Exoplanets Found to be Plentiful in the Galactic Bulge

I mentioned yesterday that we are just opening up the discovery space when it comes to exoplanets. It’s an obvious observation for those who follow these things, but I suspect most casual observers don’t realize that almost all the planetary systems we’ve found thus far are located relatively close to the Sun, almost always within no more than a few thousand light years. Most of the stars the Kepler mission observed in Cygnus, Lyra and Draco were about the same distance from galactic center as the Earth. The average distance to the target stars of this most productive of all exoplanet missions yet was 600 to 3,000 light years.

Kepler, like TESS, worked by studying the transits of planets across their host stars, and in Kepler’s case, the method was unable to detect transits at distances any larger than these. In fact, we have only one method that can detect exoplanets at a wide range of distances in the Milky Way, and that is gravitational microlensing, which can take us into the galactic bulge. Here the planet is detected during an occultation, with its host star moving in front of a far more distant star. The pattern of gravitational lensing this produces reveals the closer star’s planet.

But these are one-off observations, not something like a transit that can be seen with regularity. Moreover, while gravitational microlensing offers a priceless chance to find planets tens of thousands of parsecs from the Sun, its utility has been somewhat compromised by the difficulty in assessing stellar distances at ranges more than 10,000 light years out. A new study in Astrophysical Journal Letters is just out that examines the issue, homing in on the relative motion of the lens and the more distance source of light that is being occulted.

Running actual microlensing events against a model of galactic motion, a team led by Naoki Koshimoto infers the galactic distribution of planets. We learn that distance from galactic center does not produce a change in planetary distribution even in the galactic bulge. This is an intriguing result, for one might think that circumstellar disk materials would be more plentiful in areas where the density of stars is as high as in the bulge, affecting a star’s content of metals. Other factors might be the intensity of radiation in high-population areas or the likelihood of differences in the number of multiple star systems.But Koshimoto finds planets everywhere:

“Stars in the bulge region are older and are located much closer to each other than stars in the solar neighborhood. Our finding that planets reside in both these stellar environments could lead to an improved understanding of how planets form and the history of planet formation in the Milky Way.”

Image: An artist’s conception of cold planet distribution throughout the Milky Way. For comparison, the cyan cone is the Kepler transit survey field. The inset shows an artistic conception of a planetary system in the galactic bulge. Credit: Osaka University.

The researchers build their analysis around a number of observed microlensing events. One factor involved in the Osaka study is the Einstein radius crossing time, which measures the lensing event as host planet and star occult the more distant object. An Einstein ring is the lens effect as light is curved by mass during the occultation (in a perfectly aligned event, the light would appear as a ring). The researchers also measured the proper motion of the lens source, meaning the star (and planet) passing in front of the distant star and bending its light.

The authors estimate planet hosting probabilities by comparing data on the proper motion in some 28 observed microlensing events with what is predicted by their own galactic model, allowing them to estimate “for the first time the dependence of the planet-hosting probability on the Galactocentric distance.” Their model includes planet-hosting probability and considers stellar mass, velocity and density distribution for randomly selected stars. The model draws on Gaia data and a range of astronomical projects building data on stellar motion and velocity.

Needless to say, everything rides on this model, and the calculations that produced it are laid out in all their complexity in the paper. Dissecting the model is a clear path forward for subsequent research into the question of planets in the galactic bulge. But I want to cut to the chase, given the novelty of having this kind of assessment of exoplanet distribution on a galactic level. Noting that the galactic bulge has a stellar density on the order of 10 or more times greater than the neighborhood of the Sun, and that its star population is older, the paper notes the tentative result that location in the galaxy does not change the frequency of planets.

With a nod to earlier research that has suggested otherwise, they add this:

Observations of the solar neighborhood have shown that there is a correlation between stellar metallicity and the occurrence of giant planets…, and have also suggested that close encounters with other stars may affect the evolution of planetary systems… Therefore, due to the abovementioned environmental differences, the planet frequency in the bulge may differ significantly from that in the solar neighborhood. Although our results are still inconclusive, they might imply that cold planets orbiting beyond the H2O snow line also commonly exist in the bulge region regardless of such differences.

