The primary objective of NASA’s Kepler mission is to determine how common rocky planets are in the habitable zone with the ability to detect Earth-size planets in Earth-like orbits around Sun-like stars 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. Now that Kepler’s primary mission is completed, the processing and analysis of this huge data set is in full swing as the spacecraft continues on with the K2 extended mission.

While the discovery of thousands of extrasolar planets have been confirmed and thousands more candidates are currently being vetted as processing of Kepler’s huge data base continues, to date only a relative handful of these worlds have been found orbiting inside of the habitable zone (HZ) – the range of distances from a star where an Earth-like planet can maintain surface conditions which can support the presence of liquid water. The Kepler Habitable Zone Working Group, consisting of selected members of the Kepler science team as well as key experts in the field outside the program, have been combing through the collection of Kepler finds to produce a catalog of HZ exoplanets to serve as a guide for the selection of promising targets for future study by scientists. The first version of this catalog has been accepted for publication in the peer-reviewed scientific journal, The Astrophysical Journal, with astronomer Stephen Kane (San Francisco State University) as the lead author.

Creating Kepler’s HZ Exoplanet Catalog

To create this catalog of HZ exoplanets, Kane et al. started with the list of confirmed and candidate exoplanets as of April 26, 2016 with the properties listed in Data Release 24 (DR24). Covering all 17 quarters of data from Kepler’s primary mission which ended in May 2013, the DR24 exoplanet data base heavily favors uniform vetting in order to calculate statistically accurate occurrence rates. Kane et al. combined the basic data on exoplanets derived from Kepler data in DR24 with the most up to date revised stellar parameters found in the newer DR25 (which consists of only target star properties) to derive the best possible values for each planet’s radius, R P , and effective stellar flux, S eff (i.e. the amount of energy a planet receives from its sun), as well as estimate the uncertainties in these derived values.

With new exoplanet parameters calculated, Kane et al. searched through the list of Kepler’s confirmed and candidate exoplanets to find those that met some basic criteria for four broad categories of exoplanets. The first, Category 1, consists of exoplanets with radii less than twice that of the Earth (or 2 R E ) orbiting inside a “conservative” definition of the HZ. Recent work on the mass-radius relation for exoplanets has shown that planets transition from being predominantly rocky planets like the Earth to volatile-rich “mini-Neptunes” at radii no greater than 1.6 R E (see “Habitable Planet Reality Check: Terrestrial Planet Size Limit”) while more recent work suggests that the gradual transition of the exoplanet population from one to the other type begins at about 1.2 R E . Although such mini-Neptunes have little prospect of being habitable in the conventional sense, Kane et al. nonetheless set their upper limit at 2 R E to take into account the uncertainties in the exoplanet radius data as well as the currently uncertain nature of the transition from rocky to volatile-rich worlds which may allow some of these largest worlds to be rocky after all.

For the conservative definition of the HZ, Kane et al. adopted the limits calculated by Ravi Kopparapu et al. (who was also part of the Kepler Habitable Zone Working Group and a coauthor of the paper by Kane et al.). By this definition, the inner limit of the HZ corresponds to the effective stellar flux, S eff , where a runaway moist greenhouse effect sets in while the outer limit is the maximum greenhouse limit where the addition of more carbon dioxide will not raise surface temperatures any further. For an Earth-mass planet orbiting a Sun-like star, the S eff values for the HZ runs from 1.11 to 0.36 times that of the Earth for the inner and outer limits, respectively. The corresponding values for M dwarf stars tends towards lower values owing to the increased amounts of energy these cooler stars radiate in the infrared where absorption by atmospheric gases enhances the greenhouse effect.

The next list generated by Kane et al. was for Kepler exoplanets designated as “Category 2”. For this category, the radius upper limit of 2 R E was retained but now more optimistic definitions of the HZ were considered. In this case, the limits correspond to the S eff for an “early Venus” (which might have been habitable before one billion years ago when its surficial geologic record begins) and “early Mars” (which appears to have been habitable several billion years ago) which spans S eff values from 1.78 to 0.32, respectively. By this definition, Category 1 exoplanets are of subset of Category 2. Next, Category 3 includes all exoplanets of any size orbiting in the conservative HZ including mini-Neptunes to Jupiter-like gas giants while Category 4 consists of Kepler’s finds of all sizes orbiting inside the more optimistic definition of the HZ.

