The fascination with finding habitable planets — and perhaps someday, a planet much like Earth — drives media coverage of each new, tantalizing discovery in this direction. We have a number of candidates for habitability, but as Andrew LePage points out in this fine essay, few of these stand up to detailed examination. We’re learning more all the time about how likely worlds of a given size are to be rocky, but much more goes into the mix, as Drew explains. He also points us to several planets that do remain intriguing. LePage is Senior Project Scientist at Visidyne, Inc., and also finds time to maintain Drew ex Machina, where these issues are frequently discussed.

by Andrew LePage

The past couple of years have been eventful ones for those with an interest in habitable extrasolar planets. The media have been filled with stories about the discovery of many new extrasolar planets that have been billed as being “potentially habitable”. Unfortunately follow-up observations and new insights into the properties of planets larger than the Earth have cast doubts on some of these initial optimistic proclamations that have been largely ignored by the media and other outlets. With all the new information available, I figured it was a good time to make an objective reevaluation of the potential habitability of a number of extrasolar planets that have made the headlines in recent years.

Basic Habitability Criteria

A thorough assessment of the habitability of any extrasolar planet would require a lot of detailed data on the properties of that planet, its atmosphere, its spin state and so on. Unfortunately, at this very early stage, the only information typically available to scientists about extrasolar planets is basic orbit parameters, a rough measure of its size or mass and some important properties of its sun. Combined from theoretical extrapolations of the factors that keep the Earth habitable, the best we can hope to do at this time is to compare the known properties of extrasolar planets to our current understanding of planetary habitability to determine if an extrasolar planet is “potentially habitable”. And by “habitable”, I mean habitable in an Earth-like sense where the surface conditions allow for the existence of liquid water on the planet’s surface. While there may be other worlds that might possess environments that could support life (e.g. Mars or the tidally heated moons Europa and Enceladus), these would not be Earth-like habitable worlds of the sort being considered here.

One of the key pieces of information we have available for extrasolar planets to assess their potential habitability is their effective stellar flux (or S_eff where Earth’s value is defined as 1). This can be readily calculated using information about a planet’s orbit and the luminosity of its sun. If this effective stellar flux falls within a range corresponding to the limits of a sun’s habitable zone (HZ), this planet has met one of the basic criteria for potential habitability.

One of the more better known definitions for the limits of the habitable zone as defined by the work of James Kasting (Pennsylvania State University) starting over two decades ago is based on an extrapolation of our knowledge of the processes that have kept our own planet habitable over the last several billion years despite a 30% increase in the Sun’s luminosity. The latest refinements of this work by Ravi Kopparapu (Pennsylvania State University) and his collaborators define the inner limit of the HZ to correspond to the S_eff where a moist runaway greenhouse effect sets in. At higher effective stellar flux values, skyrocketing surface temperatures and the loss of a planet’s allotment of water in a geologically brief period of time will result. For an Earth-size planet orbiting a Sun-like star, this limit corresponds to an S_eff of about 1.11. The S_eff corresponding to this inner limit of the HZ would be slightly higher for planets more massive than the Earth and slightly lower for stars cooler than the Sun.

There have been models proposed over the past decade and more with higher effective stellar flux values for the inner limit of the HZ in cases of synchronous rotation (which would be common for planets orbiting in the HZs of red dwarfs) and a range of other special circumstances. Such definitions have been attractive to some hoping to maximize the chances that a new find might be considered to be habitable. However, these sometimes involve extreme extrapolations from conditions here on Earth or contrived special circumstance. In general, these definitions require more study and some reliable empirical observations to be on a firmer theoretical footing like the work by Kopparapu et al.. In a recent paper by Kasting and Kopparapu et al., it is argued that while there is certainly genuine uncertainty on the precise inner limit of the HZ as a result of limitations of the simple models used to date, some of the most optimistic inner limit definitions involve scenarios that are physically unrealistic. As result, I personally tend to favor the more conservative definition of the inner limits of the HZ.

The outer limit of the HZ, as defined by Kopparapu et al., corresponds to the maximum greenhouse limit beyond which a CO₂-dominated greenhouse is incapable of maintaining a planet’s surface temperature. The latest work suggests an S_eff value of about 0.36 for a Sun-like star with cooler stars having slightly lower values. As with the inner limit of the HZ, there are some slightly more optimistic definitions of the outer edge of the HZ such as the early-Mars scenario or evoking some sort of super-greenhouse where gases other than just CO₂ contribute to warming a planet. But these more optimistic definitions do not change the S_eff for the outer limit of the HZ significantly.

