On June 19, 2017, NASA held a press conference at Ames Research Center located in the heart of California’s Silicon Valley. At this event, Mario Perez (Astrophysics Division – NASA Science Mission Directorate) along with Susan Thompson (SETI Institute), Benjamin Fulton (University of Hawaii at Manoa/Caltech) and Courtney Dressing (NASA Sagan Fellow at Caltech) presented the results relating to the latest catalog of extrasolar planets found during the primary mission of NASA’s highly successful Kepler spacecraft. According to the latest tally, there are now 4,034 planet candidates identified with 2,335 of them verified by various means including follow up observations by ground-based telescopes. Of greatest importance to those interested in finding out how common life outside of our solar system might be, roughly 50 near-Earth size habitable zone candidates have been detected by Kepler with more than 30 of them currently verified (see “Habitable Zone Exoplanets from NASA’s Kepler Mission” for a summary of these earlier finds).

Kepler Planet Candidates in the Habitable Zone

The primary objective of NASA’s Kepler mission is to determine how common rocky planets are in the habitable zone (HZ) with the ability to detect Earth-size planets in Earth-like orbits around Sun-like stars (i.e. true “Earth twins”) being the primary driver for the design of the spacecraft and its observation strategy. Assuming that our Earth is typical of life-bearing planets in the universe, this is the best place to start looking for habitable worlds outside of our solar system. Kepler looked for tiny dips in a star’s brightness from an orbiting planet transiting from our point of view. Since such transits only occur if the planet’s orbit is by chance aligned nearly edge-on to our line of sight, they are relatively rare. For example, a planet in an Earth-like orbit around a Sun-like star has only about a 0.5% chance of producing an observable transit. Planets in smaller orbits have proportionally higher chances of producing transits.

In order to detect a significant number of these relatively uncommon planetary transits, Kepler observed the brightness of almost 200,000 stars in a single patch of sky straddling the border of the constellations Cygnus and Lyra covering 115 square degrees over a four year primary mission which started on May 13, 2009. With the end of that primary mission coming when Kepler lost its ability to stay fixed on its target as the result of the loss of a second of its four reaction wheels, Kepler eventually moved on to an extended mission. Known as “K2”, this extended mission was designed to use the remaining pair of reaction wheels and the pressure from sunlight to maintain its attitude in order to observe a succession of star fields located along the ecliptic for three months at a time to expand the search for transiting exoplanets.

As the K2 mission continues, the Kepler science team has been using increasingly sophisticated software tools to search the data base from the primary mission to detect the sometimes subtle signature of a transiting exoplanet. The purpose of the Kepler team’s press conference on June 19, 2017 was to present the results from the eighth catalog of Kepler finds as the project wraps up its work on the primary mission. For this latest release, members of the Kepler science team reprocessed the entire data set from Kepler’s primary mission using their latest tools. In addition, they performed simulations to determine how complete their survey results were – a necessary step to convert the raw statistics derived from the Kepler catalog into useful information on the occurrence rate of various sized exoplanets.

In the course of assembling this new catalog, the Kepler science team found another 239 planet candidates – objects of interest which require follow up observations in order to verify their planetary nature. Of these, ten are roughly Earth-size objects orbiting inside the habitable zone of their host stars. Table 1 below summarizes the key properties of these ten recently announced Kepler planet candidates identified by their KOI designations (Kepler Object of Interest). These planet candidates have radii less than twice that of the Earth (or 2 R E ) and orbit within an optimistic definition of the habitable zone (HZ). All of the data in this table are taken from the Data Release 25 (DR25) data set found in the NASA Exoplanet Archive. The amount of energy each candidate planet receives from its sun, or stellar flux, was calculated from those data.

Table 1: Summary of Latest Kepler Habitable Zone Candidates

KOI Number Period (days) Orbit Radius (AU) Planet Radius (Earth=1) Stellar Flux (Earth =1) 238.03 362.98 1.001 1.96 +0.33/-0.29 1.81 7706.01 42.05 0.186 1.19 +0.08/-0.16 2.00 7711.01 302.78 0.841 1.31 +0.34/-0.12 0.87 7882.01 65.42 0.273 1.31 +0.08/-0.12 1.79 7894.01 347.98 0.958 1.62 +0.49/-0.15 0.97 7923.01 395.13 1.001 0.97 +0.12/-0.10 0.44 7954.01 372.15 0.976 1.74 +0.46/-0.14 0.69 8000.01 225.49 0.686 1.70 +0.43/-0.14 1.20 8012.01 34.57 0.122 0.42 +0.17/-0.12 0.37 8174.01 295.06 0.774 0.64 ±0.07 0.70

While the Kepler science team has purposely used the most optimistic assessment of habitability to cast as wide a net as possible for targets for future study by scientists, realistically what are the chances that any of these planet candidates are actually habitable given what we know about them?

