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 Seff 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 Seff 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 Seff of about 1.11. The Seff 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 Seff 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 Seff 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 MP) derived from precision radial velocity measurements or its radius (or RP) calculated from observations of planetary transits. In the case of the radial velocity measurements, we actually only know the planet’s MPsini 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 RE) 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 ME). 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: RP=1.5 RE, Seff=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 RE, 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: MPsini=7.3 ME, Seff=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: MPsini=8.3 ME, Seff=1.23
GJ 180c: MPsini=6.4 ME, Seff=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 ME and 6.4 ME 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: MPsini=9.9 ME, Seff=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 ME 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: MPsini=3.8 ME, Seff=0.88
GJ 667Ce: MPsini=2.7 ME, Seff=0.30
GJ 667Cf: MPsini=2.7 ME, Seff=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 Seff 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 ME 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 ME 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: MPsini=8.7 ME, Seff=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 ME 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: MPsini=5.4 ME, Seff=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 ME minimum mass of GJ 832c gives it a 90% probability of exceeding the 6 ME 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: MPsini=8.6 ME, Seff=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 ME which already exceeds the 6 ME 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: MPsini=7.1 ME, Seff=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 ME derived from radial velocity measurements and an unconstrained inclination, it is unlikely that HD 40307g is a potentially habitable planet.
Kapteyn b: MPsini=4.8 ME, Seff=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 ME. With an unconstrained orbit inclination, there is an 80% probability that its actual mass exceeds 6 ME 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: RP=2.4 RE, Seff=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 RE, which easily exceeds the 1.6 RE 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: RP=2.2 RE, Seff=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 RE 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: RP=1.6 RE, Seff=1.10
Kepler 62f: RP=1.4 RE, Seff=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 RE, 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 RE, which is comfortably below the 1.6 RE 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: RP=2.2 RE, Seff=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 RE, it is much more likely that Kepler 174d is a volatile-rich mini-Neptune with poor prospects of being potentially habitable.
Kepler 186f: RP=1.2 RE, Seff=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 RE, which is comfortably below the 1.6 RE 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: RP=1.8 RE, Seff=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 RE, 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: RP=1.5 RE, Seff=1.22
Kepler 296f: RP=1.8 RE, Seff=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 RE, which is smaller than the 1.6 RE 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 RE, 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: RP=2.5 RE, Seff=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 RE 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: RP=1.1 RE, Seff=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 RE 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: RP=1.9 RE, Seff=1.43
Another one of the new discoveries announced by Torres et al. is Kepler 440b. Given its rather large radius of 1.9 RE, 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: RP=1.3 RE, Seff=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 RE, 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: RP=2.3 RE, Seff=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 RE, 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: RP=1.8 RE, Seff=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 RE, 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: MPsini=4.3 ME, Seff=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 ME 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 ME 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).
Robert A. Wittenmyer et al., “GJ 832c: A super-Earth in the habitable zone”, The Astrophysical Journal, Vol. 791, No. 2, Article id. 114, August 2014 (preprint).
Thank you Andrew LePage…
GJ677Cc, e and f are out of the race…
Kepler 61f, 186f, and 442b are in…although I don’t care for the 1,100 ly distance for 442b…I believe the most advanced Star Trek science is 2,000 years in the future…but I keep quiet among my friends about that debate…
The latest issue of Scientific American sets forth hopes for red dwarf earth-like planets existing…Humanity may one day have to accept living on Mars like worlds if we’re going to become long distance runners in the civilization process…
Which brings me back to the concept of Arthur C. Clarke’s billion year old city named Diaspar. Those citizens avoid excessive radiation dangers like those found on Mars like worlds…Diaspar on Mars would work well too…But many generations born on Mars would find human physiology altering quite a bit…
Such is life…
Among its other problems, HD 40307g seems to also belong on the “probably doesn’t exist” list, according to this presentation given last year. Not sure if a paper covering the updated HD 40307 dataset has been published yet.
I wanted to thank Paul Gilster for giving me an opportunity to discuss this important topic with Centauri Dreams readers. Since I submitted this essay two weeks ago, I have performed a more in depth analysis of the potential habitability of EPIC 201367065 which can be found here:
http://www.drewexmachina.com/2015/01/20/habitable-planet-reality-check-keplers-new-k2-finds/
During the past couple of weeks, the first initial analysis of the full data set from Kepler’s primary mission was also submitted for publication. This analysis suggests that the Kepler’s final extrasolar planet tally could reach 20,000 (!!!) after follow-up observations confirm the planetary nature of these finds. Among the new planet candidates are some nearly-Earth-size planets orbiting inside the habitable zone of Sun-like stars that might eventually be added to the growing list of potentially habitable planets.
http://www.drewexmachina.com/2015/01/26/first-look-at-keplers-complete-primary-mission-data-set/
Excellent post, Drew. I have two questions:
1. When you say that a planet beyond the 1.5 Earth radius size limit has a “4.9% chance of being rocky”, do you mean that there’s a small chance that it’s smaller than the estimated radius, or that it’s rocky despite being that large?