Another important parameter we have available today to gauge the potential habitability of an extrasolar planet is its mass (or M_P) derived from precision radial velocity measurements or its radius (or R_P) calculated from observations of planetary transits. In the case of the radial velocity measurements, we actually only know the planet’s M_Psini value where i is the inclination of the orbit with respect to our line of sight. Since the inclination can not be determined directly from radial velocity measurements alone, we can only know the planet’s minimum mass or the probability that the actual mass is in some range of interest. By definition, the actual mass of a planet with an unconstrained orbit inclination is most likely larger than this minimum mass – in some case it can be much larger.

A series of analyses of Kepler data and follow-up observations published over the last year have shown that there are limits on how large a rocky planet can become before it starts to possess increasingly large amounts of water, hydrogen and helium as well as other volatiles making the planet a Neptune-like world with no real prospect of being habitable. Work performed by Leslie Rogers (a Hubble Fellow at the California Institute of Technology) has shown that planets with radii greater than no more than 1.6 times that of the Earth (or R_E) are most likely mini-Neptunes. This and other recent work suggests that this transition corresponds to planets with masses greater than about 4 to 6 times that of the Earth (or M_E). As a result, planets larger or more massive than these empirically-derived thresholds are unlikely to be rocky planets never mind habitable. On the other hand, recent work submitted for publication by a team led by Courtney Dressing (Harvard-Smithsonian Center for Astrophysics) strongly suggests that worlds smaller than this threshold will usually have an Earth-like composition. For a more thorough discussion of this work, see The Composition of Super-Earths and my earlier Centauri Dreams post The Transition from Rocky to Non-Rocky Planets.

Image: This diagram illustrates how the boundaries of the HZ as defined in the work of Kopparapu et al. vary as a function of star temperature and planet mass. Several potentially habitable extra solar planets are included. Credit: Chester Harman/PHL/NASA/JPL.

With these basic criteria available, it is possible to start to gauge the potential habitability of an extrasolar planet. For this review, I wanted to use a well-regarded catalog of potentially habitable planets. The University of Puerto Rico at Arecibo Planetary Habitability Laboratory maintains a web site which currently lists 28 extrasolar planets in 23 systems in their Habitable Exoplanets Catalog along with many more currently unconfirmed planets that will not be considered here. The reviews that follow use this list of confirmed extrasolar planets and the data it contains except where noted.

EPIC 201367065d: R_P=1.5 R_E, S_eff=1.51

This extrasolar planet is among the first new worlds found during Kepler’s extended “K2” mission with its discovery just announced by Crossfield et al. in a paper submitted for publication. As I write this, it has yet to make it into the “Habitable Exoplanet Catalog” but I am including it here because it is bound to be added shortly since it seems to have properties similar to other worlds already in the catalog.

With a radius of 1.5 R_E, this EPIC 201367065d is just below the threshold dividing rocky and Neptune-like planets making it more likely to have an Earth-like composition. Unfortunately, its high effect stellar flux places it well beyond the inner boundary of the HZ as it is more conservatively defined for a red dwarf star. But given the uncertainties in its properties, I estimate that there is still about a one in eight chance of it actually orbiting inside even the conservatively defined HZ. While I consider EPIC 201367065d to be a poor candidate for being potentially habitable at this time, its sun is relatively nearby and bright making it a good candidate for follow up observations that can provide some hard data about the properties of worlds like this.

GJ 163c: M_Psini=7.3 M_E, S_eff=1.40

GJ 163c was discovered in 2012 using precision radial velocity measurements. As a result, we only know that its minimum mass is 7.3 ME. Given that this value is already exceeds the 6 ME threshold that seems to divide large rocky planets from mini-Neptunes and that this planet’s actual mass is probably higher still, it is unlikely that GJ 163c is a rocky planet. Combined with its high effective stellar flux that is larger than the more conservative definitions of the HZ, it seems improbable that GJ 163c is a potentially habitable, Earth-like world.