Or, to state the matter more directly:

One of the largest uncertainties when comparing the exoplanet population discovered via other exoplanet detection methods with microlensing planets is a possible large dependence of the planet frequency on Galactic location. Our results show that such a dependence is not very large, and one might be able to compare them without considering the difference in Galactic location.

Bear in mind the context here. A 2016 paper from Matthew Penny (Louisiana State University) and colleagues has suggested, based on published microlensing data and a different galactic model, that the galactic bulge could be devoid of planets. Koshimoto and team reject this work on the basis of “an inhomogeneous sample and incorrect microlens parallax measurements,” but it’s a measure of how little we know about planets in the inner regions of the galaxy that early papers like these should reach such profoundly different conclusions.

The paper is Koshimoto et al., “No Large Dependence of Planet Frequency on Galactocentric Distance,” Astrophysical Journal Letters Vol. 918, No. 1 (26 August 2021), L8 (abstract / preprint). The Penny paper is “Is the Galactic Bulge Devoid of Planets?” Astrophysical Journal Vol. 830, No. 2 (19 October 2016), 150 (abstract).


Into the Brown Dwarf Desert

It’s a measure of how common exoplanet detection has become that I can’t even remember the identity of the object I’m about to describe. Back in the early days (which means not long after the first main sequence detection, the planet at 51 Pegasi), I was at a small dinner gathering talking informally about how you find these objects. A gas giant was in the news, another new world, or was it really a brown dwarf? And just what was a brown dwarf in the first place? Back then, with just a handful of known exoplanets, introducing the idea of a brown dwarf raised a lot of questions.

Now, of course, we have planets in the thousands and are just opening up the discovery space. Brown dwarfs are plentiful, with some estimates at one brown dwarf for every six main sequence stars. A 2017 analysis of a cluster called RCW 38 by Koraljka Muzic and team concluded that the galaxy contains between 25 and 100 billion brown dwarfs. So we have plenty to work with as we home in on the still controversial borderline between planet and brown dwarf, as well as between brown dwarf and star.

We know that brown dwarfs are not massive enough to support hydrogen fusion but do fuse small amounts of deuterium, an isotope of hydrogen whose nucleus holds one proton and one neutron. The contrast could not be more stark: Low-mass M-class stars can burn hydrogen for tens of billions of years, with longevity far beyond that of our Sun. Brown dwarfs keep getting cooler after going through a short period of deuterium burning. The problem is that only about 30 brown dwarfs have been accurately characterized, as University of Geneva researcher Nolan Grieves points out.

“…we still do not know exactly where the mass limits of brown dwarfs lie, limits that allow them to be distinguished from low-mass stars that can burn hydrogen for many billions of years, whereas a brown dwarf will have a short burning stage and then a colder life. These limits vary depending on the chemical composition of the brown dwarf, for example, or the way it formed, as well as its initial radius.”

The scholarship on brown dwarf mass generally sets a limit of 13 Jupiter masses (MJup) as the dividing line between a gas giant and a brown dwarf. This is the approximate mass that an object must reach to ignite deuterium fusion in its core. The upper limit, dividing brown dwarf from star, has been commonly set at 80 Jupiter masses, which is where the object is massive enough to begin hydrogen burning, fusing hydrogen nuclei into helium nuclei. Exactly where these boundaries occur, though, depends on the chemical composition of the particular object in question.

Thus the value of the work Grieves and team have produced, a study of five brown dwarfs that were found in TESS data through their transit signature and later analyzed using radial velocity methods. They are ‘companion’ objects, as opposed to unbound brown dwarfs. All are near the hydrogen-burning mass limit. Their main sequence host stars were originally identified as TESS Objects of Interest (TOI), and the companions orbit with periods ranging between 5 and 27 days. Because they transit, we can also detect their radii, between 0.81 and 1.66 times Jupiter’s radius. They range in mass between 77 and 98 Jupiter masses, which nudges them into M-dwarf territory.

