J013656.5+093347 Is Likely a Planetary-mass Object in the Carina-Near Moving Group,” Astrophysical Journal Letters Vol. 841, No. 1 (15 May 2017). Abstract available.

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Plate Tectonics: Necessary for Habitability?

Just how important is plate tectonics for the development of complex life? We’ve learned that its continual churn, with material pushing up from ocean rifts and being subducted as it meets continental shelves, can moderate the Earth’s climate. Increasing temperatures are tamped down through the capture of excess carbon dioxide in rocks, which reduces potential greenhouse conditions. Lowering temperatures will produce the reverse effect. The result is a mechanism for maintaining stable temperatures that some have seen as necessary for life.

“Volcanism releases gases into the atmosphere, and then through weathering, carbon dioxide is pulled from the atmosphere and sequestered into surface rocks and sediment,” said Bradford Foley, assistant professor of geosciences at Penn State University. “Balancing those two processes keeps carbon dioxide at a certain level in the atmosphere, which is really important for whether the climate stays temperate and suitable for life.”

And indeed, most of the volcanoes on our planet are found on the border between tectonic plates. Here, too, plates being driven deeper into the subsurface through subduction push carbon deep into the mantle as the cycle continues. But we also know that the Earth is the only planet in the Solar System on which plate tectonics has been confirmed. Planets without such plates are known as stagnant lid planets, coping with a crust made up of a single spherical plate floating on the mantle. Should we rule out such planets as candidates for life?

Penn State’s Foley, cited above, has been working with colleague Andrew Smye on computer models that probe the idea. The scientists wanted to learn whether climate regulation through chemical weathering could be sustained on a stagnant lid planet,a place where there is no subduction and the recycling of surface material back into the mantle would be limited. On such a world, the development of continents and their productive collisions would not occur, although as the paper points out, volcanism can still release some mantle CO2 to the atmosphere, allowing for at least some degree of surface recycling through lava flows.

Foley and Smye’s simulations are restricted to planets that are Earth-like in size and composition, given the steep uncertainties in our knowledge of volcanism and outgassing on planets with different mantle compositions. Their work models conditions on Earth-like stagnant lid planets in terms of weathering, CO2 outgassing, and changing heat retention in the crust. The question is whether chemical weathering can balance the CO2 being released on such worlds, and whether there is enough outgassing to keep the surface from freezing over. And we learn that ruling out life on stagnant lid planets would be premature.

For given volcanic outgassing in the right amounts and a suitable planetary composition, clement conditions on the surface can be produced and sustained, all without plate tectonics:

Models of the thermal, magmatic, and degassing history of rocky planets with Earth-like size and composition demonstrate that a carbon cycle capable of regulating atmospheric CO2 content, and stabilizing climate to temperate surface temperatures, can potentially operate on geological timescales on planets in the stagnant lid regime. Plate tectonics may not be required for habitability, at least in regard to sustaining a stable, temperate climate on a planet.

In fact, the authors find the potential for moderate climates that can last up to 5 billion years, after which volcanism and CO2 outgassing would cease and the climate would cool below the freezing point of water, inducing glaciation on the surface. Everything depends upon the planet’s supply of CO2, and that takes us back to its formation:

At CO2 budgets lower than ∼1020 mol, a planet’s climate is estimated to be in a snowball state for its entire history, while above ∼1022 mol weathering would become supply-limited. With supply-limited weathering, CO2 outgassing overwhelms CO2 drawdown, such that an inhospitably hot, CO2-rich atmosphere forms. Thus, the amount of carbon accreted to a planet during formation is critical for whether it can sustain habitable surface conditions in a stagnant lid regime.

The amount of carbon most conducive for habitable conditions turns out to be somewhere between Earth’s total amount of carbon to about 10 times less carbon than is found in today’s atmosphere, mantle and crust combined. Below that, the planet does not stay warm enough for liquid water to survive on the surface.

Image: This is Figure 5 from the paper. Caption: Time when the total degassing flux, Fd + Fmeta, falls below Earth’s present-day degassing flux (A) and below 10% of Earth’s present-day degassing flux (B). Labeled contours give this time in billions of years. To the right of the dashed line (shaded region), weathering will be supply-limited assuming an eruption efficiency of 0.1. Credit: Bradford Foley / Andrew Smye.