The Best Picks

Since I generally favor the more conservative definitions of the HZ, I was initially drawn to the Category 1 HZ candidates compiled by Kane et al.. Below in Table 1 are what seem to be the most promising Category 1 candidates identified by Kane et al. which have radii less than 1.5 R E . Exoplanets smaller than this more stringent size limit have the highest probability of being rocky, Earth-like planets given what we know about similarly sized exoplanets elsewhere. Also excluded from Table 1 are eight unconfirmed exoplanet candidates or Kepler Objects of Interest (KOIs) which, based on a closer inspection by others, are highly likely to be instrumental artifacts instead of bona fide exoplanets.

Table 1: Most Promising “Category 1” HZ Candidates from Kane et al.

Name Period (days) Orbit Radius (AU) Planet Radius (Earth=1) Stellar Flux (Earth=1) Kepler 62f 267.3 0.718 1.41±0.07 0.42±0.05 Kepler 186f 129.9 0.432 1.17±0.08 0.26±0.04 Kepler 442b 112.3 0.409 1.34±0.14 0.73±0.11 Kepler 1229b 86.8 0.290 1.25±0.21 0.37±0.11 Kepler 1512b 20.4 0.129 0.61±0.13 0.87±0.38 KOI 2626.01 38.1 0.158 1.27±0.45 0.91±0.31 KOI 3138.01 8.7 0.038 0.57±0.04 0.47 KOI 7223.01 317.1 0.835 1.50±0.28 0.55±0.19

The first four extrasolar planets listed in Table 1 have already been reviewed in detail previously.

Given our limited knowledge, these four Kepler finds are among the best candidates for being potentially habitable, keeping in mind that the larger exoplanets are less likely to be habitable due to the increasing probability that they are mini-Neptunes instead. The new parameters derived by Kane et al. for these four extrasolar planets do not deviate from those published in earlier work by more than the quoted uncertainties and, as a result, do not require any major reassessment of their status.

The only question at this point concerns Kepler 1229b which is listed as KOI 2418.01 in Kane et al. because it had yet to be confirmed when their paper was being prepared for submission. In the discovery paper for Kepler 1229b by Morton et al., it was noted that Kepler’s CFOP (Community Follow-up Observation Program) archive included high-resolution images of the host star which indicates the presence of close stellar companion. The presence of this companion might be affecting the derived properties of the star and its planet by an amount that is greater than the current formal measurement uncertainties. While Kane et al. had flagged a handful of stars in their lists as being part of wide binaries, it is not certain whether or not they were aware of the findings of Morton et al. or if they took the presence of a close companion into account in their analysis.

Notable Category 1 candidates identified by Kane et al. not in Table 1 that have been reviewed earlier include:

In addition to the four confirmed Kepler discoveries, Kane et al. included four Kepler exoplanet candidates with KOI designations which had yet to be confirmed at the time their paper was being prepared but show some promise for being potentially habitable:

Kepler 1512b (KOI 3497.01)

Listed as KOI 3497.01 by Kane et al., the existence of Kepler 1512b has been confirmed by Morton et al.. Based on the planet properties calculated by Kane et al. using the DR25 star properties, this exoplanet has a radius of 0.61±0.13 R E and a S eff of 0.87±0.38 which places it comfortably inside the conservative HZ. Assuming that it has an Earth-like composition, this “sub-Earth” would have a mass on the order of twice that of Mars which is near the theoretical lower limit for a potentially habitable planet – the opposite of the situation we have typically dealt with in recent years when prospective habitable exoplanets might be too large to be habitable.

But before we invest too much time in a discussion about the potential habitability of this world, it needs to be noted that the DR25 stellar properties for Kepler 1512 used by Kane et al. do not seem to be entirely self consistent. They list the star with a temperature of 3419±72 K and a radius of 0.34±0.06 times that of the Sun, or R Sun , which leads to a luminosity of 0.014 times the Sun’s, or L Sun . While these parameters seem to agree with models for an early-M dwarf star, the mass of 0.7 times that of the Sun, or M Sun , implied by the orbit parameters listed by Kane et al. (and similar to the 0.730 +0.022/-0.095 times the Sun mass value in DR24) seems to be too high for such a star by a factor of about two. This difference in mass has a significant effect on the planet’s calculated orbital radius and hence its S eff value. If we assume a lower host star mass of 0.35 M Sun while retaining the other DR25 stellar properties, the orbital radius for Kepler 1512b with an orbital period of 20.35973 days (as measured by Kepler) would be a smaller 0.10 AU resulting in a S eff of approximately 1.3 – about half again as much as calculated by Kane et al. and outside of the conservative definition of the HZ (but still inside more optimistic definitions of the HZ).

