Habitable Planet Reality Check: 8 New Habitable Zone Planets

NASA’s Kepler mission has now reached a new milestone with 1,000 confirmed planets to its credit. In addition, another 554 candidate planets requiring follow-up study have also been added bringing the tally up to 4,175. With scientists continuing to process data from Kepler’s primary mission and with new data coming in from the extended “K2” mission, even more planetary discoveries are sure to come.

As exciting as reaching this milestone has been, the real interest in recent days has come with the announcement of the discovery of an additional eight planets orbiting inside the habitable zone (HZ) of their respective systems. The announcement of these new discoveries was made by Guillermo Torres (Harvard-Smithsonian Center for Astrophysics) in a press conference at a meeting of the American Astronomical Society on January 6 along with the submission of a paper by Torres et al. for publication in The Astrophysical Journal. As expected, this announcement has dominated the news over the past few days especially in the space-related media. But having a healthy skepticism about overhyped claims of habitability, I decided to take a closer look at these newly confirmed planets.

Diagram showing the major components of NASA Kepler spacecraft. (NASA/Kepler Mission/Ball Aerospace)

New Discoveries

One common method for confirming Kepler planet candidates is to look for variations in the host stars’ radial velocity indicating the presence of an orbiting planet. But many of Kepler’s finds have such low masses and orbit stars which are so dim that it is often impossible to detect them using this method. Instead, Torres and his team used a new program called BLENDER developed by Torres and Francois Fressin (Harvard-Smithsonian Center for Astrophysics) to process data from NASA’s Kepler mission in an attempt to statistically validate the existence of a dozen planets candidates (including three that had been confirmed earlier by others). Combined with a year’s worth of new follow-up observations such as high-resolution optical and near-infrared spectroscopy, adaptive optics imaging as well as speckle interferometry, the team was able to confirm 11 of these planets to a 3σ certainty or better effectively eliminating the possibility of false positives. Five of these candidates proved to be in multiple star systems whose blended images would lead to erroneous results. The team used BLENDER and the supplementary follow-up observations to determine which of these stars the planets actually orbited and correct their derived properties accordingly.

What impressed me the most about the announcement and discovery paper by Torres et al. was the fairly cautious tone about the nature of their discoveries (a tone that is not always reflected in the media hype over discoveries like this). Taking into account the sometimes large uncertainties in the parameters they derived for the properties of these planets’ orbits and their suns, Torres et al. calculated the probability that their finds actually orbited inside the HZ. This provides a quantitative measure in the confidence in the potential habitability of these new worlds.

My only complaint over their approach is the overly optimistic definition used by Torres et al. to define the limits of the HZ. For their paper, Torres et al. defined the outer edge of the HZ to correspond to the optimistic “early Mars” limit derived by Kopparapu et al. which has a minimum effective stellar flux (or S_eff) of 0.32 times that of the Earth for a Sun-like star. A more conservative limit set by Kopparapu et al. is the maximum greenhouse limit where a CO₂-dominated greenhouse can no longer maintain above-freezing temperatures on a planet. This limit corresponds to a minimum S_eff of 0.36 for a Sun-like star. Likewise, Torres et al. adopted a very optimistic limit for the inner edge of the HZ based on a dry desert-world model described by Zsom et al. which is defined by a maximum S_eff of about 2.9 – a stellar flux that is 50% higher than that for Venus (which is most certainly not a habitable planet!). A more conservative definition for the inner edge of the HZ is the runaway greenhouse limit as described by Kopparapu et al. which has a maxium S_eff of 1.11 for a 1 Earth-mass (or M_E) planet orbiting a Sun-like star. The exact S_eff values for the limits of the HZ can vary with planet mass and star temperature.

Another aspect of the paper by Torres et al. that impressed me was the fact that they also calculated the probability that the new planets have a rocky composition. Several analyses of Kepler data and subsequent follow-up observations over the past year have shown that planets with radii greater than about 1.6 Earth radii (or R_E) are much more likely to be mini-Neptunes which have little prospect of being potentially habitable. The low-density planets of the Kepler 11 system and the recent discovery of PH 3c hint that smaller planets can also have high volatile contents but they quickly become much less common with decreasing planet size. Although somewhat different from the method used by Rogers, the approach used by Torres et al. to calculate the probability that a planet with a particular radius is rocky gave qualitatively similar results (see “Habitable Planet Reality Check: Terrestrial Planet Size Limit” for a full discussion of Rogers’ work).

