Are Planets with Continuous Surface Habitability Rare?

Put a rocky, Earth-sized planet in the habitable zone of a Sun-like star, and good things should happen. At least, that seems to be the consensus, and since there are evidently billions of such planets in the galaxy, the chances for complex life seem overwhelmingly favorable. But in today’s essay, Centauri Dreams associate editor Alex Tolley looks at a new paper that questions the notion, examining the numerous issues that can affect planetary outcomes. Just how long does a planetary surface remain habitable? Alex not only weighs the paper’s arguments but runs the code that author Toby Tyrrell used as he examined temperature feedbacks in his work. Read on for what may be a gut-check for astrobiological optimists.

by Alex Tolley

The usual course of the discussion about planet habitability assumes that the planet is in the habitable zone (HZ), probably in the continuously habitable zone (CHZ). The determination if the planet is inhabitable concerns the necessary composition and pressure of the atmosphere to maintain a surface temperature able to support liquid water. As stars usually increase their luminosity over time, we see charts like the one below for Earth showing the calculated range of average surface temperature. Atmospheric pressure and composition can be modified to determine the inner and outer edges of the HZ or CHZ.

Image: Life on an exoplanet in a globular cluster. Credit: David Hardy (

However, these charts say nothing about the various factors that may upset the stability of the climate, especially the geological carbon cycle, where volcanic outgassing of carbon dioxide (CO2) is approximately balanced by weathering of rocks to ultimately sequester carbon as carbonate rocks such as limestone. Imbalances can create significant changes in greenhouse gas (GHG) composition of the atmosphere with temperature impacts [4].

Figure 1. Possible bounds of Earth surface temperature over 4.5 By of changing solar output and the impact of atmosphere. The early Earth would have required a different energy trapping atmosphere to maintain an inhabitable temperature during its early history to prevent freezing. Continued increase in solar luminosity will render the surface too hot for life even with no atmospheric trapping of the sun’s heat. Source: Kasting 1988 [3]

We have much evidence of widely fluctuating average surface temperatures for the Earth, from a possibly hot Archaean eon, several local temperature maximums including the end of the “Snowball Earth”, the Permian/Triassic extinction, and the Eocene thermal maximum. Earth has also cooled, most notably during the “Snowball Earth” period that lasted millions of years and several extreme glaciations over its history.

Before even considering the many other possible factors that may preclude an inhabitable planet, there is a question of just how stable are planetary surface temperatures, especially when subjected to shocks due to excess CO2 emissions from very active volcanism, or conversely from excess weathering depleting the atmospheric CO2 pressure.

The Journal Paper

A new paper [1] by Prof. Toby Tyrrell looks at this question in a very different way. He posits that for the many planetary types and conditions in the galaxy, we should assume a wide variety of possible temperature feedbacks and simulate the average surface temperature of a planet over a long period of geologic time (3.0 By) to simulate how frequently planets could continuously maintain an inhabitable surface temperature throughout this period.

Tyrrell on randomly configured feedbacks:

“It is assumed that there is no inherent bias in the climate systems of planets as a whole towards either negative (stabilising) or positive (destabilising) feedbacks. In other words, it is assumed here that the feedback systems of planets are the end result of a set of processes which do not in aggregate contain any overall inherent predisposition either towards or against habitability.”

His model is very simple. He assumes a randomly chosen number of feedback values (change in temperature over time for a specified temperature) within an inhabitable temperature range. Figure 2 below shows one example of the model showing random feedbacks, calculated temperature attractors, runaway temperature zones, and a time course of temperature impacted by random temperature perturbations. The values for temperatures between each feedback node are interpolated from the 2 surrounding nodes. If the feedback slope is negative and if the 2 points straddle a 0 feedback, the model calculates a temperature attractor at that point, so that temperatures between the 2 feedback nodes tend to stabilize the temperature at the attractor position.

If, however, the slope is positive, the temperature will be destabilized and driven towards the upper node if above the 0 feedback level, and conversely to the lower node if below the 0 level. He sets a minimum (-10C) and maximum(60C) surface temperature range that if the calculated temperature extends beyond those boundary temperatures or is in feedback that will lead to runaway feedback to either a very low or very high temperature, the model assumes the planet is no longer habitable on the surface. The model adds 2 other important elements. Firstly there is a long-term forcing (e.g. increased solar output), which for the Earth is a positive one as the sun continues to increase its output over time. The second is to introduce small, medium, and large temperature perturbations (i.e. shocks) that introduce noise into the model and can flip the climate between attractor temperatures and also into runaway temperature conditions where the feedbacks positively reinforce the temperature change. Figure 2 below is extracted from the paper to indicate an example. [Annotations added for clarity.]

