The Frequency and Composition of Small Exoplanets

Fri, January 29, 2016

Over the past twenty years, ground- and space-based investigations have revealed that our galaxy is teeming with planetary systems and that Earth-sized planets are common. I will focus on the results of the NASA Kepler mission and describe two investigations of the frequency and composition of small planets. First, I analyzed Kepler observations of small stars and measured a cumulative planet occurrence rate of 2.45 +/- 0.22 planets per small star with periods of 0.5-200 days and planet radii of 1-4 Earth radii. Within a conservative habitable zone based on the moist greenhouse inner limit and maximum greenhouse outer limit, I estimated an occurrence rate of 0.15 (+0.18/-0.06) Earth-size planets and 0.09 (+0.10/-0.04) super-Earths per small star habitable zone. Second, I explored the compositional diversity of small planets by using the HARPS-N spectrograph on the Telescopio Nazionale Galileo (TNG) to measure the masses of transiting planets.  Concentrating on the set of small planets with well-constrained radii from space-based observations with Kepler or CoRoT and precise mass estimates from TNG/HARPS-N or Keck/HIRES, I found that all close-in dense exoplanets with masses of approximately 1-6 Earth masses are consistent with the same fixed ratio of iron to rock as the Earth and Venus. Future measurements of the masses and radii of a larger sample of planets receiving a wider range of stellar insolations will reveal whether the fixed compositional model found for these highly-irradiated dense exoplanets extends to the full population of low-mass planets.