Precise Demographics of Kepler Exoplanets in the Gaia Era

Abstract

A major bottleneck for transiting exoplanet demographics has been the lack of precise properties for most of the observed stars, as the transit method measures exoplanet radii relative to their host's radii. We live in a golden era of host star characterization because of access to Gaia photometry, parallaxes, and proper motions, large-scale spectroscopic surveys, and ground-based photometric and spectroscopic follow-up. I have used all of this data to sharpen our view of exoplanet demographics. First, I constrained the stellar radii of Kepler targets using Gaia DR2, which allowed the first comprehensive classification of main sequence, subgiant, and giant stars in Kepler target sample; I also identified ~4000 low-mass main sequence binary systems. With these precise stellar radii, I was the first to use Gaia to revise planet radii and incident fluxes and corroborate the existence of the planet radius gap. I discovered planets within the hot sub-Neptunian desert (2.2–3.8 Earth radii, > 650 Earth fluxes), presented an updated census of habitable zone planets, and identified a hot Jupiter inflation trend for Kepler planets. I also performed isochrone modeling for the entire Kepler target sample and produced the Gaia-Kepler Stellar Properties Catalog, the first homogeneous catalog to include stellar ages, in addition to precise radii, masses, and mean stellar densities for Kepler target stars. Using these homogeneously derived properties, I found the first observational evidence of a stellar age dependence of the planet radius gap, where sub-Neptunes (1.8–3.5 Earth radii) become super-Earths (1.0–1.8 Earth radii) on roughly Gyr timescales. This result built upon my previous work, where I measured lithium abundances to separate old and young Kepler stars (using the Hyades's empirical A(Li)-effective temperature isochrone at ~650 Myr) and discovered that the young planets were statistically larger than the old planets. In addition, I investigated the stellar mass dependence of the planet radius valley and provided stringent constraints that will be required to discern between the theories of core-powered mass-loss and photoevaporation. I also confirmed the existence of planets within the hot sub-Neptunian desert, discovered that most desert planets entered recently because of their host's evolution, investigated Jupiters at low incident fluxes with radii larger than the theoretical maximum, and demonstrated that planets in single and multiple transiting systems share the same age distribution.