Figure 2. Extracted from the Tyrrell paper and annotated. The left chart shows 9 randomly created feedbacks. Where 2 adjacent feedbacks are connected by a negative slope and cross the zero feedback line, a temperature attractor is created, in this example there are 2 attractors. At either end are zones where the temperature would cause a runaway increase or decrease in temperature and these are indicated by grayed areas. The chart also indicates that over the long term, there is a negative forcing that reduces the average temperature over time. The right-hand chart shows the temperature over time. The 2 attractors are shown, as is the starting temperature [blue square]. The various perturbations are indicated both in time and size by the red triangles below the chart axis. The gray bands show the runaway temperature conditions and the black bands the start of uninhabitable conditions. The 500 My display shows the temperature flipping between the 2 attractors, with each flip due to larger temperature perturbations. A few temperature perturbations approach but do not cross into the runaway temperature zones.

With the parameters he uses, the model demonstrates that with repeated runs, only a few percent of planet runs enjoy a 3 billion year period where surface temperatures stay within the inhabitable temperature range. Once the range is exited, surface life ends and the planet becomes lifeless on the surface.

Figure 3 shows how rarely planets can maintain inhabitable conditions over the entire 3 billion year time period.

Figure 3. The probability of a planet always surviving as inhabitable over 3 billion years over several runs with the same feedback conditions but with no temperature perturbations and with random temperature perturbations. The gray (H1 – chance alone) hypothesis is pure random perturbations withwout feedbacks and the red (H2 – mechanism alone) – feedbacks but without large perturbations – are compared with the simulation results. The most important result is that a planet that can maintain surface inhabitable conditions is quite rare.


“The initial prospects for Earth staying habitable could have been poor. If so, this suggests that elsewhere in the Universe there are Earth-like planets which had similar initial prospects but which, due to chance events, at one point became too hot or too cold and consequently lost the life upon them. As techniques to investigate exoplanets improve and what seem at first to be ‘twin Earths’ are discovered and analysed, it seems likely that most will be found to be uninhabitable.”

His conclusion is ominous for astrobiologists. Even if we discover many planets that are in the HZ, and confirm that their atmospheres could support an inhabitable surface, those planets are either frozen or too hot to allow life to exist on the surface. The vast majority of apparently suitable worlds will prove lifeless and appear as if abiogenesis (or even panspermia) has failed to ignite an evolutionary progression to complex life and even possibly technological civilization.

This conclusion is enough to dampen any astrobiologist’s day and suggests that the search for biosignatures may be as disappointing as the results of SETI.

While the paper shows the results using the values of the published model and code, the supplementary information includes a considerable analysis of the model, for example, extending the inhabitable range, and several other parameters. However, the broad conclusion remains robust. Maintaining an inhabitable temperature over 3 billion years is unlikely.

Tyrrell acknowledges the simplicity of his approach and suggested in a recent SETI Institute webinar that he hopes to apply his approach with a more sophisticated planetary climate model to determine if his findings hold up.

Given the proxy indications of Earth’s paleotemperatures (see figure 4 below) showing wide ranges and some close misses to survival shown by the mass extinctions, why did Earth life survive? Tyrrell argues that the anthropic principle has to be invoked. Just as the universe we live in needs the exact constants for life and we couldn’t be in any universe without those conditions, so we technological humans cannot investigate unless the Earth had maintained a continuous inhabitable surface temperature.

A Critique

Figure 4 below shows the estimated temperature fluctuations in the paleotemperature proxy data. Have we just been lucky that there do not appear to be any clear multiple attractor temperatures?

Figure 4. A chart of paleotemperature of Earth. For 3 billion years Earth’s average surface temperature has fluctuated in a range of less than 30C.

An obvious question is whether his model reflects reality. The random nature of the feedbacks coupled with the temperature perturbations might lead to many situations where even small temperature perturbations will tip the surface temperature beyond the acceptable range. As we can see from Figure 2, the upper-temperature attractor is within 10C of a runaway temperature increase, making habitability susceptible to even relatively small temperature perturbations.