Image: This artist’s illustration represents the five brown dwarfs discovered with the satellite TESS. These objects are all in close orbits of 5-27 days (at least 3 times closer than Mercury is to the Sun) around their much larger host stars. Credit: CC BY-NC-SA 4.0 – Thibaut Roger – UNIGE.

Are we sure these detections are not of very low mass stars? One way of approaching the matter is to look at the relationship between their size and age. A brown dwarf will lose deuterium as it burns up its reserves, thus cooling down and shrinking over time. The fact that the two oldest TESS Objects of Interest — TOI 148 and TOI 746 — have smaller radii while the younger objects are larger points to the likelihood that the former are brown dwarfs.

These age estimates are based, in the case of the comparatively young TOI-587, on what is known as ‘isochrone stellar modelling,’ which essentially fits data onto the Hertzsprung-Russell diagram to draw conclusions about the ages of stars in clusters. The young age of TOI-681 is likewise drawn from inferences due to its membership in a cluster. But even these rough estimates are compromised by the fact that TOI-681b makes only a grazing transit, as does another of the TESS finds, TOI-1213b.

I mention the age issue as just one factor in working out the nature of these objects, and one that is itself not well constrained. Given the uncertainties, the authors’ conclusion points to the need to compile a larger dataset for brown dwarfs:

The sample of transiting brown dwarfs and low-mass stars we analyzed is still too small to make significant statistical claims; however, their eccentricity and metallicity distributions are still consistent with previous suggestions of two separate populations for lower and higher mass brown dwarfs. These companions are all near the hydrogen-burning mass limit and add to the statistical sample needed to distinguish the population differences between brown dwarfs and low-mass stars.

Note that comment on two separate populations of brown dwarfs depending upon mass. We’re just getting a handle on these issues. The questions that hovered over my post-dinner conversation on brown dwarfs back in the 1990s continue to vex astronomers as we probe this class. Defining separate populations for these objects seems to be where we’re headed as we craft formation models.

Bear this in mind: Brown dwarfs rarely occur in close orbits around main sequence stars, the word ‘close’ in this case meaning orbits at 5 AU or closer to the primary. Thus the phrase ‘brown dwarf desert’ to characterize orbits that brown dwarfs rarely occupy as a companion object. The authors think this lack of brown dwarf companions relates to formation mechanisms, though the jury is out:

The relative lack of brown dwarf companions may be related to a transition of the formation mechanisms required to form giant planets and low-mass stars. In this case, lower mass brown dwarfs may form similar to giant planets via core accretion (Pollack et al. 1996) or disk instability (Cameron 1978; Boss 1997) and higher mass brown dwarfs may form similar to stars from gravitational collapse and turbulent fragmentation of molecular clouds (Padoan & Nordlund 2004; Hennebelle & Chabrier 2008). The boundary of these formation mechanisms is unclear and certainly depends on an object’s initial environment.

Brown dwarfs that can be well characterized are the key to resolving these issues, especially those whose radius can be precisely determined, as in the new TESS discoveries. Here we have the kind of robust photometric and spectroscopic measurements that will help astronomers test current formation models.

The paper is Grieves et al., “Populating the brown dwarf and stellar boundary: Five stars with transiting companions near the hydrogen-burning mass limit, Astronomy & Astrophysics Vol. 652, A127 (August, 2021). Abstract / Preprint.


‘Hycean’ Worlds: A New Candidate for Biosignatures?

We’ve just seen the coinage of a new word that denotes an entirely novel category of planets. Out of research at the University of Cambridge comes a paper on a subset of habitable worlds the scientists have dubbed ‘Hycean’ planets. These are hot, ocean-covered planets with habitable surface conditions under atmospheres rich in hydrogen. The authors believe they are more common than Earth-class worlds (although much depends upon their composition), and should offer considerable advantages when it comes to the detection of biosignatures.