Basic data taken from Torres et al. concerning the potential habitability of their eight newly confirmed planets, along with the ranges corresponding to the 1σ measurement uncertainties, are shown below in Table 1 with a discussion of each planet following.

Table 1: Data on New Habitable Zone Planets (from Torres et al.)

Kepler 436b

With only a 0.6% probability being a rocky planet, Torres et al. consider that Kepler 436b is most likely a mini-Neptune and not a rocky planet. If we assume a more conservative definition of the inner limit of the HZ than used by Torres et al. (i.e. corresponding to a runaway greenhouse limit as defined by Kopparapu et al.) the maximum S_eff value for the Kepler 436 system’s HZ is 1.05 for a 5 M_E planet. Assuming the uncertainty in S_eff for Kepler 436b is Gaussian, I estimate that there is only a 21% probability that Kepler 436b orbits within the HZ – much lower than the 72.3% probability calculated by Torres et al. using their more optimistic assumptions about the limits of the HZ. The excessive size of Kepler 436b makes it highly unlikely that it is potentially habitable. The low probability that it orbits inside the HZ makes it unlikely that it possesses a potentially habitable moon assuming any exist that are large enough.

Kepler 437b

Torres et al. estimate that there is only a 11.7% probability that Kepler 437b is a rocky planet. Assuming a more conservative definition for the inner limit of the HZ, the maximum S_eff would be 1.05 for a 5 M_E planet orbiting Kepler 437. Given the high S_eff for this planet, I estimate that there is only an 11% chance of Kepler 437b orbiting within the HZ of this system. It is therefore unlikely that Kepler 437b is a potentially habitable planet or could possess a potentially habitable moon.

Kepler 438b

Kepler 438b is much more likely to be a rocky planet with a probability of 69.6% calculated by Torres et al.. In fact, it has the highest probability of being a rocky planet out of all the worlds they studied for this paper. The 71.8% probability that this planet orbits inside the HZ of Kepler 438 is based on their very optimistic definition of the HZ. Assuming a more conservative definition of the runaway greenhouse limit, the maximum S_eff for the inner edge of the HZ for a 1 M_E planet orbiting Kepler 438 would be about 0.94. Given the uncertainty in the S_eff of Kepler 438b, I estimate that there is a 27% probability that Kepler 438b orbits inside the HZ. While Torres et al. consider Kepler 438b to be a good habitable planet candidate, I can not agree. Unless the scenario for habitable desert-planets as described by Zsom et al. proves to be correct, I feel that it is unlikely that Kepler 438b is a habitable planet and it is more probable to be a slightly cooler and larger version of Venus instead.

Kepler 439b

By the estimation of Torres et al., there is only a 6.5% chance that Kepler 439b is a rocky planet and is much more likely to be a mini-Neptune. Using a more conservative definition of the inner limit of the HZ, the maximum S_eff for a 5 M_E planet orbiting Kepler 439 would be 1.14. Given the uncertainty of S_eff of Kepler 439b and assuming that it is Gaussian, I estimate that there is only a 13% chance that Kepler 439b orbits inside the HZ. Given its size and orbit, it is unlikely that Kepler 439b is potentially habitable.

Kepler 440b

Torres et al. estimate that there is only a 29.8% probability that Kepler 440b is a rocky planet making it more probable to be a mini-Neptune instead. The inner limit of the HZ for Kepler 440, assuming a more conservative definition for the HZ, corresponds to a S_eff of 1.02 for a 5 M_E planet. I estimate that there is only a 39% probability that Kepler 440b orbits inside this more conservative HZ. Taking into account these facts, Kepler 440b is, at best, only a fair candidate for being potentially habitable.

Kepler 441b

Unlike the first five new planets discussed which have high S_eff values and orbit near the inner edge of the HZ, the last three have much lower effective stellar flux or S_eff values. Kepler 441b, with an S_eff of 0.21 is estimated by Torres et al. to have a 48.6% chance of orbiting inside the HZ. However, this is based on the optimistic “early-Mars” definition for the outer limit of the HZ. Using the more conservative maximum greenhouse limit, which has an S_eff of 0.28 for Kepler 441 according to Kopparapu et al., I estimate that there is only a 27% probability of Kepler 441b orbiting inside the HZ. Combined with the 45.0% probability that Kepler 441b is a rocky planet, this world is only a fair candidate at best for being potentially habitable.