Potential Habitability

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, the evolution of its volatile content and so on. Unfortunately, at this very early stage, the only information typically available to scientists about extrasolar planets are basic orbit parameters, a rough measure of its size and some important properties of its sun. Combined with extrapolations of the factors that have kept the Earth habitable over billions of years (not to mention why our neighbors are not currently 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 in an Earth-like sense where the surface conditions allow for the existence of liquid water on the planet’s surface – one of the presumed prerequisites for the development of life as we know it. While there may be other worlds that might possess environments that could support life, these would not be Earth-like habitable worlds of the sort being considered here.

The first step in assessing the potential habitability of any exoplanet is to determine what sort of world it is: is it a rocky planet like the Earth or is it a volatile-rich mini-Neptune? Although examples of this newly recognized class of planet are of interest to scientists (in part because they are absent in our solar system), mini-Neptunes have poor prospects of being habitable in an Earth-like sense. If we know the radius and mass of an exoplanet, its mean density can be readily calculated which in turn can be used to constrain its bulk composition. While the radii of transiting exoplanets can be determined from an analysis of the transit light curves, unfortunately the mass can not be directly determined. Other methods, such as the analysis of precision radial velocity measurements, are needed to do that – a process that will take time assuming that the host star is bright enough and the orbiting planet massive enough for current instruments to detect.

Without any information on the masses of Kepler’s new batch of planet candidates currently available, we are forced to rely on statistical arguments based on the observed mass-radius relationship of other exoplanets whose radii and masses have been measured. A series of analyses of Kepler data and follow-up observations published over the last several years 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. Rogers has shown that planets have a higher probability of being mini-Neptunes at a radius of no greater than 1.6 R E although 1.5 R E seems more probable (see “Habitable Planet Reality Check: Terrestrial Planet Size Limits”).

A more recent analysis of the mass-radius relationship with a much larger collection of exoplanetary data by Chen and Kipping suggests that that the gradual transition from rocky to volatile-rich exoplanets starts at about 1.2 R E again with the probability that a planet is rocky decreasing with increasing radius. Hints of this transition in exoplanet populations from primarily rocky to volatile-rich exoplanets is evident in the statistical analysis of Kepler finds which shows comparatively few exoplanets in the 1.5 to 2.0 R E size range. This result suggests that once rocky planets reach a radius of 1.5 R E , they become massive enough to retain more volatiles including a puffy envelope of hydrogen which allows them to jump the size gap to radii of 2.0 R E and greater. Only the largest rocky planets and the smallest mini-Neptunes fill this gap.

The next criterion that can be used to determine if a rocky exoplanet is potentially habitable is the amount of energy it receives from its sun known as the effective stellar flux or S eff . According to the work by Kopparapu et al. (2013, 2014) on the limits of the habitable zone (HZ) based on detailed climate and geophysical modeling, the inner limit of the HZ for an Earth-like rocky planet is conservatively defined by the runaway greenhouse limit where a planet’s temperature would soar even with no CO 2 present in its atmosphere resulting in the loss of all of its water in a geologically brief time in the process. For an Earth-size planet orbiting a Sun-like star, this happens at an S eff value of 1.11 times that of Earth. For stars with effective surface temperatures lower than the Sun’s 5780 K, this S eff value for the inner limit of the HZ becomes lower because more of the stars’ energy is radiated in the infrared where there are numerous atmospheric absorption bands which help to heat the atmosphere. For planets more massive than the Earth, this S eff limit becomes somewhat higher because the atmospheric column is compressed by the planet’s higher gravity.

As a planet receives less energy from its sun, various processes such as the carbonate-silicate cycle allow more CO 2 to build up in the atmosphere which helps to increase the greenhouse effect and maintain surface temperatures. The outer limit of the conservative HZ, as defined by Kopparapu et al. (2013, 2014), corresponds to the maximum greenhouse limit beyond which a CO 2 -dominated greenhouse is incapable of maintaining a planet’s surface temperature. Instead of helping to heat the atmosphere, the addition of more CO 2 beyond this point makes the atmosphere more opaque causing the surface temperatures to drop instead of increase. The latest work suggests an S eff value of about 0.36 for the outer limit of the HZ of a Sun-like star with cooler stars having slightly lower values. 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 CO 2 contribute to warming a planet. But these more optimistic definitions do not change the S eff for the outer limit of the HZ significantly.