Hycean worlds give us another habitable zone, this one taking in a larger region than the liquid water habitable zone we’ve always considered as the home to Earth. In every respect they challenge our categories. Not so long ago a Cambridge team led by Nikku Madhusudhan found that K2-18b, 2.6 times Earth’s radius and 8.6 times its mass, could maintain liquid water at habitable temperatures beneath its hydrogen atmosphere. The team has now generalized this work with a full investigation of the planetary and stellar properties making life possible on such planets.

Planets between the size of Earth and Neptune are thus far the most common type of planet we’ve found, generally being labeled as ‘super-Earths’ or ‘mini-Neptunes.’ There are no analogues to planets like this in the Solar System; they are classed as super-Earths or mini-Neptunes largely on the basis of their density as inferred by their mass and radius. Some may be predominantly rocky, while others are closer to the ice giants in our system. Some may be water worlds. Some of them are in the habitable zones of nearby M-dwarf stars, making them good candidates for atmospheric studies and possible biosignature detections.

For planets with a hydrogen atmosphere surrounding a layer of high-pressure water covering an inner core of rock and iron may become astrobiologically interesting. It’s true that too dense a hydrogen envelope would create temperature and pressure at the surface that would preclude life. But if the atmosphere is not too thick, life-sustaining temperatures can exist.

The Hycean planets thus represent a new category of potentially habitable worlds, and can be up to 2.6 times the size of Earth, with atmospheric temperatures up to 200 degrees Celsius, while still remaining habitable. They are defined not only by size but also by mass, temperature and atmospheric pressure. Conditions in their oceans may allow at least microbial life.

We are looking at a wide habitable zone as well. Its range takes in planets with orbital separations so large that the only energy source would be internal heat. It also extends to planets orbiting so close to the host star that they are tidally locked, but can support life on their dark sides. The span of possible temperatures allowing life to exist is thus substantial. About the tidally locked worlds the authors refer to as ‘Dark Hycean,’ for example, we learn this:

…we nominally consider the planet-wide average surface and atmospheric temperature to be 500 K. The choice of this temperature is motivated by the atmospheric models for nightsides of Dark Hycean planets…. In particular, we find that planets with equilibrium temperatures of ?510 K with inefficient day-night energy redistribution can lead to dayside temperatures of ?500-600 K but nightside surface temperatures ?400 K. Therefore, while a 510 K temperature is not considered to be habitable, it represents a planet wide average and still allows a nonnegligible fraction of the nightside ocean surface to be at habitable surface temperatures, i.e., below 400 K.

Image: Artist’s conception of the surface of a Hycean planet. Credit: Amanda Smith, Nikku Madhusudhan.

The researchers believe these interesting worlds are common. In their paper, they present a sample of potential Hycean targets that could be useful fodder for next-generation telescopes. All of these orbit red dwarf stars close enough to be suitable targets for the James Webb Space Telescope; none are more than 150 light years away. JWST observations of K2-18b are already being considered and could conceivably provide a biosignature detection. For having looked at five potential biomakers in Hycean atmospheres, the authors note:

Hycean atmospheres may offer even better opportunities for detecting these biomarkers than those of rocky super-earths… For a 10 M? planet, the Hycean radius range is ?2-2.6 R? compared to the super-Earth radius of 1.75 R? considered in Seager et al. (2013b). The increased radii and lower gravities lead to larger, more easily detectable spectral signatures for Hycean planets. Second, considering that prominent sources of the above biomarkers are thought to be aquatic microorganisms, we expect them to be even more abundant on Hycean worlds compared to predominantly rocky worlds.

