The Prevalence and Compositions of Small Planets

Abstract

This thesis describes three investigations of the galactic abundance and properties of small planets. First, I revised the properties of the smallest Kepler target stars and searched their light curves for transits using a custom transit detection pipeline. Combining the detected population of 156 planet candidates (including one previously undetected candidate) with an empirical estimate of the search completeness based on transit injection and recovery simulations, I found occurrence rates of 0.24 (+0.18/-0.08) Earth-size planets (1− 1.5 Earth radii) and 0.21 (+0.11/-0.06) super-Earths (1.5−2 Earth radii) per M dwarf habitable zone. Consequently, the most probable distances to the nearest non-transiting and transiting potentially habitable planets are 2.6 ± 0.4 pc and 10.6 (+1.6/-1.8) pc, respectively. Second, I conducted an adaptive optics imaging survey of 87 bright Kepler target stars with ARIES at the MMT to search for nearby stars that might be diluting the depths of the planetary transits. I identified visual companions within 1” for 5 stars, between 1” and 2” for 7 stars, and between 2” and 4” for 15 stars. For all stars observed, I placed limits (typically delta Ks = 5.3 at 1” and delta Ks = 5.7 at 2”) on the presence of undetected nearby stars. Third, I investigated the composition of Kepler-93b, a 1.478 ± 0.019 Earth radius planet with a 4.7-day orbit around a bright (V = 10.2) asteroseismically-characterized host star with a mass of 0.911 ± 0.033 solar masses and a radius of 0.919 ± 0.011 solar radii. Based on two seasons of observations with HARPS-N at the Telescopio Nazionale Galileo and archival observations from Keck/HIRES, I found a mass of 4.02 ± 0.68 Earth masses and a density of 6.88 ± 1.18 g/cc. Comparing Kepler-93b to the other nine exoplanets smaller than 2.7 Earth radii with well-constrained parameters, I found that all 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. There are currently no such planets with masses greater than 7 Earth masses. 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 seven highly-irradiated dense exoplanets extends to the full population of dense 1 – 6 Earth mass planets.