Thus carbon dioxide can still escape from rocks in a degassing process that takes it to the surface, a process that depends on the types and quantities of heat-producing elements found in the planet. The planet’s initial composition tells the tale. The work indicates that high internal heating rates favor long-term habitability, and points to young planets as more likely to experience sufficient rates of CO2 outgassing. In the hunt for biosignatures, the authors argue for planets orbiting stars with high thorium or uranium abundance, because these are the most likely to contain the necessary constituents to produce sufficient internal heat.

The paper is Foley, B. J. and A. J. Smye (2018), “Carbon cycling and habitability of Earth-size stagnant lid planets,” Astrobiology, Vol. 18, Issue 7 (2018), 873-896. Full text.

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Toward An Archaeology of Exo-Civilizations

Light of the Stars: Alien Worlds and the Fate of the Earth, by Adam Frank. W.W. Norton & Co. (2018), 272 pp.

Although he has published several previous books and is well represented in the technical literature, Adam Frank (University of Rochester) found himself suddenly thrust onto the public stage with an op-ed he wrote in the New York Times in 2016. Chosen by the paper’s editors, the title “Yes, There Have Been Aliens” injected a certainty Frank didn’t intend, but it brought up an intriguing point: We may not know whether other technological civilizations exist now, but the odds are exceedingly good that at some point, somewhere, they once did.

Frank and colleague Woody Sullivan had written the original idea up for Astrobiology, the result of their pondering how exoplanet data now streaming in could be used to refine the original Drake equation, which sets up the factors thought to determine the prevalence of technological societies in the universe. In his new book Light of the Stars, Frank explains by way of background that the duo realized they needed to change Drake’s focus by removing the question of the average lifetime of such a culture.

For the time being, then, the L factor that Drake used to cover the average lifetime of a technological civilization would be ignored (though it permeates the book in ways we’ll soon see). For now, let’s look at what our exoplanet data are telling us. For thanks to missions like Kepler and the effort going into other planet hunting methods from radial velocity to gravitational microlensing, we’re learning how to answer the astronomical terms in the Drake equation.

Ponder: We have a good read on the number of stars with planets, something Drake couldn’t have known in his early formulation (the value is now thought to be about 1, meaning just about every star we can see will have at least one planet). We can also begin to make estimates of the number of habitable zone planets around each star, another factor in the Drake equation, and Frank goes with the figure of one in five stars hosting a planet with a chance for life as we know it. That means planets with liquid water on their surface, the classic habitable zone definition.

Frank and Sullivan chose to re-write the Drake equation with the new values incorporated but without reference to L. They pursued the probability that humans might be the only civilization the universe has yet produced. Their answer: 10-22, which means one in ten billion trillion. The number identified what the author calls ‘the pessimism line.’ From the book:

To understand how to think about the pessimism line, imagine you were handed a very big bag of Goldilocks-zone planets. Our results say the only way human beings are unique as a civilization-building species would be if you pulled out ten billion trillion planets and not one of them had a civilization. That’s because Kepler has shown us that there must be ten billion trillion Goldilocks-zone planets in the universe. So the pessimism line is really telling us how bad the probability of a civilization forming would have to be in order for ours to be [the] only one that has ever existed.

Image: Astrophysicist and author Adam Frank. Credit: University of Rochester.

So while we can’t know how many civilizations exist right now, which was the Drake question (the relevance to SETI is obvious), we can get a sense for the odds that somewhere, somewhen, civilization has formed. As the author points out, your chance of being killed by lightning in any given year is about one in ten million. And that would be a thousand trillion times more likely than our species being the only civilization in the history of the cosmos.

You can see why this caught the attention of journalists and the public at large. We’ve looked at Frank’s work quite a few times on Centauri Dreams, as a look through the archives will show (the best place to start is probably Perspectives on Cosmic Archaeology). A criticism Frank addresses straightforwardly in the book is the obvious one: We have no information on the odds for abiogenesis itself, without which we can’t rule out the prospect that we are alone in the universe. Likewise we have no data on how likely intelligence is to develop once life has begun. We are dealing here solely in terms of probabilities, and probability is not proof.