The fact that Hycean planets open up the discovery space for worlds that could support life makes them noteworthy. We begin to consider planets of higher mass and radius than before, provided they have a rocky core that, according to the paper, is at least 10% of planetary mass and is of Earth-like composition. Their wide habitable zone expands the area for detection, while presenting a range of challenges that is likewise wide. Mass and radius help us spot a Hycean candidate, but they alone are not sufficient. We also need to learn more about temperatures and pressures in the ocean, and basic properties of the atmosphere:

,,,even if a candidate Hycean planet is in the Hycean HZ it may not necessarily have the right conditions for habitability, e.g., the internal structure and atmospheric properties may be such that the ocean surface pressure and/or temperature is too high… [T]he detection of H2O in the atmosphere does not guarantee the presence of an ocean on the planet, as H2O can be naturally occurring in H2-rich atmospheres as the prominent oxygen bearing species. Conversely, the nondetection of H2O does not preclude the presence of an ocean, since at low atmospheric temperatures H2O can rain out and not be detectable in the atmosphere. Nevertheless, in all these aspects Hycean candidates offer better prospects for establishing their habitability compared to habitable rocky exoplanets, which are inherently harder to characterize.

Given the wide range of transiting worlds we’ve discovered between 1 and 2.6 Earth radii, Hycean worlds offer no shortage of targets and if nothing else provide opportunities for atmospheric characterization that, according to the authors, should be less challenging than similar work on rocky exoplanets. Their large radius and thick atmospheres seem made to order for JWST and future instruments like the Extremely Large Telescope. Although not similar to Earth, Hycean planets can be valuable venues for detecting trace biosignatures. That alone contributes to the larger quest of finding life on planets more similar to our own.

The paper is Madhusudhan et al., “Habitability and Biosignatures of Hycean Worlds,” in process at The Astrophysical Journal (2021). Preprint. The paper on K2-18b is “The Interior and Atmosphere of the Habitable-zone Exoplanet K2-18b,” Astrophysical Journal Letters Vol. 891, No. 1 (27 February 2020), L7 (abstract). And I’ve just become aware of (but haven’t yet read) Benncke et al., “Water Vapor and Clouds on the Habitable-zone Sub-Neptune Exoplanet K2-18b,” Astrophysical Journal Letters 887:L14 (10 December 2019). Abstract.


A Huge Population of Interstellar Comets in the Oort Cloud

TAOS II is the Transneptunian Automated Occultation Survey, designed to spot comets deep in our Solar System. It may also be able to detect comets of the interstellar variety, of which we thus far have only one incontrovertible example, 2I/Borisov. And TAOS II, as well as the Vera C. Rubin Observatory (both are slated for first light within a year or so) could have a lot to work with, if a new study from Amir Siraj and Avi Loeb (Center for Astrophysics | Harvard & Smithsonian) is correct in its findings.

I cite Borisov as thus far unique in being an interstellar comet because the cometary status of ‘Oumuamua is still in play. On my way to looking at his paper on Borisov, I had an email exchange with Avi Loeb, from which this:

Observations with the Spitzer Space Telescope of `Oumuamua placed very tight limits on carbon-based molecules in its vicinity, implying that it was not made of carbon or oxygen. This led to suggestions that perhaps it is made of pure hydrogen or pure nitrogen, but these would be types of objects we had never seen before. Borisov appeared to be just like a regular comet that we had seen many times before. Clearly, `Oumuamua and Borisov are of very different composition and origin (irrespective of whether `Oumuamua is natural or artificial in origin).

Image: Comet 2I/Borisov. Credit: NASA, ESA and D. Jewitt (UCLA).

The paper refers to Borisov as “the first confirmed interstellar comet with a known composition,” but if this comet is alone in our catalog, it’s unlikely to remain that way long. Siraj and Loeb argue that there exist more interstellar objects in the Oort Cloud than objects born in the Solar System. Indeed, Loeb in his email cited “a hundred trillion Borisov-like interstellar comets” in this vast space, which extends from roughly 2,000 AU perhaps as far out as 50,000 AU, with some sources citing an outer edge as far as 200,000 AU. That should ring a few bells — Alpha Centauri is 268,000 AU from the Sun, meaning our Oort Cloud could mingle with any similar cloud in that system.