With these two criteria based on readily observable properties of transiting planets available, it is possible to make an initial assessment of the potential habitability of the latest planet candidates found by the Kepler science team.

KOI 238.03

This exoplanet candidate is part of the Kepler 123 system which was discovered in 2014 to contain two confirmed super-Earth size exoplanets in short-period orbits of 17.23 and 26.70 days. Since the host star has a luminosity of about 1.82 times that of the Sun, Kepler 123b and c would have S eff values many tens of times greater than that of the Earth and could not be habitable. While the new planet candidate in this system, designated KOI 238.03, orbits much farther from Kepler 123 than its two confirmed neighbors, unfortunately its prospects for being habitable are still not very promising.

With an effective temperature of 6086±133 K, the inner edge of the HZ around Kepler 123 has an S eff of 1.23 for a 5 M E planet, according to Kopparapu et al. (2013, 2014). With a Venus-like S eff of 1.81, KOI 238.03 is far outside of the conservatively defined HZ. In addition, the 1.96 +0.33/-0.29 R E radius of this planet candidate makes it highly unlikely that it is a rocky planet like the Earth but is a volatile-rich mini-Neptune instead. Taken together, it seems that KOI 238.03 has very poor prospects for being a habitable exoplanet given our current knowledge of its properties.

KOI 7706.01

With a radius of 1.19 +0.08/-0.16 R E , KOI 7706.01 is right on the 1.2 R E border found by Chen and Kipping where the population of exoplanets begins to transition from being rocky to volatile-rich. At the same time it is still well below the radius found by Rogers where the majority of exoplanets would be expected to be volatile-rich. While there is some possibility that KOI 7706.01 is a mini-Neptune, the odds seem to heavily favor it being a rocky planet like the Earth.

Issues seem to arise when considering the S eff of this planet candidate. For an Earth-mass planet orbiting KOI 7706 with an effective temperature of 4281 +115/-140 K, the conservatively defined inner edge of the HZ is found at an S eff value of 0.96, based on Kopparapu et al. (2013, 2014). The calculated S eff for KOI 7706.01 of 2.00 is over twice that value. However, given the tight orbit of this planet candidate around its comparatively dim host star, it seems likely that KOI 7706.01 will be a synchronous rotator with the same side always facing its sun. Increasingly detailed climate models over the last two decades have shown that not only is synchronous rotation not an impediment to habitability but that the inner edge of the HZ can be much closer to the sun than it is for fast rotators like the Earth. This is because feedback mechanisms promote the formation of clouds on the perpetual daylit side of the planet which reflects away energy to help moderate the surface temperature.

A recent model by Yang et al. suggests that the inner edge of the HZ of a slow or synchronous rotator has an S eff of 1.83. More recent work by Kopparapu et al. (2016) which makes more realistic assumptions about the rotation rate and its effects on global circulation suggests a similar value. While the S eff of 2.00 for KOI 7706.01 is still about 9% greater than this more optimistic definition of the inner edge of the HZ, the uncertainty in the parmeters leading to this this S eff value is large enough to suggest that this candidate may straddle the inner edge of the HZ. Until the properties of KOI 7706.01 can be more precisely determined, it can be considered a fair candidate for being a potentially habitable planet. No matter what kind of world this turns out to be, a more detailed characterization of this exoplanet’s properties would allow scientists to probe the limits of planetary habitability.

KOI 7711.01

KOI 7711.01 is definitely one of the better candidates for being potentially habitable. The host star seems to be a slightly smaller version of the Sun with 62% of its luminosity. While the radius of 1.31 +0.34/-0.12 R E is well into the transition region between rocky and volatile-rich planets, the odds still seem to favor KOI 7711.01 being a rocky planet. The S eff of 0.87 is much lower than 1.10 value for the conservatively defined inner edge of the HZ for an Earth-mass planet orbiting a star with an effective temperature of 5734±154 K strongly suggesting that it orbits comfortably inside of the HZ. Among this recent batch of Kepler planet candidates, KOI 7711.01 comes the closest to being considered a true “Earth twin” – i.e. an Earth-size planet in an Earth-like orbit around a Sun-like star. It certainly deserves closer scrutiny once its planetary nature is confirmed.