Fortunately, the model code has been placed online and the source code available to experiment with.

Observing several runs it became clear that the model would quickly fail if the current temperature at an attractor temperature was near the boundary range so that even a modest temperature perturbation could push the temperature outside the range. How serious was this effect?

Figure 5. The most benign model. The 2 feedbacks are at the temperature range extremes and result in an attractor at 25C that is maintained across the temperature range. The long-term temperature forcing is set to 0. Only the infrequent large temperature perturbations, average size 32C are operative.

I created an experiment (also suggested at bottom of page 4 of the paper) where the planet would always have the most favorable conditions for a stable surface temperature. Just 2 feedbacks were created at each end of the range, with the calculated attractor in the middle of the range at 25 C, so that any perturbation would have to exceed 35C in either direction to exit the inhabitable conditions. I removed the long-term feedback too. I also removed both the small and medium-sized perturbations, leaving just the rare, large perturbations. The probability of timing and size of the perturbations was left as per the model. By starting the planet’s temperature at the attractor, the inhabitable conditions would be maintained at the attractor temperature unless a random large perturbation exceeded the 35C size.

The results for different perturbation probabilities are shown in figure 6. The average survival time of planetary runs and the %age survival plotted against perturbation probability demonstrate what might be intuitively guessed. Tyrrell’s purely mechanistic run (H2) with optimal feedback and no perturbations had all planet runs complete the 3 By survival. This is consistent with figure 6 where expected large perturbations = 0.

Figure 6. Survival times and %age survival of planets without an attractor and with a single attractor at 25C. With an attractor, survival times are greatly enhanced, especially as the expected number of perturbations increases.

Inspection of the model’s large temperature perturbation distribution indicated that the average size was 32C with a standard deviation of 16C. For the stable model planet I was testing, the temperature would be perturbed beyond either range boundary by a value of just 0.25 standard deviations, i.e. that about 40% of all randomly selected sample perturbations would trigger a surface temperature outside the inhabitable range. When that happened depended on the random timing and would dictate the survival time of inhabitable temperatures. As a control to determine the frequency of perturbations, a model world was created with no attractor temperature so that it sat on a temperature knife edge. Any perturbation would cause runaway heating or cooling. The impact of the lack of the stabilizing temperature attractor on survival time and average % of planets surviving for 3 By is evident.

Given the importance of the large perturbations, just how reasonable are the size of the perturbations and the maximum inhabitable temperature range.?

It is hypothesized that the “Snowball Earth” temperature ranged from deepest glaciation to a temperature maximum could have been as high as 100C (-50C to 50C). The chart of paleotemperature suggests for over the last few billion years an average surface temperature range of 26C (-10C to16C). That the Cryogenian glaciation period encompassing the “Snowball Earth” could have had a surface temperature of -50C, yet life quickly reemerged more vigorous than ever (the Cambrian “explosion”) after the glaciers melted, suggests that the lower temperature bound of -10C may be too conservative. As for the upper bound, it has been suggested that the Archaean eon may have had surface water temperatures of 70-80C. While most complex life has an effective upper limit of 60C, extremophiles have been found at 122C. For complex life, while the resilient tardigrades can withstand extreme temperatures for short periods, the inhabitable surface range is reasonable for complex life. However, we should bear in mind that ecological refugia can provide safety for complex life, for example around undersea vents to resist freezing, and migration to the poles to escape the equatorial temperatures and hence live in regimes that remain below the average surface temperature.

These points were acknowledged in the Tyrrell paper that discussed the limitations and caveats to the model.


Geographical variability implies that more extreme average global surface temperatures might be required to force extinction everywhere. Microbial life can potentially survive periods of inhospitable surface conditions within refuges, such as in subsurface rocks or deep in an ice-covered ocean at hydrothermal vents, emerging later to recolonise the surface; evidence from Neoproterozoic Snowball Earth events suggests however that eukaryotic photosynthetic algae persisted through the events and therefore that surface habitability was maintained at some locations. Other environmental conditions can affect habitability, but only temperature (and therefore water availability) are considered here.