Kepler 442b

By far, the most promising candidate for a potentially habitable planet 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. Kepler 442b, with a radius of 1.34 R_E, is estimated to have a 60.7% probability of being a rocky planet. With an S_eff of 0.66, Torres et al. estimate that there is a 96.9% probability that this planet orbits inside their optimistic definition of the HZ. Even assuming a more conservative outer limit for the HZ corresponding to the maximum greenhouse limit as defined by Kopparapu et al. which has an S_eff of 0.28 for Kepler 442, I estimate that there is an 82% probability that it orbits inside the HZ – only slightly lower than the estimate by Torres et al.. Given what is currently known about this world, I agree with the assessment by Torres et al. that Kepler 442b ranks as one of the most promising potentially habitable extrasolar planets found to date.

Kepler 443b

The last new confirmed planet announced by Torres et al. is Kepler 443b. With a 4.9% probability of being a rocky planet, it is much more likely that this newly discovered world is a mini-Neptune. Torres et al. estimate that there is a 89.9% probability of Kepler 443b orbiting inside the HZ of this system. With a more conservative limit for the inner edge of the HZ of Kepler 443 corresponding to a S_eff of 1.06 for a 5 M_E planet, I estimate that there is a 76% probability of it orbiting inside the HZ. Still, given the low probability of it being a rocky planets, Kepler 443b is a poor candidate for being potentially habitable. If Kepler 443b has a sufficiently large moon, it does have a reasonable probability of being potentially habitable. Assuming a 0.1 M_E moon, the inner edge of the HZ for Kepler 443 corresponds to an S_eff of 0.88. I estimate that there are about even odds that such a moon would be in the HZ.

Other Planets

In addition to the eight newly confirmed planets just discussed, Torres et al. also studied three previously confirmed extrasolar planets thought to orbit inside the HZ as well as one additional Kepler candidate whose planetary status remains unconfirmed. Basic data taken from Torres et al. concerning the potential habitability of these worlds, along with the ranges corresponding to the 1σ measurement uncertainties, are shown below in Table 2.

Table 2: Data on Other Habitable Zone Planets (from Torres et al.)

Kepler 186f

Before the most recent announcement, Kepler 186f was already considered to be one of the better habitable planet candidates. Discovered just last year, the new work by Torres et al. further refines the properties of this world. With a radius now estimated to be 1.17 R_E, Torres et al. calculate that there is a 68.4% probability of it being a rocky world. Using their optimistic definition for the limits of the HZ, they estimate that there is a 98.4% chance of Kepler 186f orbiting inside the HZ. Assuming that the outer limit of the HZ for Kepler 186 instead corresponds to an S_eff of 0.26 for the more conservative maximum greenhouse limit of Kopparapu et al., I estimate that there is a still promising 62% probability that Kepler 186f orbits inside the HZ. It appears that Kepler 186f remains one of the best potentially habitable extrasolar planet candidates discovered to date (for a more detailed discussion of the potential habitability of Kepler 186f, see “Habitable Planet Reality Check: Kepler 186f”).

Kepler 296e and f

When its discovery was first announced, Kepler 296f was considered by some to be a potentially habitable planet candidate. But follow-up observations by Star et al. using the Hubble Space Telescope revealed that Kepler 296 was not a single star, as had been assumed, but a pair of M-dwarf stars instead. As a result, the properties derived under the assumption that Kepler 296 was single proved to be incorrect. Torres et al. made their own analysis of this system and have been able to derive corrected properties for the planets of this system. Their analysis led them to conclude that Kepler 296 has two potentially habitable planets – Kepler 296f and e.

Hubble Space Telescope image of the Kepler 296 showing it to be a close binary. Kepler saw this as a single star. (Adapted from Star et al./STScI/NASA)

According to the new properties derived by Torres et al., Kepler 296e has a 50.7% probability of being a rocky planet and a 82.0% probability of orbiting inside their optimistic definition for the HZ. But using a more conservative definition of the limits of the HZ, the situation is not as promising. Assuming a runway greenhouse limit for the HZ as defined by Kopparapu et al., the inner edge of the HZ for Kepler 296 has an S_eff of 1.00 for a 5 M_E planet. I estimate that there is only a 27% probability that Kepler 296e orbits inside the HZ. It seems that it is more likely that this planet is a larger but slightly cooler version of Venus instead.