Even so, these probabilities really are striking. The passages in Light of the Stars covering this work are among the most compelling in the book. Frank compares the one in ten billion trillion number to the pessimistic predictions of others who have addressed the probability of intelligent life arising. Ernst Mayr, for example, ruled it out because while he believed life could be common, only one of approximately 50 billion species that have existed on Earth had produced a civilization. But even given Mayr’s strictures, the sheer number of stars and planets implies at least 10 million high-tech civilizations have at some point developed somewhere.

The universe has had plenty of time and opportunity, in other words, to produce civilizations. Combine that with the fact that we have begun to understand, through our missions to other planets in our own Solar System, something about how planets grow and change over time. Thus the challenges we face in our own anthropocene era as we adapt our civilization to the planet around us and inescapably cause change to it — as all life has done — have likely been faced by many cultures, who have either found a way to make civilization an enduring process or have succumbed to planetary catastrophe driven by their growing demands for energy.

Life and Planetary Evolution

Life itself is a game-changer for planetary environments, operating on our world long before humans emerged. The single-celled organisms of the Archaean eon so early in the history of our planet would gradually have to cope with photosynthetic organisms. Eventually cyanobacteria began producing huge amounts of oxygen in what is known as the Great Oxidation Event. The sharp increase in atmospheric oxygen proved poisonous to pre-existing life but also allowed the evolution of creatures with far more interesting metabolisms.

So, what does the GOE, with all its power and reach, teach us about the Anthropocene? It demonstrates that life is not an afterthought in the planet’s evolution. It didn’t just show up on Earth and go along for the ride. The GOE makes it clear that, at an earlier point in Earth’s history, life fully and completely changed the course of planetary evolution. It shows us that what we are doing today in driving the Anthropocene is neither novel nor unprecedented. But it also tells us that changing the planet may not work out well for the specific forms of life that caused the change. The oxygen-producing (but non-oxygen-breathing) bacteria were forced off the Earth’s surface by their own activity in the GOE.

What Frank would like to arrive at is a ‘theoretical archaeology of exo-civilizations,’ a goal that seems philosophical at best, but one which can be interestingly modeled through computer simulations. The Second Law of Thermodynamics gives us insights into the feedbacks that energy use and the production of waste have on planetary systems. Frank would like to model young civilizations using combustion, wind and tidal power, geothermal methods, solar energy and nuclear in a huge variety of planetary environments to calculate the planetary impact.

A planet that lives close to the inner edge of its habitable zone might be so highly sensitive to runaway greenhouse warming that its civilization barely has time to progress before it faces its own version of the Anthropocene and collapses. Another world, farther out from its star, may be less sensitive to planetary change but have a civilization that refuses to recognize the change until the die-off has already begun. A different species on a different world could manage to build its project of civilization using only lower-impact forms of energy and make a gentle soft landing to a sustainable state that lasts thousands of millennia.

Now you can see how we circle back around to Drake’s final factor, L. It was useful for SETI because it helped to define the odds of discovering a currently existing civilization through our various methods of detection. But if we can develop simulated histories for hundreds of thousands of exoplanets in order to tweak the parameters of their civilizational responses to the changes they create on their home worlds, we can calculate an average civilization lifetime.

Frank sees this discussion as leading beyond this early research to increasingly realistic models fed by exoplanet studies, models that can be run for as many iterations as needed to simulate the possible trajectories of a vast number of worlds. These models can give us a sense for likelihoods that can have relevance to our own civilization’s future as we grapple with what must be universal issues of energy use and the inexorable scaffolding of thermodynamics. Don’t spend much time looking for a way around the Second Law of Thermodynamics, Frank advises. Most likely it’s simply ‘baked into the structure of the universe.’

Frank is thus calling for the development of a roadmap for civilizations to rise to Nikolai Kardashev’s Type 1 classification, meaning a society capable of capturing all the light energy reaching its planet from the host star, and perhaps, having survived this journey, able to rise even higher up the Kardashev scale. That scale, first presented at a famous meeting in Byurakan Observatory in Armenia, is another piece of the history of astrobiology that Frank examines in Light of the Stars, presenting a useful backgrounder for anyone hoping to discover how our views of the interactions between technology and our planet have been understood over time.