Also of concern is how much the star properties in DR25 differ from those in DR24. The earlier Kepler data base lists this star with a temperature of 4372 +59/-64 K and a radius of 0.67 +0.02/-0.06 R Sun which leads to a luminosity of 0.15 L Sun – indicative of a much larger and brighter K-type dwarf star. These stellar properties lead to a much larger derived radius of 1.18 +0.07/-0.11 R E for Kepler 1512b in DR24 making it a larger and comfortably Earth-size compared to what Kane et al. have calculated. Unfortunately, the S eff using DR24 properties is also much larger at 8.5 placing this Kepler find well outside of any definition of the HZ. Given this big change in stellar properties from DR24 to DR25 (which is not all that uncommon but does raise a flag) and a mass used by Kane et al. which seems inconsistent with an M-dwarf host star, this system requires closer scrutiny to verify its stellar properties before we can properly assess this planet’s potential habitability.

KOI 2626.01

The rest of the exoplanets in Table 1 are Kepler candidates which still require confirmation by various means. According to DR25, the host star KOI 2626 has an effective temperature of 3554±76 K and a radius of 0.40±0.05 R Sun which results in a luminosity of 0.022 L Sun . These properties, along with the 0.36 M Sun mass assumed by Kane et al., are consistent with an early-M dwarf star and are similar to the parameters listed in DR24. With a radius of 1.27±0.45 R E , there is a good probability KOI 2626.01 is a rocky planet while the S eff of 0.91±0.31 places it near the inner edge of the conservative HZ. The high uncertainty in S eff , however, suggests that there is a fair likelihood that it orbits outside the HZ. But since it is highly likely that tidal effects would have made this exoplanet a slow or even synchronous rotator. Increasingly sophisticated climate models suggest that the S eff of the inner edge of the HZ for slowly rotating planets will be much higher owing to cloud feedback effects. The model by Yang et al. finds that the S eff of the inner edge of the HZ for slow or synchronous rotators is 1.67 suggesting that KOI 2626.01 is comfortably inside this expanded definition of the HZ even with the uncertainties in its properties.

While promising, this assessment needs to be viewed with caution. Kane et al. note that KOI 2626 seems to have a wide binary companion which might affect the habitability of the exoplanet or even its derived properties. Morton et al. have also calculated a false positive probability (FPP) of 28% for this unconfirmed planet. Although it is more likely to be a bona fide exoplanet than not, this FPP is far larger than the <0.3% value typically required to be considered “confirmed”. Follow up observations are required to verify this find as well as its properties.

KOI 3138.01

Based on the DR25 data (which are identical to the DR24 release), the host star KOI 3138 has an effective temperature of 2703 K and a radius of 0.12 R Sun which results in a miniscule luminosity of 0.0007 L Sun . These properties, along with the 0.096 M Sun mass, suggest an ultracool dwarf star like the nearby TRAPPIST-1 (see “Habitable Planet Reality Check: The Seven Planets of TRAPPIST-1”) . With a radius of 0.57±0.04 R E , this exoplanet is likely to be a sub-Earth (or maybe a “super Mars”?) exoplanet near the low end of size range for habitable planets. Morton et al. find an FPP of 2.7% so this interesting exoplanet candidate still requires follow up observations to confirm. If confirmed, future study of this small world may help define the lower size limits of potentially habitable worlds.

KOI 7223.01

According to DR25, the host star KOI 7223 has an effective temperature of 5366±152 K and a radius of 0.71±0.09 R Sun which results in a luminosity of 0.37 L Sun . Along with the 0.77 M Sun mass used by Kane et al., these properties suggest it is a late-G dwarf slightly smaller, cooler and dimmer than the Sun. All of the DR25 stellar parameters differ by less than the stated uncertainties from those in DR24. While this is the only candidate in Table 1 to orbit a Sun-like star (the others listed by Kane et al. are believed to be instrument artifacts instead), its prospects for being habitable should not be oversold. While its S eff value of 0.55±0.19 places this exoplanet comfortably inside the conservative HZ, its radius of 1.50±0.28 R E suggest its chances of being a rocky planet are probably only even at best.

With an FPP of 1.8% determined by the analysis of Morton et al., this candidate still requires more follow up observations and analysis to confirm. And before we get too carried away with the prospects of any of the KOIs listed here, experience has shown that their properties can change substantially when better observations become available as well as prove to be unverifiable or outright false positives. In early 2015, for example, KOI 2194.03 and 5737.01 were considered promising candidates for Earth-like planets orbiting inside the HZ of Sun-like stars (see “Earth-Twins on the Horizon?”). Unfortunately, they have not survived the subsequent vetting process and were not included in the lists by Kane et al.. The same fate could await any of the KOIs listed here.