Dynamic models are often unstable without tuning. The simplest example is Wolfram’s linear cellular automata with 3 cell states determining the next cell state. With just 8 possible rules for the 3 state combinations, there are 256 combinations of rules, yet just 6 (2.3%) do not converge on static states. The random feedback combinations may reflect a similar outcome, but where the majority of conditions will easily slip out of the inhabitable temperature range, rather than the benign experimental planet conditions I tested.


Dr. Malcolm (Jurassic Park):

“Life Finds a Way”

Given the results from my experiment with the optimal feedbacks for a stable climate, if feedbacks are more stabilizing on average than the hypothesized randomly assigned feedbacks, planets with inhabited surfaces possibly may not be quite as rare as the author’s model indicates. Tyrrell notes that average surface temperatures hide the variability of temperatures and exclude possible refugia, such as undersea hot vents, and lithosphere life warmed by the planet’s core. If we can accept that the Earth was populated by unicellular bacteria and eukaryotes for most of its history, and that the Earth’s complex biota may have even taken a major loss during the Cryogenian period, it seems likely that inhabitable worlds will have some sort of life assuming abiogenesis is easily achievable. While our climate history may be a lucky chance, history does not seem to indicate some attractor temperatures, but rather a single attractor that is subject to GHG source and sink imbalances that last for some time. The hypothesized extreme volcanism that ended the Permian resulted in the greatest extinction event in the fossil record and lasted for 2 million years. Our current fossil fuel burning that is increasing the atmospheric CO2 levels while very much like a temperature shock is not believed to be able to cause a runaway heating as happened on Venus. However, it is suggested that sometime in the next billion years, the Earth’s atmosphere will need to have no CO2 to stay habitable. Well before then, autotrophs will not be able to fix carbon and the complex life biosphere will collapse.

Once life starts, it is tenacious. A reset back to extremophiles may well be recoverable given time allowing new complex life forms to emerge under the right conditions and genetic “accidents”. However, there may be many more possible wrinkles to the sustainability of habitability, and eventually, surface life may be unable to survive. For subsurface life, the story may be very different. The lithospheric life might survive all other life until our sun destroys Earth billions of years in the future.

It would be interesting to modify the model so that rather than stopping when the temperature is outside the range, that the instances of these periods are recorded as possible reset conditions for refuge (e.g. lithosphillic) life to restart the evolutionary process rather than assuming the planet is “sterile”.

Time will tell when astrophysicists have cataloged and characterized a statistically useful sample of Earth-like worlds in the HZ that can test the model hypothesis of rare survival of surface inhabitablity over billions of years.


1. Tyrrell, Toby. “Chance Played a Role in Determining Whether Earth Stayed Habitable.” Communications Earth & Environment, vol. 1, no. 1, (2020), doi:10.1038/s43247-020-00057-8.

2. Ibid Supplementary Information

3. Kasting, James, et al “How Climate Evolved on the Terrestrial Planets”, Scientific American, (1988)

4. Berner, R. A. & Caldeira, K. “The need for mass balance and feedback in the geochemical carbon cycle”. Geology 25, 955–956 (1997).


Seafloor Volcanoes on Europa?

What’s going on on the floor of Europa’s ocean? It’s hard to imagine a place like this, crushed under the pressure of 100 kilometers or more of water, utterly dark, although I have to say that James Cambias does wonders with an ice moon ocean in his novel A Darkling Sea (Tor, 2014). Science fiction aside, Europa Clipper is in queue for a 2024 launch, and we can anticipate a flurry of new studies that feed into plans for the mission’s scientific investigations. The latest of these puts Clipper on volcano watch.

The work deploys computer modeling to show that volcanic activity seems to have occurred recently on Europa’s seafloor. The concept is that there may be enough internal heat to cause melting — at least in spots — of the rocky interior, which would produce the needed results.

How this heating affects the moon is deduced from the 3D modeling of heat production and transfer in the paper, which was recently published in Geophysical Research Letters. The lead author is Marie B?hounková (Charles University, Czech Republic), who describes the astrobiological import of the team’s results:

“Our findings provide additional evidence that Europa’s subsurface ocean may be an environment suitable for the emergence of life. Europa is one of the rare planetary bodies that might have maintained volcanic activity over billions of years, and possibly the only one beyond Earth that has large water reservoirs and a long-lived source of energy.”