Drawing on the findings of the space program and exoplanetary research, Frank presents the Earth as one more example of a planet that needs to be understood as a tightly coupled system. Our civilization presents our world with only the latest of its challenges. The book is an eloquent call for harnessing our mathematical tools coupled with an exoplanet data influx to gain insight into our own transition to a stable pattern of growth. That pattern, if we assume ever increasing energy use, could well take a culture like ours off planet and into its local stellar system.

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On to Ultima Thule

I am now back on the job, and somewhere beyond Pluto seems a good place to go. Somehow it seems safer out there. While vacationing here on Earth, I was bitten by a brown recluse spider, spent two weeks with a swollen and painful foot, and came down with the most intense flu-like symptoms I’ve ever experienced. The final indignity: I received my monthly report on Centauri Dreams reader statistics. Since I had done no posting for a large portion of this report, I was curious to find out how much traffic had slowed in my absence. It turned out that traffic increased right after I stopped posting and stayed robust the entire time.

I am trying to figure out what this means…

But back to New Horizons, putting my tumultuous vacation experience behind me. Anyone who remembers how hard it was to find a suitable Kuiper Belt Object to serve as New Horizons’ next target will understand how challenging it would be to observe MU69 from the ground. The distant object, perhaps a binary, must be made to yield as many of its secrets as possible as our spacecraft continues the journey that will culminate in a New Year’s Day 2019 flyby. The effort proceeds by the observation of stellar occultations by hardworking mobile teams.

Image: Practicing for the August 2018 Senegal MU69 occultation. Credit: NASA/Johns Hopkins University Applied Physics Laboratory.

What good news, then, to hear from the New Horizons scientists that the latest effort to catch MU69 (recently dubbed ‘Ultima Thule’) passing in front of a much more distant star has been successful. An earlier effort took scientists to Argentina and South Africa in June of last year, while in July of 2017 the NASA Stratospheric Observatory for Infrared Astronomy (SOFIA) probed the area around the object for debris that could pose a collision hazard to the spacecraft. The Hubble instrument also examined the region that same month.

But it’s the ground-based effort that fascinates me. Because when I think of eclipses, I think about large bodies like the Moon passing in front of the Sun, or Earth’s shadow darkening the Moon. Yet the New Horizons effort is likewise the attempt to track an eclipse, one so demanding that it takes pinpoint geographical precision and sheer luck to pull in the data. The strips of shadow cast by MU69 were no more than 30 kilometers wide, with the teams’ telescopes placed at multiple points to catch the moment when the KBO passed in front of the star. Almost 50 researchers made the trip to Senegal and Columbia for the event.

The earlier occultations yielded measurements indicating that MU69 is an ‘extreme prolate spheroid,’ meaning an object shaped something like an American football. A close binary or even a ‘contact’ binary can’t be ruled out, with two objects actually touching each other.

Image: This is one artist’s concept of Kuiper Belt object 2014 MU69, the next flyby target for NASA’s New Horizons mission. This binary concept is based on telescope observations made at Patagonia, Argentina, on July 17, 2017, when MU69 passed in front of a star. New Horizons scientists theorize that it could be a single body with a large chunk taken out of it, or two bodies that are close together or even touching. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker.

We now turn toward weeks of analysis of the data gathered in Senegal and Columbia on August 3-4. Weather was an issue in the new work just as it was at the team’s multiple sites in Patagonia in 2017.

“Gathering occultation data is an incredibly difficult task,” said New Horizons occultation event leader Marc Buie of the Southwest Research Institute, Boulder, Colorado, who also discovered Ultima Thule about a year before New Horizons flew past Pluto in July 2015. “We are literally at the limit of what we can detect with Hubble and the amount of computer processing needed to resolve the data is staggering.”

Image: A photo from the previous occultations. Four members of the observation team scan the sky while waiting for the start of the 2014 MU69 occultation, early on the morning of June 3, 2017. The target field was in the Milky Way, seen here from their observation site in the Karoo desert near Vosburg, South Africa. They used portable telescopes in an attempt to observe MU69, a small Kuiper Belt object (now nicknamed Ultima Thule) and the next flyby target the New Horizons spacecraft, pass in front of a star. New Horizons team members made similar observations of Ultima this week in Senegal. Credit: NASA/Johns Hopkins Applied Physics Laboratory/Southwest Research Institute/Southwest Research Institute/Henry Throop.