The prospect of studying interstellar objects without leaving our own system is enhanced by these results, even if the calculations contain significant uncertainties. There should be many Borisovs, a small number of which should enter the inner system. This is a reversal of earlier thinking that interstellar visitors should be rare, all part of a reevaluation of the subject forced by the detection of ‘Oumuamua and 2I/Borisov in recent years, and the coming upgrades in equipment and surveys mentioned above.

We are only now getting into position to be able to see these objects and identify their true nature. The detection of Borisov in 2019 allowed scientists to calculate a number density for such objects per star based on a statistical analysis of the likelihood of a single object like this being within 3 AU of the Sun. Other researchers had applied this kind of calibration to ‘Oumuamua, with the number density implied by both being approximately the same. Similarly, the population of bound Oort Cloud comets can be inferred through observations of long-period comets. Figure 1 in the paper shows the comparison.

Image: This is Figure 1 from the paper. Caption: Comparison of the relative abundance per star of bound Oort cloud objects, as implied by the observed rate of long-period comets (Brasser & Morbidelli 2013), and interstellar objects, as implied by the detection of Borisov (Jewitt et al. 2020), with a differential size distribution for power-law index, q, values of 2.5, 3, and 3.5, displayed for reference. The error bar indicates the 3? Poisson error bars for the implication of a singular interstellar object detection on the abundance. The shaded band correspond[s] to the plausible range of nucleus radii for Borisov, given the central value for Borisov’s abundance. The error bounds on the abundance of bound Oort cloud objects are not resolvable on this plot. Credit: Siraj and Loeb.

These are calibrations with, as the paper notes, uncertainties of several orders of magnitude, but even adjusting for these, interstellar objects still prevail in the Oort. They also come with limits that can be tested by observation. Interstellar objects experience negligible gravitational focusing because of their speed (on the order of 30 kilometers per second) and the nature of their orbits as related to their distance from the Sun. Bound Oort Cloud objects should have characteristic orbits that can be differentiated from the orbits of objects that have entered the Oort from elsewhere.

Note: ‘Gravitational focusing’ refers not to gravitational lensing but to the likelihood that two particles will collide based on their mutual gravitational attraction. The authors are saying that bound Oort objects are significantly affected by gravitational focusing. We wind up with a wide dispersion in these two populations:

Given that the number density of interstellar objects may be ?103 larger than that of bound Oort cloud objects far from the Sun, the Oort cloud objects may be still a factor of ?10 more abundant than interstellar objects in the inner Solar system, due to the unequal influence of gravitational focusing on the two populations. The fact that interstellar objects outnumber Oort cloud objects per star is consistent with the Oort cloud having lost most of its initial mass. However, the degree to which interstellar objects outnumber Oort cloud objects is still very uncertain. Stellar occultation surveys of the Oort cloud will be capable of confirming the results presented here, by differentiating between the two populations through speed relative to the Sun…

Thus we can look to planned surveys of the sort mentioned above to test the abundances of the two classes of objects, and can expect more visitors of the Borisov kind, even if such comets are far more common in the Oort Cloud than in the inner system. Siraj points out that such an abundance of interstellar objects indicates that planetary formation leaves a great deal more debris than previously thought:

“Our findings show that interstellar objects can place interesting constraints on planetary system formation processes, since their implied abundance requires a significant mass of material to be ejected in the form of planetesimals. Together with observational studies of protoplanetary disks and computational approaches to planet formation, the study of interstellar objects could help us unlock the secrets of how our planetary system — and others — formed.”

The paper is Siraj & Loeb, “Interstellar objects outnumber Solar system objects in the Oort cloud,” Monthly Notices of the Royal Astronomical Society Vol. 507, Issue 1 (October, 2021) L-16-L18 (abstract).


