New Planets for the New Year

2011 was an exciting year for exoplanets, mainly thanks to the Kepler mission and its abundance of results, and 2012 looks set to be even better. Even though we’re not quite half-way through January, a number of exoplanet discoveries have already been announced.

Yesterday the three smallest exoplanet candidates to date were unveiled at a NASA press conference. The three planets orbit an M-type star, denoted ‘Kepler Object of Interest’ (KOI) 961, which has about 60% the mass of the Sun but only one sixth its diameter. The three planets, KOI 961.01, KOI 961.02 and KOI 961.03 follow a diminutive orbit analogous to the moon system of a gas giant: the most proximate planet has a semi-major axis of roughly 1.5 million kilometres, or 0.010 AU, which is slightly more distant than the orbit of Titan around Saturn. All three planets’ orbits could be roughly contained within the orbit of Iapetus, Saturn’s third largest moon, which has a semi-major axis of 3.5 million kilometres.

The planetary system of the star 'KOI 961' consists of the three small planets in a very tight orbit. The size of the 'planet' is representative of it's radius (x-axis is on a logarithmic scale). Also included is the habitable zone (inner boundary in yellow, outer in blue), Gliese 581d (a larger planet that also orbits an M-type star) and Earth and Mars for scale.

The planets themselves are well within the ‘hot zone’ of the star, outside of the inner boundary of the habitable zone, with equilibrium temperatures (excluding greenhouse effects) of between 960 (KOI 961.01) and 1150 °K (KOI 961.02).  Their radii are reported as 0.57, 0.78 and 0.73 Earth radii respectively, the smallest of which is comparable to Mars. Kepler continues to surprise; these small planets in their miniature orbits are a testament to the sensitivity of the instrument, as well as an indication of the variety of  planetary systems and possible orbital configurations across the galaxy.

Circumbinary Planets

Artists rendition of the circumbinary planet Kepler 35b orbiting the binary Kepler 35(AB)

Also announced yesterday were the planets Kepler 34b and 35b, unique in the fact that they are both ‘circumbinary’ planets. This means that, along with Kepler 16b first announced in December 2011, they orbit two stars  in an orbital configuration now thought to be common throughout the galaxy. Weighing in at ~70 Earth masses Kepler 34b orbits two Sun-like (1.04 and 1.02 Solar masses) stars over the course of a 284 day orbit. The stars themselves orbit one another in 28 days and are separated by 0.22 AU, or ~32 million kilometres. At ~40 Earth masses, Kepler 35b is slightly smaller and orbits the binary pair Kepler 35A and B, with 0.80 and 0.88 solar masses respectively, at a distance of 0.60 AU. The stars are separated by 0.17 AU, or 25 million kilometres. Both planets appear to be within the HZ distance of their respective stars, but their complex orbits are likely to complicate this metric and any conclusions regarding their habitability will have to await further study. However, considering their masses and densities (0.61 and 0.41 g cm-3 respectively) they are likely to be low-density gas giants with little potential for habitability despite their residence within the habitable zone. Nevertheless, the possibility of orbiting habitable moons cannot be excluded.

Principal investigator Dr. William Welsh of San Diego State University says that the unique orbital configurations of  circumbinary planets would make for novel and interesting climate dynamics due to large multi-periodic variations in insolation over the course of a single orbit. Speaking at the American Astronomical Society meeting on behalf of the Kepler Science Team he said, “It would be like cycling through all four seasons many times per year, with huge temperature changes. The effects of these climate swings on the atmospheric dynamics, and ultimately on the evolution of life on habitable circumbinary planets, is a fascinating topic that we are just beginning to explore.”