The spacecraft itself continues to receive commands from Earth in preparation for the Ultima Thule flyby, including memory updates, as subsystem and science instruments are checked out. In September, New Horizons will begin to make observations of the target, which will once again help to refine the spacecraft’s course for the flyby. The distance between spacecraft and KBO for the event is now planned to be 3,500 kilometers. When it occurs, at 6.5 billion kilometers from Earth, tiny Ultima Thule will become the most distant object ever explored.

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Revising the Classical ‘Habitable Zone’

With my time-out period over (more about this next week), I want to get back into gear with the help of Ramses Ramirez, a specialist on planetary habitability whose work has now taken him to Japan. Born and raised in New York City, Ramses tells me he is much at home in his new position as a research scientist at the Earth-Life Science Institute (ELSI) in Tokyo, where opportunities for scientific collaboration abound and the chance to learn a new language beckons. We’ve looked at Ramses’ papers a number of times in these pages, and I was delighted when he offered this description of his work to our readership. A student of James Kasting, he received his Ph.D. from Penn State in 2014 and went on to postdoc work with Lisa Kaltenegger at Cornell’s Carl Sagan Institute. A fascination with astrobiology and the issues involved in defining habitable zones continues to be a primary focus. Ramses’ new paper ponders whether we are best served by looking for life similar to Earth’s because this is what we know, or whether there is a broader strategy, one that probes all the assumptions in our ideas of the classical habitable zone.

By Ramses Ramirez

I am glad to have the opportunity to formally introduce myself and give a summary of the recent work on planetary habitability and the habitable zone. I define the habitable zone (HZ) as the circular region around a star(s) where standing bodies of liquid water could exist on the surface of a rocky planet [1]. The inclusion of the phrase “standing bodies of water” excludes dry worlds that may exhibit small outpourings of seasonal surface water (e.g. possibly Mars). Defined this way, the HZ is properly focused for detecting worlds that have large surface bodies of water (e.g. seas, big lakes, oceans) that are in direct contact with the atmosphere. If life is present on such a world, potential atmospheric biosignatures could be detected with current technology. However, in the absence of such large water bodies, life would not be detectable even if it were present. Likewise, although life may be possible within the seas of a Europa or Enceladus exoplanetary analogue, the global ice layer covering their oceans would prevent the detection of such subsurface life. Such observational issues keep the HZ within an orbital region that is placed somewhat closer to the star.

When I first started my Ph.D. in 2010 under my mentor Professor James F. Kasting, I did not know the exact topic that I would be working on at first, but with my interest in the search for extraterrestrial life, I knew that I wanted to work on planetary habitability. Fortunately, I got involved as one of the two lead authors on the 2013 HZ paper [2], where we updated the seminal Kasting et al. [3] HZ limits [2,3]. Although I was fascinated by the HZ concept as a navigational tool to find potentially habitable planets, my investigations soon led me to realize that the HZ definition described in these earlier works, what I dub the “classical HZ”, would be insufficient for capturing the diversity of such planets. From that point on, it became my personal mission to turn the HZ into an even more capable navigational tool.

The classical HZ assumes that CO2 and H2O are the key greenhouse gases on potentially habitable planets, following the carbonate-silicate cycle on the Earth, which is thought to regulate CO2 between the atmosphere, surface, and the interior (e.g. [3]). Given that the concept of a universal carbonate-silicate cycle on habitable exoplanets is itself far from proven (e.g., [4]), I always thought that this assumption was needlessly restrictive and geocentric. After all, HZ planets with atmospheres consisting of different gas mixtures can also support standing bodies of liquid water and potentially be habitable. Reduced greenhouse gases, like H2 and CH4, have been considered as major atmospheric constituents on both early Earth and early Mars (e.g., [5,6]). It has even been suggested that planets with primordial hydrogen envelopes may be habitable in some circumstances [7]. If hypothetical planets consisting of dense 10-bar CO2 atmospheres near the outer edge of the classical HZ are potentially habitable (e.g. [2][3]), then why not worlds composed of these other atmospheric constituents (Figure 1)?