2. I thought the flaring issue was mostly with the smaller M-dwarf stars. Is it a big problem with the K-stars as well?
I’m unclear on whether it is possible (with other current instruments) to confirm/deny the existence of all the potential planets identified by Kepler. Is there some number of possible planets for which the only available data is from Kepler, and we’ll just have to wait for additional missions or technological advances to get more information?
Planets that are too large, but in the habitable zone, could also have smaller moons that might be habitable. Although we haven’t found one extrasolar moon yet, there are indications even in our own solar system that they might be quite common, especially among gas giants. Just pointing out habitability doesn’t have to be limited to planets and the numbers of such habitable worlds could be a lot larger than the number of planets being considered. We’ll have to wait and see on the moon issue, but we are fairly close to finding the first, and with the next generation more powerful telescopes, we may find that the number moons in and out of the habitable zone outnumber the planets.
My point above is, when looking for habitability, the gas giants shouldn’t just be ignored, they may be more interesting targets to look at than we might think.
@Brett January 30, 2015 at 13:59
I’m glad you liked the essay. To address your questions:
1) More specifically, what I said was “with a radius of 2.3 RE, Torres et al. calculate that there is only a 4.9% probability of Kepler 443b being a rocky planet.”. This probability that Torres et al. calculated using their model of the planetary mass-radius relationship gives a 4.9% probability that the planet is a rocky planet given the radius of Kepler 443b (as well as its associated measurement uncertainty). This same model predicts ~50% chance of a 1.5 RE planet being rocky. This mass-radius relationship has its own statistical uncertainties associated with it because of the measurement errors contained in the data used to derive it. The work done by Rogers (which is slightly different than model used by Torres et al. but comparable) suggests that a step-wise transition at 1.5 RE is mildly favored over a more gradual one like that used by Torres et al.. *IF* that proves to be the case, then there is a 0% chance that Kepler 443b is a rocky world. More data will be needed to define the mass-radius relationship more accurately. I discuss Rogers’ work in more detail and its statistical uncertainties in the following:
http://www.drewexmachina.com/2014/07/24/habitable-planet-reality-check-terrestrial-planet-size-limit/
2) Yes, it is true that flaring is potentially a larger issue for planetary habitability the smaller and dimmer the star is. Small M-dwarf stars would have a larger issue with this than larger K-type stars but exactly where that transition is is still TBD.
Thanks for your questions!
@NS January 30, 2015 at 14:58
There are a number of ways the Kepler science team have used to confirm the planetary nature of their planet candidates (a subject that is worthy of an essay in its own right!). But, briefly, some of those finds can be independently confirmed using precision radial velocity (RV) measurements. However, some of the smaller finds are simply too small to generate measurable RV variations. In those cases, a number of statistical methods have been developed to confirm the existence of these worlds. For example, the presence of multiple transit event signatures can frequently only be explained by the presence of multiple coplanar planets. No conceivable form of stellar activity or other phenomena could reproduce the observed signature. The observation of transit timing variations indicating gravitationally interacting planets only further supports the planetary interpretation for Kepler’s photometric data.
@andrew. Given this analysis, what is your best estimate of potentially habitable planets, the n sub e term in the Drake equation?
Okay, so there planets that are candidates for life. So how about we, as a species, take the 1 to 2 trillions dollars we spend each year murdering each other, and build some true space ships? ie. Mega-Orions, as in the kind that we use nuclear explosives to launch. I imagine you can store quite a few large telescopes on board a vessel that has 5-7 million tons of cargo space.
Then, even though those worlds will still be out of reach, we could at least get a better look at them, see if they do bear life, and if so let it provide us with an incentive to reach outward.
Okay, so I like to dream big, and that will never happen. :(
@Ross Turner January 30, 2015 at 15:04
This essay deals with planets known (or strongly suspected by some) of actually existing. As you rightly point out, habitable moons could exist possibly orbiting some of the planets discussed here but no exomoon (habitable or otherwise) has yet to be discovered. I wrote about habitable exomoons (a favorite topic of mine for decades) in Centauri Dreams a few months ago:
https://centauri-dreams.org/?p=31557
When any potentially habitable exomoons are discovered, I’ll be glad to write about them.
@Alex Tolley January 30, 2015 at 16:36
At this point in time, I have no favored general value for n_sub_E since most of the analyses published to date have glaring problems or limitations (e.g. they have included planets with radius values greater than 1.5 RE that are likely to be mini-Neptunes, they include planets far inside the conservative limits of the HZ, they do not include planets smaller than the Earth but are still likely to be habitable or any planets with effective stellar fluxes found in the outer HZ, etc.).