KOI 7882.01

Like KOI 7711.01, KOI 7882.01 has a radius of 1.31 R E suggesting that the odds favor it being a rocky planet. But with a smaller uncertainty in that measurement, +0.08/-0.12 R E compared to +0.34/-012, it would seem that it is slightly less likely that KOI 7882.01 is a mini-Neptune thus improving its habitability prospects at least in this regard. For a star like KOI 7882 with an effective temperature of 4348±130 K, the inner edge of the conservatively defined HZ for an Earth-mass planet found by Kopparapu et al. (2013, 2014) would have a S eff of 0.96 which is much smaller than the current calculated value of 1.79. But like 7706.01, this exoplanet is likely to be a synchronous rotator. Based on the model by Yang et al., the inner edge of the HZ for a slow or synchronous rotator would have an S eff of 1.85. This places KOI 7882.01 just inside of the HZ for this type of exoplanet. It would appear that this planet candidate has some good prospects for being potentially habitable and would be the type of target scientists could use to probe the true limits of the HZ.

KOI 7894.01

With an Earth-like S eff of 0.97, KOI 7894.01 seems to orbit comfortably inside the HZ whose inner edge would correspond to an S eff of 1.14 for a host star with an effective temperature of 5995 +163/-181 K. Unfortunately, with a radius currently estimated to be 1.62 +0.49/-0.15 R E , it seems that this Kepler planet candidate is more likely to be a mini-Neptune with poor prospects of being habitable like the Earth. While it is certainly worth watching, it would seem that KOI 7894.01 is not likely to be potentially habitable.

KOI 7923.01

Of the latest batch of Kepler planet candidates, KOI 7923.01 would seem to have the best prospects of being potentially habitable. Its radius of 0.97 +0.12/-0.10 R E is essentially identical to that of the Earth (to within current measurement uncertainties) and is therefore likely to be rocky like the Earth. For a host star like KOI 7923 with about 40% of the Sun’s luminosity and an effective temperature of 5060 +192/-174 K, the conservatively defined HZ for an Earth-mass planet would span S eff values from 1.02 out to 0.31 at the outer edge using the model of Kopparapu et al. (2013, 2014). With a S eff calculated to be 0.44, KOI 7923.01 is comfortably inside the outer part of its sun’s HZ. Although the S eff is lower than that of the Earth, it would seem that KOI 7923.01 is the closest any current Kepler find, confirmed or otherwise, has come to being considered a true Earth twin. As such, this planet candidate deserves further attention.

KOI 7954.01

With an S eff currently estimated to be 0.69, KOI 7954.01 seems to orbit comfortably inside of its sun’s conservative HZ which, with a Sun-like effective temperature of 5769 +155/-172 K, spans S eff values from 1.11 to 0.36 for an Earth-mass planet. Unfortunately, this Kepler planet candidate with a radius of 1.74 +0.46/-0.14 R E is most likely a uninhabitable mini-Neptune. While certainly worthy of study for what it can tell us about this newly recognized class of exoplanet, KOI 7954.01 seems to have poor prospects for being habitable in an Earth-like sense.

KOI 8000.01

The prospects for KOI 8000.01 being habitable are even poorer than those of KOI 7954.01. With a radius of 1.70 +0.43/-0.14 R E , this planet candidate is most likely a mini-Neptune with little chance of being habitable. With a S eff of 1.20, KOI 8000.01 seems to orbit right at the edge of the HZ which, for its host star with an effective temperature of 5663 +169/-152 K, has an S eff of 1.17 for a 5 M E planet as calculated by Kopparapu et al. (2013, 2014). Again, it seems that KOI 8000.01 has poor prospects for being habitable although it is certainly worth additional study to characterize this new class of exoplanet.

KOI 8012.01

With a Mars-like radius of 0.42 +0.17/-0.12 R E , KOI 8012.01 is the smallest planet candidate being considered here. This candidate also has a Mars-like S eff of 0.37 which places it just inside of its host star’s HZ which, with an effective temperature of 3374 +112/-82 K, has an S eff of 0.24 at its outer limit, according to the models of Kopparapu et al. (2013, 2014). Unlike far too many other exoplanets which some have claimed to be habitable but are too big, this Kepler planet candidate may be too small to be habitable, if Mars in our own solar system is to serve as any sort of a guide. While there are questionable prospects for KOI 8012.01 being potentially habitable due to its small size, it would be an ideal target for probing the lower size limits of planetary habitability.

KOI 8174.01

Based on an initial assessment, it would seem that KOI 8174.01 has better prospects for being habitable than the comparatively diminutive KOI 8012.01. Its measured radius of 0.64 ±0.07 R E places this candidate between being Earth-size and Mars-size possibly giving this world an edge on staving off the worst effects of atmosphere loss the Red Planet has experienced. The S eff of 0.70 also places this candidate comfortably in the middle of the host star’s HZ which, with a effective temperature of 5332 +160/-144 K, spans S eff values from 1.05 out to 0.33 for an Earth-mass planet. Given what little we know about sub-Earth size planets, this will be another ideal target for further examination and, at first blush, seems to have fairly good prospects for being potentially habitable.

