The news this week has been filled with stories about Kepler reaching its milestone of 1,000 confirmed planetary discoveries with eight of those newfound planets advertised as orbiting inside the habitable zones (HZs) of their suns (see “Habitable Planet Reality Check: 8 New Habitable Zone Planets”). Also announced this week were the addition of 554 new planet candidates bringing Kepler’s total of these worlds to 4,175. Kepler planet candidates are just preliminary detections that require follow-up observations to confirm that they are actually planets and not “false positives” resulting from eclipsing binaries or other phenomena. Based on past experience, it is estimated that about 90% of Kepler candidates will prove to be bona fide extrasolar planets.
Diagram showing the major components of NASA Kepler spacecraft. (NASA/Kepler Mission/Ball Aerospace)
As scientists continue to process the data from Kepler’s primary mission using increasingly sophisticated analysis software, they have been finding smaller, more Earth-sized planets orbiting larger, more Sun-like stars. Buried in the coverage of the discovery of the eight new HZ planets was also the announcement of two especially interesting Kepler planet candidates currently designated KOI 2194.03 and KOI 5737.01. Unlike most of the other potentially habitable planets found to date by Kepler which orbit dim K and M-dwarf type stars, these planet candidates appear to orbit in the HZ of Sun-like stars.
While I usually like to wait for Kepler candidates to be confirmed and their properties more accurately determined, I figured I would quickly analyze the known properties of these two candidates that have been in the news to gauge their potential habitability for myself. These candidates also offer a foretaste of the Kepler discoveries yet to come in the months ahead.
KOI 2194.03
KOI 2194 appears to be an F-type star with a temperature of 6,308° K (+144° K/-179° K), a radius of 0.922 (+0.357/-0.082) RSun and a mass of 1.000 (+0.113/-0.128) MSun. The luminosity derived from these parameters is about 1.1 (+0.8/-0.2) LSun. Based on the work by Kopparapu et al. which conservatively defines the inner edge of the HZ to be the runaway greenhouse limit and the outer edge to be the maximum CO2 greenhouse limit, the effective stellar flux or Seff for the HZ of this star runs from 1.18 to 0.39, respectively, for an Earth-size planet.
Based on the first 16 quarters of Kepler data, the planet candidate KOI 2194.03 is in a 445.2-day orbit with a mean distance of 1.14 AU. The Seff for this world would be about 0.84 with an uncertainty that I roughly estimate to be about +0.60/-0.18. I do not have enough information at this time on the interdependence of the measurement errors of the various parameters to derive a better estimate on the uncertainty of Seff but this figure is good enough to provide a feel for the uncertainty. Given these uncertainties, I roughly estimate that there is on the order of a 70% chance that KOI 2194.03 orbits inside the HZ of this system.
This same Kepler data set also indicates that the radius of KOI 2194.03 is about 1.4 (+0.6/-0.1) RE. Since this is just below the 1.6 RE size threshold found by Rogers (below which planets are more likely to be have rocky compositions instead of being mini-Neptunes), there is a fair probability that KOI 2194.03 is a rocky planet (see “Habitable Planet Reality Check: Terrestrial Planet Size Limit”). While more precise parameters for this system are definitely required (not to mention the planetary nature of KOI 2194.03 confirmed), these initial results indicate that KOI 2194.03 is a fairly good candidate for being a potentially habitable Earth-size planet orbiting a Sun-like star.
KOI 5737.01
Based on the best data currently available, KOI 5737 appears to be a G-type star with a temperature of 5,916° K (+153° K/-167° K), a radius of 0.960 (+0.374/-0.099) RSun and a mass of 0.976 (+0.126/-0.0.99) MSun. The luminosity derived from these parameters is about 1.0 (+0.8/-0.2) LSun. Like KOI 2194, this star appears to be a slightly hotter and brighter version the Sun. Based on the work by Kopparapu et al., the conservative definitions of the inner and outer edges of the HZ correspond to Seff values of 1.04 and 0.32, respectively, for an Earth-size planet.
The analysis of the first 16 quarters of Kepler data indicates that the planet KOI 5737.01 is in a 376.2-day orbit with a mean distance of 1.01 AU. The Seff for this world would be about 0.99 with an uncertainty that I estimate to be about +0.78/-0.23. Given the uncertainties, I roughly estimate that there is around a 50% chance that KOI 5737.01 orbits inside the HZ of this system. This is a somewhat lower probability than for KOI 2194.03, but given the uncertainties, KOI 5737.01 is still a reasonable candidate for a planet orbiting inside the HZ worthy of additional study.
While KOI 5737.01 has only even odds of orbiting in the HZ, it appears to have slightly better chances of being a rocky planet than KOI 2194.03. The current best estimate of the radius of KOI 5737.01 is 1.32 (+0.51/-0.14) RE which is below the 1.6 RE threshold derived by Rogers for the dividing line of planets that are predominantly rocky versus predominantly mini-Neptunes. Once again, more accurate data on the parameters of this planet and its sun are needed as well as confirmation of its planetary status, but it appears that KOI 5737.01 is a reasonable candidate for being a potentially habitable planet orbiting a Sun-like star.
Conclusion
While there has been too much hype about too many extrasolar planets concerning their potential habitability, this initial look at the very preliminary results for KOI 2194.03 and KOI 5737.01 are indeed promising. Even though their planetary status needs to be confirmed and more work done to determine more accurate values of key properties of these planets and their suns, it does appear at this time that potentially habitable extrasolar planet candidates have been found orbiting two Sun-like stars. This is just a hint of the discoveries yet to come from the continued analysis of the data from NASA’s Kepler mission.
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Related Reading
“Habitable Planet Reality Check: 8 New Habitable Zone Planets”, Drew Ex Machina, January 8, 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]
General References
“KOI 2194.03”, NASA Exoplanet Archive [Link]
“KOI 5737.01”, NASA Exoplanet Archive [Link]
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”, arVix 1407.4457 (submitted to The Astrophysical Journal), July 16, 2014 [Preprint]