That being said, there was an excellent paper recently submitted for publication by Courtney Dressing and David Charbonneau (Harvard-Smithsonian Center for Astrophysics) that deals with the occurrence of potentially habitable planets orbiting red dwarf stars. They provided statistics (and associated uncertainties) for occurrence rates for a range of planet sizes, orbital periods and effective stellar flux values. They found that the occurrence rates of planets with radii in the 1.0 to 1.5 RE range in a conservatively defined HZ which I and others favor was 18% (+18%/-7%). For planets in the 0.5 to 1.4 RE range (which encompasses most of the size range for likely habitable planets), the occurrence rate is 29% with the larger uncertainty range of (+25%/-12%) reflecting the poorer statistics available for smaller planets. A full discussion of this paper with references can be found here:
http://www.drewexmachina.com/2015/01/12/occurrence-of-potentially-habitable-planets-around-red-dwarfs/
I am certain that a more general value of n_sub_E, which is likely to be similar for the red dwarf values, will be forthcoming in a future analysis of Kepler data.
Some other issues that should be mentioned are the volatile loss from the enhanced EUV flux of M & late K dwarfs relative to Sol like stars, and the enhanced hydrogen retention of planets massing more than 1.5 Earths. These will counteract each other to some extent.
A final point is that a multi-bar CO2 atmosphere isn’t “habitable” for Earth-like lifeforms. The “Earth-like Zone” is significantly narrower than the Habitable Zone.
As I understand it, even without the flares the insolation HZ of a red dwarf would still be a very hostile environment in terms of EUV flux (the MUSCLES survey found that even red dwarfs which do not appear to suffer from optical flares are still active at ultraviolet wavelengths) and stellar wind, which would subject the planets to rapid atmospheric erosion. The long pre-main sequence evolution does not help matters either.
There is also the problem of increased space rocks velocities and therefore energy of impacts as we get closer to Red dwarfs or low heat emission bodies. We have to get closer to get warm but we also get deeper into their gravity wells at a higher rate. Red dwarfs may be more numerous than other types of stars but they have many drawbacks for the emergence of life, they would be great places to live for advanced space faring species though.
The wikipedia article brought up the “sunspot” issue with red dwarf stars as well. It’s not good for your planet’s habitability if your star suddenly drops 40% in luminosity for a while because of sun spots.
Thank you Andrew for a very comprehensive review of the data . Such as it is. Getting more of it and relying less on computer simulations is a priority. Obviously TESS and especially TESS in combination with JWST NIRSpec will be a very potent addition to that data base. I have been fortunate enough to correspond with both Professor Kasting and Ravi Kopparapu who are both very approachable and modest individuals apart from being brilliant scientists. They tell me that even JWST will not be able to constraint the atmospheres of Earth like planets , in or out of the HBZ. What they will do though is tell us a lot more about larger planets and hopefully that point , if indeed it is a point at which planets be one Neptune like. So the 2017/2018 period is obviously a date for our calendars. Another critical date is Feb 10 this year when the NASA Exo-PAG ( Exoplanet programme analysis group ) , the subcommittee tasked with exoplanet policy , meets to discuss their contribution to the next Decadel survey. I’m told it is likely exoplanets will feature centrally and it is possible that when NASA assigns its Decadel Science definition team later this year that Professor Kasting is central to that. He was involved in the sadly defunct TPF concept too but it is likely that this will be reborn ( under a new name of course !) in the next Decadel. There will thus be a push for a large Earth finding telescope in the early 2030s. There have been many breakthroughs with telescope design and material science that combined with flagship funds will make for a great telescope and with the development of Falcon Heaavy and SLS future telescope will be serviceable and long lived too. ( there is a very informative white paper on telescope servicing authored by Marc Postman in 2010 ! Co authored with some heavyweight NASA administrators like John Grunsfeld. It gives clear insight into future plans this way including JWST and surprisingly , possibly Hubble too) .Sadly one big mission per decade for NASA these days and JWST and WFIRST have taken the next two decades. Both at least will make significant contributions to exoplanet science .There may be room for a Discovery class transit spectroscopy mission meantime and many exoplanet astronomers will push for this but it won’t be big enough to analyse Earth like atmospheres unfortunately. The Exo-PAG committee has public meetings twice yearly that are open to registration and audience participation via the NASA website . The next one is in Chicago in June and should be interesting as exoplanets move to centre stage again. I would encourage everyone to register on line and sign up for the Exo-PAG newsletter. All this helps as it draws attention to exoplanets and increases the likelihood of future missions. Prof Kasting et Al will be there so will be very informative. It is possible to sign up for all these meetings via webinar too , even the Feb crucial policy decider. I know that for all their over optimism , the exoplanet astronomy community welcome media interest/support as it is central to getting Congress attention for funding.