Figure 1: The classical HZ extended to A-stars (blue) with CO2-CH4 (green) and CO2-H2 (red) outer edge extensions for stars of stellar effective temperatures between 2,600 and 10,000 K (reproduced from Ramirez [1]).

Moreover, the classical HZ really targets potentially habitable planets orbiting main-sequence stars. However, this approach ignores the importance of the temporal evolution of the HZ, particularly a star’s pre-main-sequence phase. It is during this early stage that many M-stars are bright and luminous enough to completely desiccate any planets that are now thought to be located within the main-sequence HZ (e.g. [8][9][10]), like Proxima Centauri-b and many of the TRAPPIST-1 planets, unless such worlds are able to accrete enough water to offset losses (e.g., [11][12]) (Figure 2).

Figure 2: The pre-main-sequence HZ for a late (M8) star. A planet that forms at ~0.03 AU undergoes a runaway greenhouse state for ~100 Myr before finally entering the HZ, settling near the outer edge after ~1 billion years (reproduced from Ramirez [1]).

In my view, we are currently ill-equipped to infer the atmospheric conditions that are most suitable for extraterrestrial life. All we know is that life did somehow arise on this planet. It is therefore pretentious to take our one poorly-understood example and assume that alien life must follow a similar trajectory. Some might argue that we should focus on finding life similar to Earth’s because it is familiar and would be simpler to find. However, this argument from the principle of mediocrity is not convincing to me because we have no idea how common or rare our particular brand of life is until we are able to find a second instance of life’s occurrence elsewhere.

It is then illogical to rely on any one iteration of the HZ to find potentially habitable planets. Our inadequate understanding of biology, the origin of life, and planetary processes require that all of these working hypotheses, including the assumptions used in the classical HZ, be tested by observations. Ideas that are later found to be unsupported by nature can then be rejected whereas those that are confirmed should be refined. Only then will we begin to truly understand life’s possibilities and make more informed – and possibly more restrictive – design decisions for subsequent missions. But only proper observations (and possibly better theoretical constraints) can tell us this, and not adherence to some ambiguous and untested geocentric philosophy. Finding extraterrestrial life will be one of the hardest endeavors undertaken by our species. To meet the challenge, we need to start with a thorough and openminded search for potentially habitable planets.

Thus, as some Centauri Dreams readers may already know, part of my work (in collaboration with Lisa Kaltenegger) in recent years has focused on improving our understanding of both the spatial [13][14] and temporal evolution of the HZ [8][15]. These papers have already been discussed in previous Centauri Dreams posts. However, in addition to our own work, other researchers and colleagues have also proposed their own revisions and extensions to different aspects of the HZ, further increasing its utility as a tool for finding life that is both similar and dissimilar to Earth’s. This gradual but steady evolution in how we think about the HZ has come to a good point (I believe) for a review paper to come along such as this one.

I hope that your readers will like my new summary of recent advances (linked below) in our understanding of the HZ. I see this work as both a primer for the student and layman as well as a guide for space missions. It includes many recommendations and suggestions for how the classical HZ, in conjunction with newer HZ formulations, can be used to maximize our chances of finding extraterrestrial life. For example, I do not think that a proper assessment of planetary habitability can be made without properly assessing a planet’s pre-main-sequence habitability. A-stars should also be considered as potential hosts for life-bearing planets. Most importantly, we should employ the various HZ formulations and rubrics to rank which planets are most likely to host life. Finally, my paper clarifies many common misconceptions about the HZ concept and explains why the search must necessarily be limited to finding surface liquid water (at least for now).

The review paper is “A more comprehensive habitable zone for finding life on other planets,” published in Geosciences 8(8), 280 (28 July 2018). Full text available here. [PG: Expect a close look at this paper in coming weeks on Centauri Dreams].

References

1) Ramirez, R.M. 2018. “A more comprehensive habitable zone for finding life on other planets.” Geosciences 8(8), 280.

2) Kopparapu, R., Ramirez, R.M. (co-primary author), Kasting, J., et al., 2013. Habitable zones around main-sequence stars: New Estimates. ApJ, 765, 2, 131

3) Kasting, James F., Daniel P. Whitmire, and Ray T. Reynolds. “Habitable zones around main sequence stars.” Icarus 101.1 (1993): 108-128.