Image: Scientists’ findings suggest that the interior of Jupiter’s moon Europa may consist of an iron core, surrounded by a rocky mantle in direct contact with an ocean under the icy crust. New research models how internal heat may fuel volcanoes on the seafloor. Credit: NASA/JPL-Caltech/Michael Carroll.

With massive Jupiter close at hand, it’s no surprise that gravitational interactions should account for heat production in Europa’s mantle, for the rocky interior flexes in the course of the moon’s orbit. The paper drills down into how this flexion operates, where the resulting heat dissipates, and how it results in melting in the mantle.

In Europa Clipper terms, it’s useful to learn that volcanic activity is most likely to occur near the poles, for this is where the most heat is produced from these effects. On top of this, we learn that the volcanic activity likely to be produced here is long-lived, giving life the opportunity to evolve. As an analogue, we can imagine hydrothermal systems like those at the bottom of Earth’s oceans, where seawater and magma interact.

In the absence of sunlight, the resultant chemical energy supports life on the seafloor and could conceivably do so on Europa. Europa Clipper will measure the moon’s gravity and magnetic field, looking for anomalies toward the poles that could confirm the presence of volcanic activity. Long-time Europa specialist and Europa Clipper project scientist Robert Pappalardo (JPL) sees all this as fodder for continuing investigation:

“The prospect for a hot, rocky interior and volcanoes on Europa’s seafloor increases the chance that Europa’s ocean could be a habitable environment. We may be able to test this with Europa Clipper’s planned gravity and compositional measurements, which is an exciting prospect.”

Europa Clipper should reach the Jupiter system in 2030, orbiting the giant planet and performing numerous close flybys of Europa as it surveys the surface, samples any gases that may have been emitted by the exchange of material from below the ocean, and possibly takes advantage of plumes of water vapor. If water is indeed welling up on occasion from below, we may be able to learn a good deal about the interior ocean without having the need to drill down through kilometers of ice.

The paper is B?hounková et al., “Tidally Induced Magmatic Pulses on the Oceanic Floor of Jupiter’s Moon Europa,” Geophysical Research Letters Vol. 48, Issue 3 (22 December 2020). Abstract.


Spiral Galaxies: A Common Path to Formation?

The galaxy UGC 10738 resonates with the galaxy described yesterday — BRI 1335–0417 — in that it raises questions about how spiral galaxies form. In fact, the team working on UGC 10738 thinks it goes a long way toward answering them. That’s because what we see here is a cross-sectional view of a galaxy much like the Milky Way, one that has both ‘thick’ and ‘thin’ disks like ours. The implication is that these structures are not the result of collisions with smaller galaxies but typical formation patterns for all spirals.

Nicholas Scott and Jesse van de Sande (ASTRO 3D/University of Sydney) led the study, which used data from the European Southern Observatory’s Very Large Telescope in Chile. As you can see from the image below, the galaxy, some 320 million light years away, presents itself to us edge on, offering a cross-section of its structure. Key to the work was the team’s assessment of stellar metallicity, as van de Sande explains:

“Using an instrument called the multi-unit spectroscopic explorer, or MUSE, we were able to assess the metal ratios of the stars in its thick and thin discs. They were pretty much the same as those in the Milky Way – ancient stars in the thick disc, younger stars in the thin one. We’re looking at some other galaxies to make sure, but that’s pretty strong evidence that the two galaxies evolved in the same way.”

Image: Galaxy UGC 10738, seen edge-on through the European Southern Observatory’s Very Large Telescope in Chile, revealing distinct thick and thin discs. Credit: Jesse van de Sande/European Southern Observatory.

Published in Astrophysical Journal Letters, the study argues that the thin and thick disks found in the Milky Way — and the similar structures found in UGC 10738 — indicate that rather than being the result of chance collisions with other galaxies, the different disks reveal a standard path of galaxy formation and evolution. They would be unlikely to be repeated elsewhere if the result of a rare and violent merger.

The ‘thick’ and ‘thin’ disks are interesting features, with the former primarily made up of ancient stars that show a low ratio of iron to hydrogen and helium. Tracing the metallicity of the thin disk stars reveals that they are younger and contain more metals. Our Sun is a thin disk star, and as the authors note, it is made up of about 1.5% elements heavier than helium (the definition of ‘metals’ in astronomical parlance). Stars of the thick disk show three to ten times less metal content, a striking difference.