Flaring has even been witnessed in sun like stars. It’s basically a big coronal mass ejection as can happen in any star .Although it is generally more common in smaller , rapidly rotating and young stars. I don’t think the theories as to its origin have been perfected yet. It is believed to be related to charged particles from star’s getting trapped inside their magnetic fields, building up until like a cork popping out of a bottle they burst out. Rapidly rotating stars tend to knit up their magnetic fields like a twisted tourniquet and thus increase the likelihood of this entrapment and subsequent release. The general idea is that the star’s magnetic field is tethered to the base if it’s outer convective zone . However stars smaller than M3 class are entirely convective with no base and yet tend to be some of the most like to flare. Alpha Centauri B, a K1/2 star has been observed to flare . In general though , larger and older stars rotate slower and flare less and less often.
Perhaps a little out of context, but you should check this site out:
http://stars.chromeexperiments.com/
A very nice interactive star map. My 6 year old loves it, he was only disappointed he couldn’t find his favorite star, COROT-7 on it. Probably in there, just hard to find.
Andrew: Rumer has it that the best KOI (4878.01) may be confermed as early as this summer! KOI4878 is estimated to be 97% as massive as the sun, but with a slightly larger radius (1.06 solar). This suggests to me that this star is conciderably older than the sun, almost ready to enter its subgiant phase. It is also somewhat hotter than the sun, so the 449 day orbital period of KOI 4878.01 would put it in the same part of the habitable zone as Earth, but, global warming on this planet would have been going on for megayears, instead of decades. What is your take on how this would affect the true habitability of this planet, should it BE confermed?
@Harry R Ray January 31, 2015 at 16:42
There are literally thousands of KOIs currently subject to follow-up observations with dozens of those having the possibility of making it onto the “potentially habitable” list… with still more being added it seems every day. As a result, it is next to impossible for me to keep up with all of them and I typically wait for KOIs to be confirmed with a discovery paper in hand before I start digging into their potential habitability (with the exception of a couple of recent essays I have written on my web site with the intent to illustrate that better habitable planet candidates are on their way – these are cited above at the end of this essay and in my first comment).
That being said, I am not surprised to hear that a confirmation of KOI 4878.01 is on the horizon. However, I would be VERY careful not to over-interpret the raw numbers from the Kepler target list for a couple of reasons. First of all, any KOI that has not received dedicated follow-up observations can have hefty uncertainties associated with their properties. For example, I checked the Kepler target list data base for the Q1 – Q17 cumulative data for KOI 4878 and it has a listed mass of 0.97 (+0.14/-0.11) M_Sun, a radius of 1.068 (+0.389/-0.145) R_Sun and an effective temperature of 6031 (+143/-168) K. Based on these parameters, I calculate that KOI 4878 has a luminosity of 1.35 L_Sun with a VERY roughly estimated uncertainty of (+0.73/-0.40) L_Sun (this is assuming the uncertainties in the parameters are independent which is probably not strictly true). KOI 4878 might be an older version of the Sun as you state but, given the large uncertainties, it could also easily be a younger, hotter and more massive version of the Sun instead. Another issue with the stellar property data in the Kepler target list is that after more detailed follow-up observations, the listed properties can be found to be off by more than the listed uncertainties. So like I said, I would be VERY careful not to over-interpret these data until a discovery paper becomes available with a full analysis of the follow-up observations. And these analyses typically include good estimates of the star’s age as well (especially for non-red dwarf stars)
That being said and keeping in mind the uncertainties, KOI 4878.01 certainly has some promise of being a potentially habitable planet. According to the Kepler data base, it has a radius of 1.04 (+0.38/-0.14) R_E so it would seem to have a high likelihood of being a rocky planet. With an orbital radius of 1.137 AU (with no uncertainty listed) it would seem to have an effective stellar flux of ~1.05 times that of the Earth which would place it near the inner edge of the conservatively defined HZ (although it might be just inside the inner limit of the HZ or just outside of it, given the uncertainties). On the surface, it would seem that KOI 4878.01 shows definite promise of being a Earth-size planet in an Earth-like orbit around a Sun-like star. But I would like to wait for the discovery paper before I’d place any bets on it. Still, like several similar KOIs I have recently written about, this planet candidate hints at the very promising discoveries that should be announced in the months to come.
In this case I really have to sound like an idiot, but WHY spend time, energy and money on looking at suns and planets hundreds or thousand light years away when we must have so many interesting suns with planets much closer? What happened to the Alpha Centauri system?
Did it just have big gas-planets? Then why arent we looking for large moons there?
Daniel Högberg writes:
It’s not an idiotic question at all. Here’s the answer: Most of the stars in the Kepler field of view are a long way out, but doing Kepler was crucial for developing a statistical model of how many stars have planets, and telling us something about those planets. Now we’re moving toward future missions that will focus in on stars closer to ours.