Enter the ‘Belatedly Habitable’ Zone

The most common objection I hear about what we call the ‘habitable zone’ is that it specifies conditions only for life as we know it. It leaves out, for example, conceivable biospheres under the ice of gas giant moons, examples of which we possibly have here in the Solar System. But there is another issue with defining habitability in terms of atmospheric pressures that can support liquid water on the surface. As Jason Wright and Noah Tuchow (both at Penn State) point out in a recent paper, the classic habitable zone concept does not take the evolution of both planet and star into account.

It’s a solid point. A planet now residing in the habitable zone could have remained habitable since the earliest era of its formation. Or it could have become habitable at a later time. Thus Tuchow and Wright make a distinction between what they refer to as the Continuous Habitable Zone (CHZ) and a class of planets they refer to as ‘belatedly habitable.’ These worlds may benefit from changes in the location of the habitable zone as stellar properties change, or they may enter the habitable zone through planetary migration. They may represent a substantial fraction of planets in the habitable zone. But are they truly habitable?

As the authors see it, there is not a single belatedly habitable zone (let’s refer to this as the BHZ), but rather two. The outer consists of the planets whose stars become more luminous over time, thus moving the habitable zone outward. The question here would be whether planets like this can successfully thaw and become habitable. I like James Kasting’s term for these worlds, coined as long ago as 1993. He calls them ‘cold start’ planets, and they represent a lively area of current research.

The inner belatedly habitable zone holds stars around which the habitable zone moves inward as the star dims. These inner BHZ planets are an intriguing lot because they orbit a wide range of lower-mass objects. Both brown and white dwarfs dim with time as they cool, making previously uninhabitable worlds more clement, though the authors note that these may lose many of their volatiles before achieving temperate conditions.

And because of their ubiquity in the Milky Way, we should pay special attention to M-dwarf planets. These worlds may spend millions of years in a greenhouse phase, with the possible loss of water, before their host star has finished the contraction that will eventually place it on the main sequence, dimming enough for habitability.

Given these distinctions, the liquid water habitable zone is actually a combination that includes the Continuous Habitable Zone as well as the inner and outer belatedly habitable zones, and as the authors point out, at any specific time in a star’s history, these regions will have different sizes and as the star evolves, may disappear entirely.

Image: This is Figure 1 from the paper. Caption: Habitable zone evolution for a 0.5?M? M dwarf (left) and a 1.0?M? solar analog (right). Continuous habitability is considered to start at the dashed vertical line, roughly representing the planet formation timescale. The green regions on the plots represent the continuously habitable zone, while the orange and blue regions represent the inner and outer belated habitable zones respectively. Credit: Tuchow & Wright.

To consider what the authors call ‘belated habitability,’ the star’s evolutionary history must be considered along with the presence of volatiles and their origins, the rates of cooling and outgassing as a young planet evolves, its related geophysical processes and more. Thus the complexity of the habitable zone deepens, taking the edge off quick claims for habitability in any given system. The fact that a planet is in the habitable zone today does not necessarily mean that liquid water exists on its surface:

A large portion of exoplanets that we find in the habitable zones of other stars will lie in the belatedly habitable zones, and future missions will greatly benefit by considering belated habitability and not assuming these planets are habitable. For example, in a search for biosignatures, the target stars and the search strategy will be affected by whether or not one considers the habitability of these planets. While the special circumstances of their habitability have been overlooked in the past, belatedly habitable planets could have major implications for future mission design and warrant future study.

I think these are useful distinctions that should come into play as our new generation telescopes come online. It’s certainly true that the press often exaggerates new discoveries of ‘habitable zone planets’ (and our friend Andrew Le Page is a shrewd judge of such claims), but from the standpoint of creating a catalog of best targets for further investigation, we need to be able to winnow the list efficiently and accurately. The study of ‘belated habitability’ should prove a productive research path.

The paper is Tuchow & Wright, “Belatedly Habitable Planets,” Research Notes of the AAS,” Volume 5, No. 8 (August, 2021). Full text.