Slow Rotating Exoplanets

In addition to the most promising Category 1 exoplanets identified by Kane et al., there are several other confirmed exoplanets and candidates worth noting. As mentioned earlier, recent models by Yang et al. and others suggest that slow or synchronous rotators can remain habitable with S eff values much higher than those corresponding to the inner edge of the more conservatively defined HZ of Kopparapu et al. (whose own more recent work largely confirms the findings of Yang et al.). Table 2 below includes all of the Category 2 exoplanets identified by Kane et al. with radii less than about 1.5 R E which orbit red dwarfs beyond the inner limits of the conservatively defined HZ and are likely to be slow or synchronous rotators.

Table 2: Promising Slow or Synchronous Rotators Among “Category 2” HZ Candidates from Kane et al.

Name Period (days) Orbit Radius (AU) Planet Radius (Earth=1) Stellar Flux (Earth=1) Kepler 296e 34.1 0.169 1.53±0.26 1.42±0.67 Kepler 560b 18.5 0.092 1.48±0.31 1.14±0.54

Here is a closer look at these two confirmed Kepler finds which have been reviewed earlier:

Kepler 296e

The properties of Kepler 296e derived by Kane et al. using DR25 are virtually the same as those published earlier. It orbits a red dwarf star located about 740 light years away with a temperature of 3740±130 K, a radius of 0.48±0.08 R Sun and a luminosity of 0.040 L Sun . The resulting S eff of 1.42±0.67 is higher than for the inner limit of the conservative HZ and which contributed to an initial pessimistic review (see “Habitable Planet Reality Check: 8 New Habitable Zone Planets”). However, this S eff value is less than the inner limit of the HZ for a slow or synchronous rotator of 1.70 found by Yang et al. improving the odds somewhat that this exoplanet might prove to be habitable. However, the large radius of 1.53±0.26 R E makes it fairly likely that Kepler 296e is a mini-Neptune especially given the large uncertainty. While this exoplanet is worth additional study, its prospects for being potentially habitable are modest at best.

Kepler 560b (KOI 463.01)

Kepler 560b, identified as KOI 463.01 by Kane et al., is similar to Kepler 296e in many regards. Based on the stellar properties from DR25, the host star has a temperature of 3395±71 K, a radius of 0.28±0.06 R Sun and a luminosity of 0.0009 L Sun . While this host star is smaller and dimmer than Kepler 296, Kane et al. calculated that Kepler 560b has a radius of 1.48±0.31 R E and a S eff of 1.14±0.54. This S eff value places Kepler 560b more safely into the HZ for slow or synchronous rotators whose inner edge, according to Yang et al., has an S eff of 1.62. While this exoplanet is even more likely to be inside the HZ despite the large uncertainty in its S eff , its radius is essentially identical to that of Kepler 296e making it fairly likely that this is a mini-Neptune. While the updated properties somewhat improve the chances that this exoplanet is habitable compared to those in Morton et al. (see “Habitable Planet Reality: Check Kepler’s Latest Finds“), all in all, it seems to have only slightly better prospects for being potentially habitable than Kepler 296e.

While these properties suggest that Kepler 560b has modest chances for being habitable based on the calculations by Kane et al., the DR25 stellar properties do not appear to be entirely consistent. The host star seems to be a little too massive for its derived radius and temperature. Using what seems to be somewhat more self consistent DR24 stellar properties, Kepler 560b would have a radius of 1.72±0.09 R E and an S eff of 1.71 making it much more probable that Kepler 560b is a very warm mini-Neptune incapable of supporting life as we know it. Follow up observations to better characterize the host star of this system will be required to pin down the important properties of this exoplanet with more certainty.

Notable Category 2 candidates identified by Kane et al. not in Tables 1 or 2 that have been reviewed earlier include:

Habitable Moons?

While we seem to have exhausted the possibilities for approximately Earth-size planets orbiting in the HZ of their suns using Kepler primary mission finds as of April 26, 2016, there is another class of potentially habitable worlds we have not considered yet: exomoons orbiting gas giants in the HZ (see “Habitable Moons”). While Kepler has yet to discover any exomoon, habitable or otherwise, it has detected a number of Jupiter-sized gas giants orbiting inside the HZ which could host large moons. Table 3 below lists the Category 3 HZ candidates identified by Kane et al. as having a Jupiter-like radius of 11.2 R E and a low FPP.