GJ 180b: M_Psini=8.3 M_E, S_eff=1.23

GJ 180c: M_Psini=6.4 M_E, S_eff=0.79

GJ 180 is a system thought by some to contain a pair of potentially habitable planets. While GJ 180c appears to orbit comfortably inside the inner part of the HZ of this system, GJ 180b seems to orbit just a little too close to be considered habitable using the more conservative definition of the HZ. Unfortunately, with measured minimum masses of 8.3 M_E and 6.4 M_E for GJ 180b and c, respectively, it is highly unlikely that either of these planets have rocky compositions. Given that these planets’ actual masses are probably much higher than this, it is more likely they are mini-Neptunes or larger with little prospect of being potentially habitable.

GJ 442b: M_Psini=9.9 M_E, S_eff=0.70

GJ 442b is yet another example of a planet that seems to orbit inside the HZ of its sun no matter how it is defined but it is too massive to likely be a rocky planet. Radial velocity measurements indicate that this planet has a minimum mass of 9.9 M_E which makes it much more likely to be a mini-Neptune. In fact, given the uncertainty in the inclination of its orbit to our line of sight, there are better than even odds that GJ 442b is Neptune-size or even larger. As a result, GJ 442b is highly unlikely to be a potentially habitable planet.

GJ 667Cc: M_Psini=3.8 M_E, S_eff=0.88
GJ 667Ce: M_Psini=2.7 M_E, S_eff=0.30
GJ 667Cf: M_Psini=2.7 M_E, S_eff=0.56

GJ 667C has been in the news a lot recently because of the belief that it contains as many as seven planets discovered using precision radial velocity measurements. Initial assessments hinted that three of the planets in this packed system might be potentially habitable – GJ 667Cc, e and f. Unfortunately, follow-up work performed on this promising planetary system now strongly suggests that it does not contain any potentially habitable planets at all.

A series of independent analyses of the radial velocity data for GJ 667C culminating in the work by Paul Robertson and Suvrath Mahadevan (Pennsylvania State University) now indicates that the radial velocity variations originally interpreted as being the result of as many as seven planets are in fact caused by only two planets. It now seems likely that surface activity on GJ 667C modulated by its 105-day rotation period is responsible for mimicking the subtle radial velocity signature of the other supposed planets including the potentially habitable GJ 667Ce and f. A similar situation was encountered last year with the habitable planets of GJ 581 which these same investigators also found to be the result of stellar activity masquerading as planets. While more follow-up work is required, it now seems likely that GJ 667C and f do not exist.

While the existence of GJ 667Cc seems to be secure, unfortunately its potential habitability appears to have been overstated. Based on its S_eff value, GJ 667Cc seems to be safely inside the inner portions of this star’s HZ. However, since this planet was discovered using radial velocity measurements, we currently only know that its minimum mass is about 4.1 M_E based on the work by Robertson and Mahadevan. Given the currently unconstrained inclination of its orbit to our line of sight, there is only a one in three chance that this world has a mass less than the 6 M_E threshold dividing predominantly rocky worlds from mini-Neptunes. It is much more probable that GJ 667Cc is a mini-Neptune with little chance of being potentially habitable.

If GJ 667Cc beats the odds and is a rocky planet after all, it is still unlikely to be a promising habitable planet candidate. Investigation of the spin state of GJ 667Cc performed by Valeri Makarov and Ciprian Berghea (US Naval Observatory) strongly suggests that this world is experiencing excessive tidal heating due to the high eccentricity of its small orbit around its primary. Makarov and Berghea estimate that if GJ 667Cc has an Earth-like composition, tidal heating would generate about 300 times the heat flow as the Earth experiences melting its mantle and crust in the process. Given the two most likely possibilities, it seems highly improbable that GJ 667Cc is a potentially habitable world. For a more detailed discussion of this system, see Habitable Planet Reality Check: GJ 667C.

GJ 682c: M_Psini=8.7 M_E, S_eff=0.37

Based on an analysis of the radial velocity of GJ 682, it appears that GJ 682c orbits near the outer limits of the HZ of this system. But once again, with a minimum mass of 8.7 M_E and an actual mass that is probably much higher, it is highly unlikely that GJ 682c is a rocky planet. Given an unconstrained orbit inclination, it has about an even chance of being Neptune-size or larger. It is therefore very unlikely that GJ 682c is potentially habitable.