Alpha Centauri is a fascinating system, but neither A nor B are likely to have gas giants. We would have spotted them by now. The focus is therefore on smaller worlds out to about 2.5 AU or so, where conditions seem possible for rocky worlds to form. We don’t know if they’re there yet, but the effort to find and confirm planetary discoveries around Centauri A and B (and, for that matter, Proxima) is ongoing. Alpha Centauri Bb is still not confirmed, but if it’s real, it’s a small world orbiting close to its star. We’ll learn more about the Centauri stars as this work continues.
@Daniel Högberg February 1, 2015 at 16:53
Just to add to Paul’s response, astronomers *ARE* looking for planets orbiting Alpha Centauri. I wrote a pair of essays about the details of the current searches performed to date that appeared on my web site last year:
http://www.drewexmachina.com/2014/08/11/the-search-for-planets-around-alpha-centauri/
http://www.drewexmachina.com/2014/09/25/the-search-for-planets-around-alpha-centauri-ii/
I also wrote an essay for Centauri Dreams last October about the efforts to confirm the existence of Alpha Centauri Bb whose discovery was announced a couple of years ago:
https://centauri-dreams.org/?p=31730
In addition to Alpha Centauri, virtually every single star within a couple of dozen parsecs of the Sun and virtually every Sun-like out to several dozen parsecs has been or is currently the subject of surveys looking for extrasolar planets. And with Kepler’s K2 mission, TESS and Gaia, the list of stars being search for planets by any means our technology allows is easily exceeding several hundreds of thousands. No matter how distant an extrasolar planet may be, its discovery adds to our knowledge.
Although these mini Neptunes might not be rocky, could they be candidates for the “Jupiter-style” biosphere envisioned by Sagan back in the ’70’s?
Regarding Alpha Centauri, there is a proposed telescope that would be solely designed to look at the Alpha Centauri AB system: http://blogs.scientificamerican.com/observations/2015/01/27/planet-hunters-bet-big-on-a-small-telescope-to-image-alien-earths/
I hope it gets funded!
@ijv February 2, 2015 at 0:18
When looking for the unknown, it is best to start with what we do know. With this principle in mind, if we wish to find extraterrestrial life, the best bet is to start looking at worlds that appear to be similar to the Earth where we know life exists. While it is certainly possible for life to exist on gas giants like Carl Sagan had speculated back in the 1970s, it is such an extreme extrapolation from what we currently know about life that there is no way to predict what combination of planetary properties would be required to support such a biosphere with any accuracy. As a result, we have no way of knowing where to begin to look for such lifeforms. Until we know a lot more about life on other planets and what maintains their environments, our best bet for finding extraterrestrial life is to be searching for Earth-like habitable planets.
ijv said on February 2, 2015 at 0:18:
“Although these mini Neptunes might not be rocky, could they be candidates for the “Jupiter-style” biosphere envisioned by Sagan back in the ’70’s?”
Such as this:
https://centauri-dreams.org/?p=6308
And this:
https://www.youtube.com/watch?v=uakLB7Eni2E
I Look at that chart and I see a rarity of planets of ANY size at
the habitable zone for F,G,K type stars. Now I know that sensitivity issues
prevented Kepler from spotting smaller worlds, but isn’t any one surprised
that we have not found but ONE, K22b, which is marginally inside HZ of these more SUN like stars. I know that this represents only about 1.5%
of the total planets of any type (aligned so kepler can spot them). On the average that means we can expect to find about 65-70 planets in the HZ of
F,G,K stars. for the entire sample of stars Kepler tracked.
Assuming there are no natural constrains in the size of Re .5 – Re 2.5 planets and with an even distribution, then about 18 planets exist in the
range of size between .8 RE to 1.2 RE. Assuming Kepler’s spotting range
is around 2,000 Ly, Then we still have result that on Average the
distance to the nearest of these .8RE-1.2Re terrestrials is 110 LY
I think it is possible to create formula for the odds that a terrestrial world exists at what ever distance you choose from Earth, in LY, assuming
the simplifications above, but my probability & Stats education have been forgotten by now.
@RobFlores February 2, 2015 at 12:33
I am not sure which “chart” you are referring to but one has to be *VERY* careful about drawing any conclusions from a simple visual inspection of any chart showing the locations and sizes of currently known extrasolar planets. I am pretty sure we have had this discussion in the comments of one of my earlier Centauri Dreams essays but it bears repeating: the radial velocity and transit methods of searching for extrasolar planets (the latter of which is employed by Kepler) have VERY strong selection biases favoring finding large planets in small orbits. Detailed statistical analysis which takes these selection biases into account is the only way to draw any conclusions about the occurrence rates of planets as a function of size and orbital radius. Attempting to draw any conclusions without this sort of painstaking analysis and relying solely on a simple visual inspection of the data is guaranteed to generate incorrect conclusions.