Finding the same differences in the two disk structures in other spiral galaxies points to the likelihood that the Milky Way formed in a way common to such galaxies. Says Scott:

“It was thought that the Milky Way’s thin and thick discs formed after a rare violent merger, and so probably wouldn’t be found in other spiral galaxies. Our research shows that’s probably wrong, and it evolved ‘naturally’ without catastrophic interventions. This means Milky Way-type galaxies are probably very common. It also means we can use existing very detailed observations of the Milky Way as tools to better analyse much more distant galaxies which, for obvious reasons, we can’t see as well.”

So we have what the authors describe as a ‘tension’ between two formation scenarios, one stochastic, one common to spiral galaxies. But let’s get into the weeds a little. The paper goes on to widen the sample into other subtypes of galaxy:

This tension is further enhanced once early-type disk galaxies with [?/Fe]-enhanced thick disks are included in the population of present day galaxies with similar formation histories (Pinna et al. 2019b; Poci et al. 2019, 2021). That early-type disk galaxies are found to contain similar abundance patterns as found in the Milky Way (i.e. increased mean [?/Fe] off the plane of the disk) is unsurprising. Such galaxies represent one plausible end point for the Milky Way’s evolutionary history, suggesting a shared and generic evolutionary pathway for disk galaxies.

All of this harks back to Edwin Hubble’s classification of galaxies (the Hubble Sequence). The phrase “early-type” galaxies” refers to elliptical and lenticular galaxies, as opposed to spiral and irregular galaxies, without implying an evolutionary path from one type to another. So the authors are speculating when they talk about an ‘evolutionary end point’ for the Milky Way, but it’s an interesting speculation:

The Milky Way is often identified as a so-called ‘green valley’ galaxy (Mutch et al. 2011), a galaxy already undergoing a transition to the red sequence. When fully quenched (and assuming no dramatic structural changes in the mean time) the Milky Way will likely resemble a lenticular galaxy similar to FCC 170 (Pinna et al. 2019a).

The ‘green valley’ refers to galaxies where star formation has been slowed, either because they have exhausted their reservoirs of gas, or (in younger galaxies) have undergone mergers with other galaxies. Both the Milky Way and Andromeda are assumed to be green valley galaxies of the first kind.

FCC 170 (NGC 1381) is a lenticular galaxy found in the constellation Fornax, about 60 million light years from Earth, a member of the Fornax Cluster (hence the FCC designation, standing for Fornax Cluster Catalogue). As a lenticular, it is in a middle stage between a spiral and an elliptical galaxy, with a large scale disk but lacking spiral arms on the scale of a true spiral galaxy. Is this the Milky Way’s future?

And then, of course, there is the merger with Andromeda to look forward to. How much we have to learn about how galaxies change over time!

Image: This is NGC 1387, a lenticular galaxy likewise in the Fornax Cluster, and better angled to show the lack of spiral structure in such galaxies than FCC 170. Credit: Fabian RRRR. Based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA). CC BY-SA 3.0.

The paper is Scott et al., “Identification of an [?/Fe]—Enhanced Thick Disk Component in an Edge-on Milky Way Analog,” Astrophysical Journal Letters Vol. 913, No. 1 (24 May 2021), L11 (abstract / preprint).


The Most Ancient Spiral Galaxy Yet Found

My fascination with the earliest era of star and galaxy formation leads me to a new paper on an intriguing find. The authors describe the distant object BRI 1335–0417 as “an intensely star-forming galaxy,” and its image as captured by the Atacama Large Millimeter/submillimeter Array (ALMA) is striking. This is a galaxy that formed a mere 1.4 billion years after the Big Bang, making it the most ancient galaxy with spiral structure ever observed.

Spirals make up perhaps 70 percent of the galaxies in our catalogs, but how they form is an open question. Indeed, the proportion of spiral galaxies declines the further back in the evolution of the universe we observe. The spiral structure observed here extends 15,000 light years from the center of the galaxy (about a third the size of the Milky Way), while the total mass of stars and interstellar matter roughly equals our own galaxy.

Image: ALMA image of a galaxy BRI1335-0417 in the Universe 12.4 billion years ago. ALMA detected emissions from carbon ions in the galaxy. There is a compact and bright area in the center of the galaxy, and spiral arms are visible on both sides of it. Credit: ALMA (ESO/NAOJ/NRAO), T. Tsukui & S. Iguchi.