GJ 832c: M_Psini=5.4 M_E, S_eff=1.00

In 2014, a team led by Robert Wittenmyer (UNSW Australia) announced the discovery of a planet orbiting GJ 832 using precision radial velocity measurements. Given the properties of this world, Wittenmyer et al. specifically stated in their discovery paper that they did not believe that their find was a potentially habitable planet and it was more likely to be a uninhabitable super-Venus instead. This candid assessment was ignored by some who argued that GJ 832c is among the most Earth-like planets known. The effective stellar flux of GJ 832c places this world just inside the inner edge of this system’s conservatively defined HZ. Even if we were to expand the HZ limits based on more optimistic definitions of the HZ, the 5.4 M_E minimum mass of GJ 832c gives it a 90% probability of exceeding the 6 M_E mass threshold dividing Earth-like and Neptune-like planets. As a result, it is improbable that GJ 832c is a rocky planet never mind a potentially habitable one. For a more detailed discussion of this planet, see GJ 832c: Habitable Super-Earth or Super Venus?.

GJ 3293b: M_Psini=8.6 M_E, S_eff=0.60

GJ 3293b is yet another example of a world that seems to orbit comfortably inside the HZ but has almost no chance of being habitable due to its excessive mass. Based on precision radial velocity measurements, GJ 3293b has a minimum mass of 8.6 M_E which already exceeds the 6 M_E mass threshold where it is more likely that a planet is a mini-Neptune instead of a rocky planet. With an unconstrained orbit inclination, there are about even odds that this planet is actually Neptune-size or larger. As a result, it is highly improbable that GJ 3293b is potentially habitable.

HD 40307g: M_Psini=7.1 M_E, S_eff=0.68

The situation with HD 40307g is comparable to that of GJ 442b, GJ 682c and GJ 3293b: the planet seems to orbit comfortably inside the HZ but it is most likely a mini-Neptune or larger planet. With a minimum mass of 7.1 M_E derived from radial velocity measurements and an unconstrained inclination, it is unlikely that HD 40307g is a potentially habitable planet.

Kapteyn b: M_Psini=4.8 M_E, S_eff=0.43

Kapteyn’s Star is an ancient, nearby red sub-dwarf only 12.8 light years away. Last year’s announcement of the discovery of two planets orbiting this star promises important insights into the planet formation process during the earliest history of our galaxy. One of those two planets, Kapteyn b, was widely claimed to be the oldest potentially habitable planet yet discovered. Looking at this world’s effective stellar flux, it seems to be comfortably inside the outer part of this star’s HZ. But since it was discovered using precision radial velocity measurements, we only have a minimum mass value of 4.8 M_E. With an unconstrained orbit inclination, there is an 80% probability that its actual mass exceeds 6 M_E making it more likely to be a mini-Neptune rather than a rocky planet. As a result, it is unlikely that Kapteyn b is a potentially habitable planet. For a more detailed discussion of this planet, see Habitable Planet Reality Check: Kapteyn b.

Kepler 22b: R_P=2.4 R_E, S_eff=1.11

Like so many planets found using radial velocity measurements, there have also been worlds discovered by NASA’s Kepler mission that were initially considered potentially habitable by some but turn out to be too large after more detailed analyses of planet properties have become available. The effective stellar flux of Kepler 22b places it just beyond the inner edge of a conservatively defined HZ. But with a radius measured to be 2.4 R_E, which easily exceeds the 1.6 R_E threshold where planets are no longer likely to be rocky, it is very unlikely that Kepler 22b is a potentially habitable planet and more likely to be a volatile-rich mini-Neptune instead.

Kepler 61b: R_P=2.2 R_E, S_eff=1.27

Kepler 61b is in a similar situation as Kepler 22b: its effective stellar flux appears to be a bit too high to be considered inside the conservative definition of the HZ and its large radius of 2.2 R_E makes it unlikely to be a rocky planet. As with Kepler 22b, Kepler 61b is very unlikely to be a potentially habitable planet and more likely to be a mini-Neptune.

Kepler 62e: R_P=1.6 R_E, S_eff=1.10
Kepler 62f: R_P=1.4 R_E, S_eff=0.39

After reading one disappointing review after another so far, the reader might begin to think there are no potentially habitable planets currently known. Fortunately, there is the multi-planet system of Kepler 62. Kepler 62e appears to orbit just beyond the inner edge of this star’s HZ and with a radius of 1.6 R_E, it has about even odds of actually being a rocky planet. Taking into account the uncertainty of the actual inner limit of the HZ, it seems that Kepler 62e is a fair candidate for being a potentially habitable planet.