4) Bean, Jacob L., Dorian S. Abbot, and Eliza M-R. Kempton. “A statistical comparative planetology approach to the hunt for habitable exoplanets and life beyond the solar system.” The Astrophysical Journal Letters 841.2 (2017): L24.

5) Wordsworth, Robin, and Raymond Pierrehumbert. “Hydrogen-nitrogen greenhouse warming in Earth’s early atmosphere.” Science 339.6115 (2013): 64-67.

6) Ramirez, R.M., Kopparapu, R., Zugger, M., Robinson, T.D.,Freedman, R., Kasting, J.F., 2014. Warming early Mars with CO2 and H2. Nat. Geosc., 7, 59 – 63

7) Pierrehumbert, Raymond, and Eric Gaidos. “Hydrogen greenhouse planets beyond the habitable zone.” The Astrophysical Journal Letters 734.1 (2011): L13.

8) Ramirez, R.M., Kaltenegger, L., 2014. Habitable Zones of Pre-Main-Sequence Stars. The Astrophysical Journal Letters, 797, 2, L25

9) Luger, Rodrigo, and Rory Barnes. “Extreme water loss and abiotic O2 buildup on planets throughout the habitable zones of M dwarfs.” Astrobiology 15.2 (2015): 119-143.

10) Tian, Feng, and Shigeru Ida. “Water contents of Earth-mass planets around M dwarfs.” Nature Geoscience 8.3 (2015): 177.

11) Levi, Amit, Dimitar Sasselov, and Morris Podolak. “The Abundance of Atmospheric CO2 in Ocean Exoplanets: a Novel CO2 Deposition Mechanism.” The Astrophysical Journal 838.1 (2017): 24.

12) Ramirez, R.M. and Levi, A. 2018. The ice cap zone: a unique habitable zone for ocean worlds. The Monthly Notices of the Royal Astronomical Society, 477, 4, 4627- 4640

13) Ramirez, R.M., Kaltenegger, L., 2017. A volcanic hydrogen habitable zone. The Astrophysical Journal Letters, 837, 1

14) Ramirez, R.M., Kaltenegger, L. 2018. A methane extension to the classical habitable zone. The Astrophysical Journal 858, 2

15) Ramirez, R.M., Kaltenegger, L., 2016. Habitable Zones of Post-Main Sequence Stars. The Astrophysical Journal, 823, 6, 14pp

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Time Out

Dave Brubeck’s Time Out album was the first jazz LP I ever bought, just after it came out in 1959, the same year that Miles Davis released Kind of Blue. Watershed moments both. Paul Desmond once said of his alto sax work that he was trying to create the sound of a dry martini, a description I certainly can’t top.

Last night, while listening to Desmond and Brubeck, I realized that the Time Out album would be emblematic for today’s post. For it’s that time of year, and I am indeed taking time out for a much needed break. Centauri Dreams will be back in the first week of August, but until then, my break will include a good bit of jazz, much catch-up reading, a lot of long walks and, perhaps, a few of those martinis Desmond talks about. I’ll keep an eye on the site to handle comment moderation as well. Meanwhile, I hope all of you are having a splendid summer.

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Unusual Companion for a Brown Dwarf Binary

A cluster of stars sharing a common origin, now gravitationally unbound, is referred to as a stellar association. I’ve written before about how useful some of these groupings can be. In the form of so-called moving groups — a stellar association that is still somewhat coherent — they help us identify stars of similar age, an aid as we discover new objects. Now we have word of an object called 2MASS 0249 c, found in the Beta Pictoris moving group, that has striking similarities to the most famous member of that group, Beta Pictoris b.

2MASS 0249 c, like Beta Pictoris b, was found by direct imaging, meaning we’re actually looking at the object under discussion in the image below. The two objects are all but identical in mass, brightness and spectrum. Images from the Canada-France-Hawaii Telescope (CFHT) showed an object moving at a large distance from its host, which turned out to be a pair of closely spaced brown dwarfs.

Follow-up observations with the Keck instrument allowed that determination, while spectroscopy at the NASA Infrared Telescope Facility and the Astrophysical Research Consortium 3.5-meter Telescope at Apache Point Telescope completed the data.