The BRI1335-0417 observations were performed by Satoru Iguchi (SOKENDAI/National Astronomical Observatory of Japan), working with graduate student Takafumi Tsukui, who is lead author of the paper reporting the work. Huge amounts of dust exist here, obscuring the object in visible light, but ALMA’s prowess at detecting radio emissions from carbon ions makes the investigation possible.

Even so, Tsukui points out that getting the size of the galaxy right is a difficult matter:

“As BRI 1335-0417 is a very distant object, we might not be able to see the true edge of the galaxy in this observation. For a galaxy that existed in the early Universe, BRI 1335-0417 was a giant.”

The biggest issue is how such a large structure with obvious spiral structure emerged so soon after the Big Bang. This is a ‘starburst’ galaxy: There are areas of active star formation here and gas instabilities in the outer parts of the disk which point to possible interactions with smaller galaxies or other interstellar matter. The paper points out that dusty galaxies with high rates of star formation, like BRI 1335–0417, are the progenitors of elliptical galaxies, with the spiral structure possibly redistributing angular momentum so as to trigger gas inflows into the center of the galaxy.

The authors are quick to note, however, that gas inflows like this are beyond the detection threshold of these observations. What we do see is the possibility of tidal interactions. The authors describe the likelihood of such events:

The high star formation rates of z > 4 [the measure of redshift] galaxies like BRI 1335–0417 are commonly explained as the result of major mergers, which could produce distorted galactic kinematics. We find that BRI 1335–0417 has only slightly disturbed, rotation dominated kinematics, which can be well described by a rotating disk model. This suggests that the high star formation rate must have been maintained long enough for the disk to form after any major merger event. The Q parameter [a measure of stability in a rotating, gaseous accretion disc] shows the outer disk of BRI 1335–0417 is unstable, which could be caused by gas accretion along large-scale filaments of the cosmic web, and/or minor mergers with accreting satellites.

So we have spiral structure, a rotating galactic disk, and a centralized mass. Is BRI 1335–0417 the ancestor of a giant elliptical galaxy? Answering that question of galactic survival and change involves issues of galaxy evolution that remain wide open.

The paper is Tsukui & Iguchi “Spiral morphology in an intensely star-forming disk galaxy more than 12 billion years ago,” published online in Science 20 May 2021 (abstract).


Lights of the Nightside City

On the matter of city lights as technosignatures, which we looked at on Friday, I want to follow up with Thomas Beatty’s work on the issue in the context of an assortment of nearby stars. Beatty (University of Arizona, Tucson) assumes Earth-like planets examined via direct-imaging by LUVOIR, a future space telescope in planning, or HabEx, a different architecture for a likewise powerful instrument. What he’s done is to take data from the Soumi National Polar-orbiting Partnership satellite to find the flux from city lights and the spectra of currently available lighting. He goes on to model the spectral energy distribution from such emissions as applied to exoplanet settings at various distances.

Why look at city lights in the first place? Because they’re another form of technosignature that may be within the realm of detection, and we’d like to find out what’s possible and what any results would imply. In particular, Beatty reminds us, the National Academies’ Exoplanet Science Strategy and Astrobiology Strategy reports are on record as recommending space-based, direct imaging that is capable of directly detecting emissions from habitable zone planets. This would obviously support biosignature searches but would also open up a hunt for technosignatures.

Technosignature searches can take place within the context of ongoing biosignature investigations on the same planets. Both LUVOIR and HabEx should be capable of this, generating data sets that can be scanned for both biological and technological returns. One area of investigation has been satellites — could we detect satellite constellations in orbit around an Earth-like planet? Large-scale photovoltaic arrays would show a different signature than vegetation on the surface. Various forms of pollution in atmospheres are within LUVOIR’s range, so the field is wide.

A lack of a specific technosignature is itself interesting, as it helps us begin to constrain the field. Just as we started searching for planets around Proxima Centauri by first ruling out gas giants of a certain mass, then Jupiter-class objects, then ice giants of Neptune size, we first learn what is not there and then can specify what remains to be searched for. The lack of SETI detections at radio or optical frequencies, for example, makes it less likely that technological civilizations are broadcasting powerful beacons aimed at us from stars near the Sun, thus paring down earlier possibilities.