In addition to this, Kepler’s primary mission only lasted for four years and requires observing three transits for a planet to be “detected”. That means that only planets with orbital periods less than ~16 months can be detected by Kepler which corresponds to a maximum orbital radius of only ~1.2 AU for a Sun-like star. This barely covers the inner part of the HZ for a Sun-like stars which can extend out to ~1.7 AU.
As for your estimates for the number of planets you expect to find in the HZ of FGK stars, they bear little resemblances to the figures I have seen published in peer-reviewed papers on the subject. A recent review of the occurrence rate of “Earth analogs” (which are defined as a planet with a radius between 1 and 2 times that of the Earth orbiting a G-type dwarf star with a period of 200 to 400 days – a definition that includes planets that are not necessarily habitable) by Daniel Foreman-Mackey, David W. Hogg and Timothy D. Morton predicts that only 9.2 (+5.9/-4.0) such planets can be expected to be found in the Kepler data set (whose analysis is still ongoing). An earlier paper by Erik A. Petigura, Andrew W. Howard and Geoffrey W. Marcy implies that maybe three times that number may eventually be found. A thorough discussion of these papers can be found in the following essay:
http://www.drewexmachina.com/2014/06/25/abundance-of-earth-analogs/
This being said, there are on the order of several dozen Kepler planet candidates of various sizes that appear to be in or near the HZ of Sun-like stars which are currently the subject of follow-up observations to confirm their planetary nature and properties. I would suggest that you take a deep breath and just wait for the results of the continuing processing of Kepler’s huge data set and the inevitable publication of more detailed statistical analyses of the occurrence rate of planets as a function of size and orbital radius before getting too concerned about the perception of a lack of planets in the HZ of FGK stars.
Thing is you’re only able to find the planets that you’re able to find. This is obviously a tautology but it is pretty much at the heart of why astronomers end up looking for planets outside our immediate neighbourhood.
For example the transit method used by the Kepler mission can only find systems that are precisely aligned with the line-of-sight. The overwhelming majority of planets will not be aligned so conveniently so you have to take a large number of stars to have a chance of finding any. Other detection methods have their own biases, but the effect is the same: to find the relatively rare systems where we can find planets, you need to survey a large sample of stars to have a good chance of finding a planet.
So it’s not that no-one’s looked, it’s just that the planets that have the right properties to be detectable (which for a long time essentially limited the detections to massive gas giants in the inner part of their solar systems) turned out not to exist around the nearby stars, so astronomers had to look further afield.
Then you have a geometric bias which comes into play: the further out you go, the more volume there is for a given distance interval. For example, nearly half the volume of a sphere is contained within the outer fifth of its radius. There is 271 times as much space between 90 and 100 light years away as there is between 0 and 10 light years away. So the majority of the planets found are likely to be far away.
Your are of course correct Mr LePage everything will change when the
final stats for Kepler are analyzed.
I was referring to the Stellar Flux and Temperature chart and restricting to FGK stars, the chart shows One Planet in the HZ of that category of stars
One has to factor in the whole 360* field of view to calculate odds of finding a closer Earth Analogue. Kepler is only looking at 10.5 x 10.5 degrees of the sky. Even if we restrict ourselves to areas near the galactic disk. So how many FGK stars are there ]within 100 LY, that is the question.
@RobFlores February 2, 2015 at 16:50
Now that I understand which chart you are talking about, I would still recommend not reading too much into it. First of all, the chart I used in this essay (which was just for illustrating the boundaries of the HZ) is hardly exhaustive since it only includes five of the 29 planets I discussed in this essay. But even if one were to include all the planets discussed, the distribution of planets as a function of effective stellar flux and temperature is going to have a lot more larger planets with higher effective flux orbiting stars with lower temperatures. Kepler, for example, will preferentially find more planets orbiting cooler stars because such stars tend to have smaller radii (which means that they generate more readily detectable transit events for a planet of a given size) and, since their luminosity is lower, have smaller HZs (which increases the probability of a planet in the HZ having its orbit aligned to produce an observable transit) with shorter orbital periods (which produces more transits in a given period of time allowing Kepler to detect them more quickly and average the results of more transits to find smaller planets).
Once the analysis of the Kepler data is much further along, there should be a lot more planets found orbiting in or near the HZ of hotter, more Sun-like stars. But even then, there will be no Kepler planets in this diagram for Sun-like stars (i.e. with a temperature around 5778 K) with effective solar fluxes less than ~0.68 because Kepler did not observe such stars long enough to definitively detect them (as I explained in my earlier post). Detailed statistical analysis of Kepler’s data will then be required to correct the results for observation biases in order to determine occurrence rates as a function of effective stellar flux, temperature, planet size and a range of other factors. Once that is done, astronomers can start making meaningful extrapolations from Kepler’s limited sample to the rest of the stars in the galaxy.