Table 3: Promising Primaries for Large Exomoons Among “Category 3” HZ Candidates from Kane et al.

Name Period (days) Orbit Radius (AU) Planet Radius (Earth=1) Stellar Flux (Earth=1) Kepler 553c 328.2 0.917 11.24±0.85 0.58±0.13 KOI 868.01 236.0 0.613 11.00±0.53 0.33±0.05 KOI 1466.01 281.6 0.752 11.35±0.60 0.51±0.08

Since gas giants are rare around M dwarf stars (see “Occurrence of Potentially Habitable Planets Around Red Dwarfs”), this exoplanet and two candidates orbit more Sun-like early-K to late-G type stars. To date, the only confirmed HZ gas giant on the list is Kepler 553c designated as KOI 433.02 by Kane et al.. Since the radius of gas giants is insensitive to mass (see “A New Definition for Gas Giants”), there is no way to estimate the masses of these planets from Kepler data alone. They could have any mass from slightly less than Jupiter’s to brown dwarfs with masses several tens of times that of Jupiter. Follow up observations, including precision radial velocity measurements, will be required to confirm the candidates and determine masses of all three gas giants as well as search for any possible exomoons that could be orbiting them and might be potentially habitable.

Summary

The new paper by Kane et al. summarizes the initial efforts by the members of the Kepler Habitable Zone Working Group to create a catalog of extrasolar planets found orbiting inside of the habitable zone (HZ). This “Catalog of Kepler Habitable Zone Exoplanet Candidates” will serve as a guide for the selection of promising targets for future study by scientists. The most promising candidates for being potentially habitable extrasolar planets includes well known names like Kepler 186f, Kepler 442b and Kepler 62f along with another seven promising worlds among recently confirmed Kepler finds (see “The Top Five Known Potentially Habitable Planets“) and candidates still requiring verification whose properties need to be verified and refined.

Unfortunately, almost all of these most promising habitable planet candidates found as of April 26, 2016 in Kepler’s primary mission data orbit stars much smaller and dimmer than our Sun. The only exception is the unconfirmed KOI 7223.01 which might be a potentially habitable super-Earth or a non-habitable mini-Neptune – assuming its existence can be verified. There are also one confirmed and two candidate Jupiter-size gas giants orbiting inside the HZ of Sun-like stars which could potentially host habitable exomoons worthy of future study.

So where are the Earth-size exoplanets with Earth-like orbits around Sun-like stars which Kepler was suppose to find? The simple fact of the matter is that these “Earth-twins” have proved to be more difficult to detect than originally anticipated. Typical Sun-like stars have turned out to be photometrically noisier than the Sun and instrumental artifacts have tended to create spurious signals with periods of 9 to 12 months – roughly the same as the orbital periods expected for Earth-twins. With this in mind, increasingly sophisticated data analysis tools are slowly overcoming these issues to uncover ever-smaller exoplanets in Kepler’s primary mission data, as the recent announcement of over 1284 newly validated exoplanets by Morton et al. demonstrates (see “Habitable Planet Reality Check: Kepler’s Latest Finds”).

Extrapolating from recent statistical studies of Kepler finds suggests that about one in ten Sun-like stars have planets with radii between 0.6 and 1.5 R E orbiting in the HZ – the sort of worlds with the best chances of being habitable in the Earth-like sense (see “The Prevalence of Earth-Size Planets Around Sun-Like Stars”). It is only a matter of time before more promising Earth-twin candidates are found in the Kepler data. In the mean time, there will be follow up observations to confirm promising HZ planet candidates already identified as well as refine our knowledge of HZ exoplanet properties not to mention the new finds to come from the extended K2 mission which is ideal for spotting more HZ exoplanets orbiting smaller M dwarf stars (see “The First Year of Kepler’s K2 Mission”). Future catalogs of HZ exoplanets created using Kepler data will contain many more promising habitable planet candidates for future study.

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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

Stephen Kane et al., “A Catalog of Kepler Habitable Zone Exoplanet Candidates”, arXiv 1608.00620 (accepted for publication in The Astrophysical Journal), August 1, 2016 [Preprint]

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

Timothy D. Morton et al., “False positive probabilities for all Kepler Objects of Interest: 1284 newly validated planets and 428 likely false positives”, The Astrophysical Journal, Vol. 822, No. 2, Article ID 86, May 10, 2016

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