But back to city lights, which Jean Schneider and colleagues first studied in 2010 (citation below). We’ve learned through the work of Avi Loeb and Elisa Tabor that artificial illumination from the nightside of Proxima b could be detected, though with great difficulty, by the James Webb Space Telescope. LUVOIR will up the ante and widen the range. Beatty points out that city lights are compelling because they would presumably be long-lived artifacts of a technological culture and would offer a unique spectroscopic signature that is unlike anything produced by natural processes.

Image: This is Figure 1 from the paper. Caption: Figure 1. The nightside of Earth shows significant emission from city lights in the optical. This is a composite, cloud-free, image of Earth’s city lights compiled using Day/Night Band observations taken using the Visible Infrared Imaging Radiometer Suite instrument on the Soumi National Polar-orbiting Partnership satellite (Roman et al. 2018). Searching for the emission from city lights is a compelling technosignature because it requires very little extrapolation from current conditions on Earth, should be relatively long-lived presuming an urbanized civilization, and offers a very distinct spectroscopic signature that is difficult to cause via natural processes. This places the emission from nightside city lights high on the list of potential technosignatures to consider (Sheikh 2020).

Beatty considers the detectability of city lights first as a function of stellar distance and the amount of a planet’s surface covered by urbanization on Earth-like planets around G-, K- and M-dwarf stars. He then calculates their detectability on planets orbiting stars within 10 parsecs of the Sun, and finally estimates detectability on two dozen known, potentially habitable planets around stars close to the Sun. The tables within this paper are worth scanning, but here are some of the highlights:

We learn that LUVOIR should be able to detect city lights on Proxima b at an urbanization level of 0.5% (10 times Earth’s). Lalande 21185 b, Luyten’s Star b and Tau Ceti e and f would show detectable emission from city lights at urbanization levels of 3% to 10% in LUVOIR imaging.

Detection of city lights should be easiest on M-dwarf planets, and Beatty notes in particular planets around Proxima Centauri, Barnard’s Star, and Lalande 21185, but he points out how quickly the habitable zone around this kind of star moves within the inner working angle (IWA) of LUVOIR with distance, making it beyond the capacity of the instrument.

Earth-analog planets around Sun-like stars can be imaged at greater distances, but the distance drives the minimum detectable urbanization fraction higher. Here Beatty suggests Alpha Centauri A and B, Epsilon Eridani, Tau Ceti and Epsilon Indi A as potential targets.

And this brings up memories of Isaac Asimov’s global city Trantor: What Beatty calls an ‘ecumenopolis’ — a planet-wide city — would be detectable at much larger distances. The author surveys 80 nearby stars on which such a city would be at least marginally detectable.

Thus the work moves from the study of Earth’s urbanization fraction (0.05%) up to an ecumenopolis, showing how detectability scales with the amount of planetary surface covered. The paper assumes 100 hours of observing time for generic Earth-class planets around stars within 10 parsecs. Earth itself would not be detectable by LUVOIR in this range, but planets around M-dwarfs near the Sun would show detection for urbanization levels of 0.4% to 3%. City lights on planets orbiting nearby Sun-like stars would be detectable at urbanization levels in the range of 10 percent.

From the paper:

The possibility of directly detecting technosignatures on the surfaces of potentially habitable exoplanets is thus starting to be in the realm of practicality. Perhaps unsurprisingly, the 15m LUVOIR A architecture would be the most capable observatory for detecting city lights on the nightsides of nearby exoplanets, though LUVOIR B [smaller than LUVOIR A) or HabEx with a starshade would also have significantly sized detection spaces. Much of this proposed capability has been spurred by the goals of characterizing the atmospheres of and detecting biosignatures on potentially habitable exoplanets, but it also would afford us the opportunity to search for other, technological, signs of life.

In short, we’re going to be looking hard at many of these planets within a few decades as we search for biosignatures. The same data may show technosignatures, the strength of which we need to examine to see what’s possible. We are simply defining the limits of the search.

The paper is Beatty, “The Detectability of Nightside City Lights on Exoplanets,” in process at Monthly Notices of the Royal Astronomical Society (abstract). The Schneider et al. paper is “The Far Future of Exoplanet Direct Characterization,” Astrobiology Vol. 10, No. 1 (22 March 2010). Abstract. Thanks to my friend Antonio Tavani for the pointer to this paper.