The situation for Kepler 62f appears even better. With a radius of 1.4 R_E, which is comfortably below the 1.6 R_E dividing line between Earth-like and Neptune-like planets, there is a good chance that Kepler 62f is a rocky planet. Combined with its effective stellar flux that places it in the outer part of even a conservatively defined HZ, it appears that Kepler 62f is among the better potentially habitable planet candidates currently known.

Kepler 174d: R_P=2.2 R_E, S_eff=0.43

Like so many other planets initially considered to be potentially habitable by some, Kepler 174d seems to orbit well inside the HZ but it appears to be too large to be a rocky planet. With a radius of 2.2 R_E, it is much more likely that Kepler 174d is a volatile-rich mini-Neptune with poor prospects of being potentially habitable.

Kepler 186f: R_P=1.2 R_E, S_eff=0.29

When its discovery was announced last year, Kepler 186f generated much attention because of its Earth-like size and its orbit inside the HZ of its red dwarf sun. Recently published refinements of its properties by a team led by Guillermo Torres (Harvard-Smithsonian Center for Astrophysics) in the same paper where they just announced the discovery of eight new habitable zone planets has only reinforced the case for the potential habitability of Kepler 186f. Its effective stellar flux places it towards the outer edge of the HZ of this system. Its radius of 1.2 R_E, which is comfortably below the 1.6 R_E limit that divides Earth-like planets from Neptune-like planets, makes it probable that it is a rocky planet. So long as no major impediments to habitability of planets orbiting red dwarfs are revealed, Kepler 186f is one of the best habitable planet candidates currently known. For a more detailed discussion of this world, see Habitable Planet Reality Check: Kepler 186f.

Kepler 283c: R_P=1.8 R_E, S_eff=0.90

Kepler 283c is one of those planets that is frustratingly close to being potentially habitable but just doesn’t quite make it. The effective stellar flux of Kepler 283c places it near the inner edge of its sun’s HZ. But with a radius measured to be 1.8 R_E, it is more likely to have a volatile-rich instead of rocky composition. Kepler 283c only has a fair chance at being potentially habitable.

Kepler 296e: R_P=1.5 R_E, S_eff=1.22
Kepler 296f: R_P=1.8 R_E, S_eff=0.34

When the discovery of planets in this system was first announced in 2014, Kepler 296f was considered by some to be a good habitable planet candidate. But follow-up observations of this star soon revealed that instead of it being a single star, it consisted of a pair of red dwarf stars instead that appear blended together as viewed by Kepler. As a result, the properties of its planets which had been derived assuming a single star were no longer valid. Additional work by Torres et al. has been able to resolve this issue and they derived properties that would make both Kepler 296f and e habitable planet candidates.

A closer look at this work, however, casts some doubt on this assessment. With a radius of 1.5 R_E, which is smaller than the 1.6 R_E size limit for rocky planets, Torres et al. calculated that there is 50.7% probability of Kepler 296e being a rocky planet. While they calculated a high probability that Kepler 296e orbits inside the HZ, they were using a very optimistic definition of the HZ that placed the inner edge of the HZ where the effective stellar flux was 50% higher than Venus experiences today. Given the uncertainties in this world’s derived orbital properties, I estimate that there is only one chance in four that it actually orbits inside the HZ as it is more conservatively defined. Unless the predictions of models of a more optimistic definition of the inner limit of the HZ are borne out, it seems more likely that Kepler 296e is a larger but cooler version of Venus and is only a fair habitable planet candidate.

The situation for the more distantly orbiting Kepler 296f is a bit more promising in some ways. The effective stellar flux for this planet places it comfortably inside the outer part of its sun’s HZ. However, with a radius of 1.8 R_E, Torres et al. estimate that there is only a 30.6% probability that Kepler 296f is a rocky world. Because of this, Kepler 296f is only a fair potentially habitable planet candidate

Kepler 298d: R_P=2.5 R_E, S_eff=1.29

This world’s high effective stellar flux places it well outside the conservative definition of the HZ. But even if more optimistic limits prove to be true, its large radius of 2.5 R_E makes it much more likely that it is a mini-Neptune. As a result, Kepler 298d has a very low probability of being a potentially habitable planet.

