EssayHabitable Planets: What Are We Learning from Keplerand Ground-Based Searches?James F. KastingHabitable, or potentially habitable, exoplanets have beengetting a lot of attention lately. First, Vogt et al. (2010)announced the discovery of Gliese 581g, the sixth planet in theGliese 581 system. (Gliese 581a is the star itself.) This putativeplanet, dubbed the ‘‘Goldilocks planet’’ by the popular press,was reported to be a 3.1 minimum Earth-mass object orbitingsmack in the middle of the habitable zone of its parent star.The habitable zone is the name given to the region around astar in which a rocky planet might support liquid water on itssurface. This is a good definition, one that I myself havehelped to quantify (Kasting et al., 1993). It’s good because itincludes two key factors that are considered essential for lifeas we know it: liquid water and a solid (or liquid) surface onthe planet. Although biochemists debate whether liquid wateris required for life (Committee on the Limits of Organic Life inPlanetary Systems, 2007), virtually everyone agrees that somekind of surface is necessary to create stable pressure-temperatureAU2 cconditions. Life on gas giant planets is probablyimpossible because it could never get started, given the lack ofa stable environment. And, despite the debate over whetherliquid water is necessary for life, the presence of water on anexoplanet’s surface may be a de facto requirement for remotelife detection because, without it, we are not likely to know forwhich spectroscopic biosignatures to look.Gliese 581 g, unfortunately, may not even exist. The planetwas discovered by the Lick-Carnegie Exoplanet Survey—aground-based radial velocity survey carried out by re-searchers at those two institutions. The radial velocity, or RV,method is a search technique that uses the Doppler shift of astar’s absorption lines to detect the presence of an unseenplanet. As soon as this planet was announced, researchersassociated with the Swiss team of Michel Mayor quicklydoused cold water on the discovery, announcing that theplanet does not appear in their more accurate RV data. Theysuggested that the assumption of circular orbits for the entireGliese family of planets by Vogt et al. may have led to anerroneous result. Planets, of course, reside on elliptical orbits,which may be circularized if they happened to form that wayor if tidal forces are strong enough to damp their eccentric-ities. According to the Swiss group, both Gliese 581 g and aseventh planet, ‘‘f,’’ disappear when the orbits are allowed tohave finite eccentricities.Whether or not this particular planet is real should not bea major issue. Its existence, or lack thereof, will undoubtedlybe tested by continued measurements by both researchgroups. If Gliese 581 g does not exist, it is only a matter oftime before another potentially habitable planet will befound. Indeed, five such candidates were announced justrecently by the Kepler team (Borucki et al., 2011). Kepler is aNASA space telescope that has been in operation for justunder 2 years. Kepler observes a field of *160,000 stars inthe constellation Cygnus, looking for evidence of planetarytransits. A transit is when a planet passes in front of a starduring part of its orbit, blocking out some of the star’s light.Earth’s diameter is about 1% that of the Sun, so it wouldblock about 1 part in 104of the Sun’s light if observed intransit from afar (because the area of a disk equals pr2).Kepler can measure stellar brightnesses to 1 part in 105,soitcan identify Earth-sized planets around Sun-like stars, givenenough time. It takes a minimum of three transits to producea reliable signal, so this process takes about 3 years for anexo-Earth orbiting a star like the Sun. But planets orbitingwithin the habitable zones of smaller stars (late K or Mdwarfs) have shorter orbital periods, so this is what theKepler scientists have reported.When we talk about five new, potentially habitable plan-ets, we should bear in mind that these represent only a smallfraction of the planet candidates that Kepler has reported. Intheir February 2011 data release (Borucki et al., 2011), theKepler team described 1235 planet candidates orbiting 997host stars. The team calls these ‘‘planet candidates,’’ ratherthan planets, because some of them, like Gliese 581g, maynot be real. Background eclipsing binary stars and other‘‘false positives’’ that mimic planetary transit signaturesmean that some of these 1235 planet candidates will likely goaway with time. Nonetheless, Kepler may still triple thenumber of known exoplanets, which had just recently sur-passed 500 prior to the February announcement. Of the 1235new planet candidates, 54 are within their host star’s habit-able zone or the Kepler team’s definition thereof. (Theirpublished definition includes planets with effective radiatingtemperatures as high as 373 K, or 1008C. This is almost cer-tainly too high, as such planets would receive more stellarradiation than does Venus.) Forty-nine of these planets,though, have estimated radii more than twice that of Earthand, hence, are probably either warm Neptunes or warmJupiters—gas giant planets that orbit relatively close to theirparent stars. Such planets are not very interesting astro-biologically, for reasons mentioned above. Indeed, dozens ofgas giant planets within stellar habitable zones were alreadyDepartment of Geosciences, The Pennsylvania State University, University Park, Pennsylvania.ASTROBIOLOGYVolume 11, Number 4, 2011ª Mary Ann Liebert, Inc.DOI: 10.1089/ast.2011.31301AST-2011-3130-Kasting_1PType: essayAST-2011-3130-Kasting_1P.3d 04/27/11 3:32pm Page 1known from prior RV surveys. Life could still exist withinthese systems if these planets have large moons (Williamset al., 1997); however, the prospects for detecting life on suchmoons are extremely slim. (One would need to separate thelight from the giant planet from that of the star and thenobserve the moon in transit in front of the giant planet.Doing these things simultaneously would be nearly impos-sible.)Astrobiologists should be thrilled by the new Kepler re-sults. (If any of you aren’t, come closer, and I will knock youon the head!) We are now finding direct evidence that po-tentially habitable planets exist around stars other than ourown. That’s the first step in finding out if there are little greenmen out there with whom we might one day converse. Thiswas a dream of the