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Habitable planets: what are we learning from Kepler and ground-based searches?

Habitable, or potentially habitable, exoplanets have been getting a lot of attention lately. First, Vogt et al. (2010) announced the discovery of Gliese 581g, the sixth planet in the Gliese 581 system. (Gliese 581a is the star itself.) This putative planet, dubbed the ‘‘Goldilocks planet’’ by the popular press, was reported to be a 3.1 minimum Earth-mass object orbiting smack in the middle of the habitable zone of its parent star. The habitable zone is the name given to the region around a star in which a rocky planet might support liquid water on its surface. This is a good definition, one that I myself have helped to quantify (Kasting et al., 1993). It’s good because it includes two key factors that are considered essential for life as we know it: liquid water and a solid (or liquid) surface on the planet. Although biochemists debate whether liquid water is required for life (Committee on the Limits of Organic Life in Planetary Systems, 2007), virtually everyone agrees that some kind of surface is necessary to create stable pressuretemperature AU2c conditions. Life on gas giant planets is probably impossible because it could never get started, given the lack of a stable environment. And, despite the debate over whether liquid water is necessary for life, the presence of water on an exoplanet’s surface may be a de facto requirement for remote life detection because, without it, we are not likely to know for which spectroscopic biosignatures to look. Gliese 581 g, unfortunately, may not even exist. The planet was discovered by the Lick-Carnegie Exoplanet Survey—a ground-based radial velocity survey carried out by researchers at those two institutions. The radial velocity, or RV, method is a search technique that uses the Doppler shift of a star’s absorption lines to detect the presence of an unseen planet. As soon as this planet was announced, researchers associated with the Swiss team of Michel Mayor quickly doused cold water on the discovery, announcing that the planet does not appear in their more accurate RV data. They suggested that the assumption of circular orbits for the entire Gliese family of planets by Vogt et al. may have led to an erroneous result. Planets, of course, reside on elliptical orbits, which may be circularized if they happened to form that way or if tidal forces are strong enough to damp their eccentricities. According to the Swiss group, both Gliese 581 g and a seventh planet, ‘‘f,’’ disappear when the orbits are allowed to have finite eccentricities. Whether or not this particular planet is real should not be a major issue. Its existence, or lack thereof, will undoubtedly be tested by continued measurements by both research groups. If Gliese 581 g does not exist, it is only a matter of time before another potentially habitable planet will be found. Indeed, five such candidates were announced just recently by the Kepler team (Borucki et al., 2011). Kepler is a NASA space telescope that has been in operation for just under 2 years. Kepler observes a field of *160,000 stars in the constellation Cygnus, looking for evidence of planetary transits. A transit is when a planet passes in front of a star during part of its orbit, blocking out some of the star’s light. Earth’s diameter is about 1% that of the Sun, so it would block about 1 part in 10 of the Sun’s light if observed in transit from afar (because the area of a disk equals pr). Kepler can measure stellar brightnesses to 1 part in 10, so it can identify Earth-sized planets around Sun-like stars, given enough time. It takes a minimum of three transits to produce a reliable signal, so this process takes about 3 years for an exo-Earth orbiting a star like the Sun. But planets orbiting within the habitable zones of smaller stars (late K or M dwarfs) have shorter orbital periods, so this is what the Kepler scientists have reported. When we talk about five new, potentially habitable planets, we should bear in mind that these represent only a small fraction of the planet candidates that Kepler has reported. In their February 2011 data release (Borucki et al., 2011), the Kepler team described 1235 planet candidates orbiting 997 host stars. The team calls these ‘‘planet candidates,’’ rather than planets, because some of them, like Gliese 581g, may not be real. Background eclipsing binary stars and other ‘‘false positives’’ that mimic planetary transit signatures mean that some of these 1235 planet candidates will likely go away with time. Nonetheless, Kepler may still triple the number of known exoplanets, which had just recently surpassed 500 prior to the February announcement. Of the 1235 new planet candidates, 54 are within their host star’s habitable zone or the Kepler team’s definition thereof. (Their published definition includes planets with effective radiating temperatures as high as 373 K, or 1008C. This is almost certainly too high, as such planets would receive more stellar radiation than does Venus.) Forty-nine of these planets, though, have estimated radii more than twice that of Earth and, hence, are probably either warm Neptunes or warm Jupiters—gas giant planets that orbit relatively close to their parent stars. Such planets are not very interesting astrobiologically, for reasons mentioned above. Indeed, dozens of gas giant planets within stellar habitable zones were already