As for your question of how many FGK stars there are within 100 LY, I do not have a number readily available. But based on a quick extrapolation from a couple of databases I glanced at, there is probably something on the order of 2,400 FGK stars within 100 LY.
Andrew LePage February 2, 2015 at 14:34;
“the radial velocity and transit methods (…) have VERY strong selection biases favoring finding large planets in small orbits”.
I have had a question about this for a while, which probably/hopefully you can answer:
Whereas the RV method indeed has very strong selection biases in favor of small orbits AND large planets, I would think that the transit method mainly has a bias in favor of small orbits (the detection chance declining linearly with orbital AU distance) but, however, can detect planets beyond a minimum threshold with more or less comparable chance.
Please correct me if I am wrong here, I would really like to know: how does planet size (beyond the minimum threshold) correlate with detection chance at given orbital distance for the transit method?
PS: thank you for your excellent and honest analysis.
I have two additional questions for Andrew LePage:
1) What is the actual Kepler stellar sample size on which the total tally is based? I read numbers varying from just over 100 thousand to almost 200 thousand.
2) Andrew LePage January 30, 2015 at 13:10: “During the past couple of weeks, the first initial analysis of the full data set from Kepler’s primary mission was also submitted for publication. This analysis suggests that the Kepler’s final extrasolar planet tally could reach 20,000”.
Are the planetary candidate data for this Kepler’s complete primary mission data set available somewhere? On Kepler’s (Caltech) website I could still only find the approx. 7350 that have been there for a while.
@Ronald February 2, 2015 at 19:22
The transit method also has a bias towards large planets because larger planets produce a larger change in the primary star’s brightness (since it covers a larger fraction of the star’s disk) that is easier to detect, all else being equal. All planet detection methods will have this bias.
@Ronald February 2, 2015 at 19:31
Your questions are answered in the review I wrote:
http://www.drewexmachina.com/2015/01/26/first-look-at-keplers-complete-primary-mission-data-set/
But to quickly answer your questions,
1) The total sample size of the Kepler primary mission was 198,675 targets.
2) To the best of my knowledge, the latest planetary candidates found by the analysis of Seader et al. have not been added to Kepler’s on-line database of Kepler Objects of Interest (I am sure that there is a formal review procedure that must be followed before these objects are considered “KOIs” which are then subject to follow-up observations). Here is the citation for their paper (which was also included in my review):
Shawn Seader et al., “Detection of Potential Transits Signal in 17 Quarter of Kepler Mission Data”, arVix 1501.03586, submitted January 15, 2015 http://arxiv.org/abs/1501.03586
@Ronald February 2, 2015 at 19:22
Just another thought on the bias of planet size and orbital radius for the transit method: you must remember that while the change in brightness of the star for a transit of a planet of a given size is independent of the orbital radius (unlike the observed RV variation which depends on orbital radius as well as planet mass), the number of transits during a given observation time increases with decreasing orbital radius. With more transits over which to average out random noise, the detection threshold improves allowing smaller planets to be found at smaller orbital radii.
wrt the planet radius of >1.6 making it less habitable by being more Neptune that Earth like. Does the recent paper speculating that the gaseous envelope of many closer in Neptunes may be stripped by their closeness to the star have any impact on your summations?
Will the Webb telescope be able to get a hold of the stats required to determine whether “envelope stripping” occurs to any great extent on the 1.6-2.5 Earth radius planets around smaller stars?
@Andrew LePage February 3, 2015 at 0:28
Thanks, that noise argument makes a lot of sense, not just with regard to number of transits but indeed also with regard to planet size, smaller planets having a relatively smaller signal/noise ratio.
I am pretty sure Kepler team people are fully aware of it.
I would like to know how great that smaller-planet-greater-noise effect is, and to what extent it is corrected for.
Further to my previous comment:
Or, in other words, I would really like to know to what extent the relative rarity of small terrestrial planets at greater orbital radius is an observational bias, and to what extent it is a real natural phenomenon.
But maybe that is the six million dollar question to all of us.
@tesh February 4, 2015 at 4:43
No, the latest work on mini-Neptunes becoming rocky planets under some circumstances has no effect on my assessments. The transition from rocky to Neptune-like planets that occurs at radii no greater than 1.6 ME is based on actual observations of the mass and radius of Kepler finds as they appear today (and with these measurements, one can determine the bulk composition of the planet). Since all of these worlds orbit mature stars over a billion years old (to the best of my knowledge), any planet that might have originally started out as a mini-Neptune would have already been stripped of its outer volatile-rich layers (which would happen during the earliest stages of a red dwarf’s formation as it settled unto the main sequence). In other words, “evaporated” mini-Neptunes are already included in the work on the planetary mass-radius relationship (indeed, some of the larger rocky worlds in the sample of planets analyzed may already include such worlds).