The situation for the more distantly orbiting Kepler 296f is a bit more promising in some ways. Given its size, Torres et al. estimate that there is only a 30.6% probability that Kepler 296f is a rocky world. On the other hand, they estimate that it has a 99.7% probability that it orbits within their optimistic definition of the HZ. Even with a more conservative definition for the outer limit of the HZ with an S_eff of 0.25, I estimate that it still has a respectable 82% probability of orbiting inside the HZ. Given its size, however, Kepler 296f is only a fair candidate for being potentially habitable.

KOI 4427.01

One of the planets studied by Torres et al. that still remains unconfirmed is a planet designated KOI 4427.01 (the first planet candidate identified orbiting the 4427^th “Kepler Object of Interest”). Although its detection has a 99.16% confidence level, it did not quite meet the 3σ false positive threshold set by Torres et al. but it still seems significant enough to likely be a bona fide planet. Based on their best estimation of the properties of this unconfirmed world, Torres et al. estimate that there is a 27.3% probability that this is a rocky world and a 71.9% probability that it orbits inside this system’s optimistically defined HZ. With a conservative outer limit of the HZ of KOI 4427 having an S_eff of 0.23, I estimate that there is only a 41% probability that this planet orbits inside the HZ. While more work is required to confirm the existence of this planet and more accurately determine its properties, it would seem that KOI 4427.01 is a poor candidate for being a potentially habitable planet.

Conclusion

The relatively cautious tone of the discovery paper by Torres et al. and the attempt to quantify the probabilities that key properties of their discoveries match those required for being potentially habitable planets are most welcomed. I do differ with the very optimistic definition for the HZ used by Torres et al. and subsequently estimate lower probabilities that their planets orbit inside the HZ than they calculate. This difference is most evident with their assessment of the potential habitability of Kepler 438b which they feel is a good candidate. The same is true about the assessment of the potential habitability of Kepler 296e. A more conservative approach to the HZ definition suggests that these two planets are more likely to be Venus-like than Earth-like.

Despite my disagreement about the nature of Kepler 438b, I do agree with the assessment by Torres et al. that Kepler 442b is potentially habitable. Along with Kepler 186f, whose properties Torres et al. further refine in their study, these two extrasolar planets are among the best candidates for potentially habitable extrasolar planets currently known. There is also some prospect that Kepler 296f and 441b might be habitable. Most of the rest of the planets considered here are likely to be mini-Neptunes with little chance of being potentially habitable. More work will be required to improve the accuracy of the derived properties of these distant worlds. The continuing analysis of the Kepler data and follow-up work of the sort performed by Torres et al. promises to reveal more and even better habitable planet candidates very soon. Buried by the news of these eight discoveries was the announcement of two Kepler planet candidate, KOI 2194.03 and KOI 5737.01, that appear to be Earth-size planets orbiting inside the HZ of two Sun-like stars (see “Earth Twins on the Horizon?“). While their planetary nature is yet to be confirmed, these worlds give a foretaste of the discoveries yet to come.

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Related Reading

“Earth Twins on the Horizon?”, Drew Ex Machina, January 9, 2015 [Post]

“The Composition of Super-Earths”, Drew Ex Machina, January 3, 2015 [Post]

“Habitable Planet Reality Check: Terrestrial Planet Size Limit”, Drew Ex Machina, July 24, 2014 [Post]

“Habitable Planet Reality Check: Kepler 186f”, Drew Ex Machina, April 20, 2014 [Post]

“The Transition from Rocky to Non-Rocky Planets”, Centauri Dreams, November 14, 2014 [Post]

General References

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

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

Kimberly M. Star et al., “Revision of Earth-Sized Kepler Planet Candidate Properties with High Resolution Imaging by Hubble Space Telescope”, arVix 1407.1057v1 (submitted to The Astrophysical Journal), July 3, 2014 [Preprint]

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

Andras Zsom et al., “Toward the Minimum Inner Edge Distance of the Habitable Zone”, The Astrophysical Journal, Vol. 778, No. 2, Article id. 109, December 2013

“Eight New Planets Found in “Goldilocks” Zone”, Harvard-Smithsonian Center for Astrophysics Press Release No. 2015-04, January 6, 2015 [Press Release]