“To date, exoplanets found by direct imaging have basically been individuals, each distinct from the other in their appearance and age. Finding two exoplanets with almost identical appearances and yet having formed so differently opens a new window for understanding these objects,” said Michael Liu, astronomer at the University of Hawai`i Institute for Astronomy, and a collaborator on this work.

Image: Image of the 2MASS 0249 system taken with Canada-France-Hawaii Telescope’s infrared camera WIRCam. 2MASS 0249 c is located 2000 astronomical units from its host brown dwarfs, which are unresolved in this image. The area of sky covered by this image is approximately one thousandth the area of the full moon. Credits: T. Dupuy, M. Liu.

The difference in formation scenarios that Liu talks about is instructive. Here we have two host stars, one of them being a binary system of brown dwarfs, that formed in the same stellar nursery, as per their membership in the moving group. But while Beta Pictoris b, a gas giant of about 13 Jupiter masses, orbits a star 10 times brighter than the Sun, 2MASS 0249 c orbits a brown dwarf pair 2000 times fainter. At a distance of 9 AU, Beta Pictoris b is relatively close to its star, while 2MASS 0249 c is a whopping 2000 AU from its brown dwarf hosts.

These distances imply different formation scenarios. Beta Pictoris b likely formed from the accumulation of dust grains in the circumstellar disk surrounding its star. 2MASS 0249 c could not have done so, given that the two brown dwarfs it orbits would not have had sufficient disk material to produce it. That implies the new planet took form through the gravitational collapse of a gas cloud found in the original stellar birth cluster.

“2MASS 0249 c and beta Pictoris b show us that nature has more than one way to make very similar looking exoplanets,” says Kaitlin Kratter, astronomer at the University of Arizona and a collaborator on this work. “They’re both considered exoplanets, but 2MASS 0249 c illustrates that such a simple classification can obscure a complicated reality.”

Image: The infrared spectra of 2MASS 0249 c (top) and beta Pictoris b (bottom) are similar, as expected for two objects of comparable mass that formed in the same stellar nursery. Unlike 2MASS 0249 c, beta Pictoris b orbits much closer to its massive host star and is embedded in a bright circumstellar disk. Credit: T. Dupuy, ESO/A.-M. Lagrange et al.

What we get out of all this is opportunity. The formation of gas giants is a key phase in the emergence of new planetary systems, and being able to use direct imaging to study such worlds means we can probe their atmospheres directly, examining the composition, surface temperature, chemistry and other physical properties of the exoplanet. Moreover, direct imaging is most effective when working with planets far from their star, as this object is. If we are after insights into gas giant formation, the different formation pathways for Beta Pictoris b and 2MASS 0249 c may provide evidence both orbital and spectral. The paper notes:

As directly imaged objects, β Pic b and 2MASS J0249−0557 c provide a new opportunity to test atmospheric compositions and angular momentum evolution for a close-in planet and a very wide companion that share a common mass and age and that formed from the same material.

And on the issue of spectra:

If different formation mechanisms produced these objects, then their spectra could contain evidence of their divergent pasts. As noted above, we suspect that 2MASS J0249−0557 c arose from a star-formation-like process of global, top-down gravitational collapse in the same way as the freefloating object 2MASS J2208+2921. On the other hand, β Pic b bears architectural resemblance to planetary systems and thus may have formed via core accretion. Core accretion models and observations of solar system gas giants show substantial metal enrichment (e.g., Stevenson 1982; Bolton et al. 2017). Thus, if β Pic b is a scaled-up gas giant (≈13 MJup), then we may expect to see substantial metal enrichment in its atmosphere.

What an interesting find 2MASS 0249c turns out to be. Is it a planet or actually a brown dwarf? One thing is for sure: The discovery puts a spotlight on the boundary between planet and brown dwarf, both in terms of composition and in terms of formation history. Maybe it’s best to fall back on how today’s paper describes the object: a ‘planetary-mass companion.’ Now we can go to work, via direct imaging, on issues like variability, rotation and atmospheric composition. How these data vary across different formation histories should occupy astronomers for some time.

The paper is Dupuy et al., “The Hawaii Infrared Parallax Program. III. 2MASS J0249-0557 c: A Wide Planetary-mass Companion to a Low-mass Binary in the beta Pic Moving Group,” accepted at the Astronomical Journal (preprint).

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