@Ronald February 4, 2015 at 6:15
Kepler is going to have a tough time trying to detect a statistically meaningful sample of planets orbiting Sun-like stars that are smaller than the Earth. It has had much better luck detecting at least some sub-Earth-size planets orbiting smaller red dwarf stars. Based on a recent statistical analysis of Kepler finds orbiting M-type stars (which takes into account Kepler’s sensitivity and selection biases), it seems that 87.5% (+5.6%/-3.6%) of red dwarfs have planets in the 1.0 to 1.5 RE range with orbital periods from 0.5 to 200-day while 98.9% (+4.3%/-0.1%) have planets in the 0.5 to 1.0 RE range. This suggests that for red dwarfs, at least, planets slightly smaller than the Earth are more common than planets slightly larger than the Earth. Only time will tell if scientists will be able to derive any statistically meaningful results for sub-Earth size planet occurrence rates for more Sun-like stars.
Here is a link to the preprint of the paper by Dressing and Charbonneau, “The Occurrence of Potentially Habitable Planets Orbiting M Dwarfs Estimated from the Full Kepler Dataset and an Empirical Measurement of the Detection Sensitivity”, arVix 1501.01623, submitted January 7, 2015 http://arxiv.org/abs/1501.01623
@Andrew LePage, February 4, 2015 at 15:57
Thanks!
From the paper:
“Within a conservatively defined habitable zone based on the moist greenhouse inner limit and maximum greenhouse outer limit, we estimate an occurrence rate of 0.18 Earth-size planets and 0.11 super-Earths per M dwarf habitable zone.”
That is pretty spectacular. I am so looking forward to seeing similar estimates for solar type stars. However, from what we (think we) know now, it looks as if solar type stars more commonly have gas dwarfs (mini-Neptunes) and ice giants (Neptunes).
Time will tell indeed.
This just in and very related (and spectacular if confirmed):
Scientists predict Earth-like planets around most stars
http://www.sciencedaily.com/releases/2015/02/150204184449.htm
http://arxiv.org/abs/1412.6230
Well-know habitability expert Lineweaver is one of the authors;
“Planetary scientists have calculated that there are hundreds of billions of Earth-like planets in our galaxy which might support life.”
“The team extrapolated from Kepler’s results using the theory that was used to predict the existence of Uranus; We used the Titius-Bode relation and Kepler data to predict the positions of planets that Kepler is unable to see”
“They found the standard star has about two planets in the so-called Goldilocks zone”.
That seems very high to me; browsing through the paper I found the following:
– The authors used 3 different HZ definitions, one of which is the recent Kopparapu et al. HZ (runaway greenhouse – max. greenhouse). However, the differences in result are small. For the most conservative HZ, the ‘Kopparapu conservative’ HZ, the average number of planets in the HZ is 1.3 – 1.5.
– The mentioned high estimate is of all planets (and in the most optimistic HZ). The authors also estimated the number of rocky planets, which they properly defined (as per Rogers 2014) as R <= 1.5 Re.
Then the number of rocky planets in the (conservative) HZ drops to 0.1 – 0.3. That is considerably lower than published in the media (as usual), but still very impressive, much higher than the roughly 1.5% of Foreman-Mackey
et al. (2014) and indeed in line with Petigura et al. (2013).
@Ronald: I always thought the Titius-Bode “law” was more descriptive than predictive. Most notably, it didn’t work so well for Neptune. See Greg Laughlin’s blog post from the first Neptunian anniversary of the discovery of Neptune.
@andy:
yes fully agreed, also see my comments to (an) older post(s):
https://centauri-dreams.org/?p=31658#comment-126325
and
https://centauri-dreams.org/?p=31658#comment-126329
@Ronald February 5, 2015 at 6:21
I’d be REALLY careful to base any conclusions on extrapolations of Kepler results using some variation of the “Titus-Bode Law” in extrasolar planetary systems. This relation has a checkered history with questionable accuracy when applied to our solar system (as Andy rightfully points out with Neptune) and there are peer-reviewed scientific journals which have explicit policies NOT to publish papers dealing with this “law”, new versions of it or predictions based on it (the well-regarded planetary science journal Icarus comes to mind immediately).
It is strange that they based an extrapolation on Titus-Bode,
As far as I remember, some of those Kepler Multi-planet discoveries
are not correlated so well, with T-B ‘laws’
Once again though, we can only infer about F,G,K, stars
and Earth sized planets existence (RE .8 – RE 1.2) in the HZ.
An while where at it, If Earth was in a Mars orbit, would it not be
a Ice world, (except for a narrow band at the Equator free of ICE),
I guess an earth in that orbit technically like our earth Earth since it would resemble Antarctica.