Kepler 438b: R_P=1.1 R_E, S_eff=1.38

Kepler 438b is one of the eight extrasolar planets recently announced by Torres et al. as orbiting inside the HZ. While they estimate that there is a very high 69.6% probability of being a rocky planet owing to its small 1.1 R_E radius, their assessment of the potential habitability of this world is based on the very optimistic definition of the HZ they adopted in their paper that would comfortably include Venus in our own solar system (which is most definitely not a habitable planet). Assuming a more conservative definition of the HZ limits and taking into account the large uncertainties in its properties, I roughly estimate that there is only one chance in four that Kepler 438b actually orbits inside the HZ. Given this, it appears that Kepler 438b is a poor candidate for being potentially habitable and is more likely to be a slightly larger and cooler version of Venus than an Earth-like planet.

Kepler 440b: R_P=1.9 R_E, S_eff=1.43

Another one of the new discoveries announced by Torres et al. is Kepler 440b. Given its rather large radius of 1.9 R_E, Torres et al. estimate that there is only a 29.8% probability that Kepler 440b is a rocky planet making it more likely to be a mini-Neptune instead. While they calculate a high probability that this planet orbits inside the optimistic definition of the HZ they used, I estimate that there is less than even odds of this planet orbiting inside the HZ as it is more conservatively defined. Taken together, it appears that Kepler 440b is a poor candidate for being a potentially habitable planet.

Kepler 442b: R_P=1.3 R_E, S_eff=0.70

By far, the most promising candidate for a potentially habitable planet recently announced by Torres et al. is Kepler 442b. The sun of this system, Kepler 442, is a relatively young K-dwarf star about 1,100 light years away with 61% of the mass of the Sun and 12% of its luminosity. With a radius of 1.3 R_E, Kepler 442b is estimated by Torres et al. to have a 60.7% probability of being a rocky planet. Even assuming a conservative definition for the outer limit of the HZ, this world seems to have a very high probability of orbiting comfortably inside this zone. When all the current observations are considered, it appears that Kepler 442b is one of the best candidates found to date for being a potentially habitable planet.

Kepler 443b: R_P=2.3 R_E, S_eff=0.89

Kepler 443b was the last of the eight newly confirmed planets announced by Torres et al. that appear in the “Habitable Exoplanet Catalog”. While its effective stellar flux is certainly in a range that places it inside the HZ with a high probability no matter how it is defined, it seems to be too large to be potentially habitable. With a radius of 2.3 R_E, Torres et al. calculate that there is only a 4.9% probability of Kepler 443b being a rocky planet. Since it is much more probable to be a mini-Neptune, Kepler 443b is unlikely to be a potentially habitable planet.

KOI 4427b: R_P=1.8 R_E, S_eff=0.24

One of the planets studied by Torres et al. that still remains unconfirmed is a planet currently designated KOI 4427.01. Although its detection has a 99.16% confidence level, it did not quite meet the 3-sigma detection threshold set by Torres et al. but it still seems significant enough to likely be a bona fide planet. Based on the radius of 1.8 R_E, Torres et al. estimate that there is only a 27.3% probability that this is a rocky world. Combined with less than even odds of this world orbiting inside the conservatively defined outer limit of the HZ, KOI 4427b appears to be a poor candidate for being a potentially habitable planet.

Tau Ceti e: M_Psini=4.3 M_E, S_eff=1.51

The Sun-like star Tau Ceti has generated much interest over the decades among scientists looking for habitable planets. Unfortunately its relatively high level of activity has complicated efforts to find verifiable planets orbiting this star using precision radial velocity measurements. Despite the outstanding issues, one of the purported planets of Tau Ceti announced two year ago has been claimed by some to be potentially habitable. Tau Ceti e was discovered using precision radial velocity measurements but remains unconfirmed. Ignoring this issue for the moment, the analysis of the available data yields a minimum mass of 4.3 M_E which appears to be near the lower end of the mass range where rocky planets transition to volatile-rich planets. Factoring in the unconstrained inclination of this planet’s orbit, there is a two in three chance that its mass exceeds the 6 M_E threshold mass making it more likely to be a mini-Neptune. Its effective stellar flux also exceeds by a fair margin that for the conservative definition of the HZ. Taking all this information together, it seems that Tau Ceti e is more likely to be a hot mini-Neptune than a potentially habitable planet. These facts along with the questionable existence of this world make Tau Ceti e to be a very poor habitable planet candidate.