Summary

When considering this list of ten planet candidates, it needs to be remembered that the Kepler science team purposely used an optimistic assessment of what constitutes a habitable planet. This was done, in part, in response to the uncertainties in the properties of these exoplanets but also because of the limitations of the current models of planetary habitability. The team’s goal was to include all planet candidates that had any chance of being habitable to serve as an input for a “short list” of targets for future investigators.

In this review, I have taken a different approach of trying to focus instead on those worlds which have reasonably realistic chances of being potentially habitable given what we know about them and using more conservative extrapolations of what it takes for a planet to maintain Earth-like habitable conditions. Based on this more conservative approach, it appears that planet candidates KOI 238.03, KOI 7894.01, KOI 7954.01 and KOI 8000.01 are probably volatile-rich mini-Neptunes instead of being rocky like the Earth. While this recently identified class of exoplanet is certainly worth detailed study, they have very poor prospects for being habitable in the Earth-like sense.

At the other end of the size spectrum, the planet candidate KOI 8012.01 seems more likely to be a slightly smaller version of Mars potentially sharing our neighbor’s habitability issues. The larger KOI 8174.01 falls neatly between Earth and Mars in size and, like KOI 8012.01, orbits comfortably inside the habitable zone (HZ) of its sun. More detailed study of these candidates promises to shed light on the lower mass limit of habitable planets.

The planet candidates 7706.01 and 7882.01 are likely to be synchronous rotators which orbit right at the inner edge of the HZ for slowly rotating planets. Studies of these worlds will also help scientists probe the limits of habitability in this part of parameter space. KOI 7711.01 and especially KOI 7923.01 come the closest to being Earth twins on this list of new Kepler planet candidates. These Earth-size candidates orbit comfortably inside of the conservatively defined HZ of their Sun-like host stars – exactly the kind of world that NASA’s Kepler mission was designed to detect.

But before we invest too much into any assessment about the potential habitability of these KOIs, it must be remembered that they are currently only planet candidates whose planetary status must be confirmed by time-consuming follow up observations. While the current tools for processing Kepler data are constantly improving, there is the possibility that some of these planet candidates are false positives with some other natural phenomenon or subtle instrumental artifact mimicking the signature of a transiting planet. This is of increasing concern as the limits of the hardware, data and software are pushed to extract ever smaller and more difficult to detect transiting planets.

It is also possible that some of these planet candidates may remain unconfirmed for years to come. Potential Earth-twin planet candidates KOI 2194.03 and KOI 5737.01 were first discussed publicly during an astronomical conference in January 2015 but remain unconfirmed 2½ years later (see “Earth Twins on the Horizon?”). It is also quite possible that some of these planet candidates will be confirmed but, as a result of the more detailed assessments afforded by follow up observations, the properties of the host star will be updated. Any such changes would inevitably trickle down to alter the derived properties of the now confirmed exoplanet so that it may no longer be considered potentially habitable. Although the Kepler science team is formally winding up its work on the primary mission’s data set, there will be years if not decades of additional work required to characterize Kepler’s discoveries, potentially habitable or otherwise.

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Related Video

Here is the video from NASA’s June 19, 2017 press conference presenting the results from the release of the eighth Kepler catalog.

Related Reading

In addition to the articles cited above, there is an ever-growing list of articles on Drew Ex Machina related to the results from NASA’s Kepler mission. A complete list of these articles can be found on this web site’s Kepler mission page.

General References

Jingjing Chen and David Kipping, “Probabilistic Forecasting of the Masses and Radii of Other Worlds”, The Astrophysical Journal, Vol. 834, No. 1, Article id. 17, January 2017

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

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

Ravi Kumar Kopparapu et al., “The Inner Edge of the Habitable Zone for Synchronously Rotating Planets around Low-mass Stars Using General Circulation Models”, The Astrophysical Journal, Vol. 819, No. 1, Article ID. 84, March 2016

Leslie A. Rogers, “Most 1.6 Earth-Radius Planets are not Rocky”, The Astrophysical Journal, Vol. 801, No. 1, Article id. 41, March 2015

Jun Yang et al., “Strong Dependence of the Inner Edge of the Habitable Zone on Planetary Rotation Rate”, The Astrophysical Journal Letters, Vol. 787, No. 1, Article ID L2, May 2014

NASA Releases Kepler Survey Catalog with Hundreds of New Planet Candidates, NASA Press Release 17-056, June 19, 2017 [Press Release]