Summary

Unfortunately, an objective assessment of the known properties of the planets in the Planetary Habitability Laboratory’s “Habitable Exoplanets Catalog” casts grave doubts about the potential habitability of the majority of the planets on this list. Most of them are likely too large to be habitable Earth-like planets and are much more likely to be mini-Neptunes or even larger volatile-rich planets with very poor prospects of being habitable. In all fairness, this fact has only just become appreciated by the scientific community over this past year based on analyses like those conducted by Rogers. As a result of this, Torres et al. actually calculated the probability that their new finds were rocky planets and gave refreshingly honest assessments of their finds’ prospects in their recent discovery paper. Hopefully we will see more of this welcome practice in the future.

In three cases, it appears that the potentially habitable planets do not exist. Especially in the case of GJ 667C, the radial velocity variations that had been interpreted as being the result of orbiting planets now appear to be “false positives” caused by previously unrecognized and very subtle forms of stellar activity modulated by the star’s rotation. The unconfirmed planets believed to orbit Tau Ceti are also strongly suspected to be false positives at this time.

Among the 28 planets in the “Habitable Exoplanets Catalog”, only three appear to be genuinely good candidates for being potentially habitable: Kepler 62f, Kepler 186f and Kepler 442b. Fair candidates worthy of further consideration include Kepler 62e, Kepler 283e, Kepler 296e and f as well as Kepler 438b. In the case of these latter five worlds, they might be too large or too hot to be potentially habitable. Further observations and theoretical work on planetary habitability should help resolve their status.

Unfortunately, all of these promising candidates for potentially habitable planets orbit dim K- and M-dwarf stars that present possible issues with their habitability such as synchronous rotation, stellar flare activity and high luminosity early in their diminutive suns’ lives to name just a few. But all hope for finding better candidates is certainly not lost. Besides the likely prospects of finding more habitable planet candidates orbiting dimmer stars, the continued analysis of Kepler data is sure to uncover more Earth-like planets orbiting in the HZ of Sun-like stars as well. In addition to the recently announced discovery by Torres et al. of eight planet HZ planets, there was the much quieter announcement of two Kepler planet candidates found in the HZ of two Sun-like stars (see Earth Twins on the Horizon?). While these and similar finds still require follow-up observations to confirm their planetary nature, they provide a foretaste of the bona fide Earth-like habitable planets yet to come.

General References

Ian J.M. Crossfield et al., “A Nearby M Star with Three Transiting Super-Earths Discovered by K2”, arVix 1501.03798 (submitted for publication in The Astrophysical Journal), January 15, 2015 (preprint).

Courtney D. Dressing et al., “The Mass of Kepler-93b and the Composition of Terrestrial Planets”, arVix 1412.8687 (accepted for publication in The Astrophysical Journal), December 30, 2014 (preprint).

James F. Kasting, Ravi K. Kopparapu et al., “Remote life-detection criteria, habitable zone boundaries, and the frequency of Earth-like planets around M and late K stars”, Proceedings of the National Academy of Sciences, Vol. 111, No. 35, pp. 12641-12646, September 2, 2014 (full text).

R. K. Kopparapu et al., “Habitable zones around main-sequence stars: new estimates”, The Astrophysical Journal, Vol. 765, No. 2, Article ID. 131, March 10, 2013 (full text).

Ravi Kumar Kopparapu et al., “Habitable zones around main-sequence stars: dependence on planetary mass”, The Astrophysical Journal Letters, Vol. 787, No. 2, Article ID. L29, June 1, 2014 (preprint).

Valeri V. Makarov and Ciprian Berghea, “Dynamical evolution and spin-orbit resonances of potentially habitable exoplanet. The Case of GJ 667C”, The Astrophysical Journal, Vol. 780, No. 2, article id. 124, January 2014 (preprint).

Paul Robertson and Suvrath Mahadevan, “Disentangling Planets and Stellar Activity for Gliese 667C”, The Astrophysical Journal Letters, Vol. 793, Article ID. L24, October 1, 2014 (preprint).

Leslie A. Rogers, “Most 1.6 Earth-Radius Planets are not Rocky”, arVix 1407.4457 (submitted to The Astrophysical Journal), July 16, 2014 (preprint).

Guillermo Torres et al., “Validation of Twelve Small Kepler Transiting Planets in the Habitable Zone”, arVix 1501.01101 (submitted to The Astrophysical Journal), January 6, 2015 (preprint).

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