Sydney Vach

The occurrence and evolution of short-period, small planets younger than 1 Gyr

We present the occurrence rates and evolution in radius and orbital properties of short-period small planets within their first billion years. We present evidence that young planets around FGK stars contract rapidly during the first 200 million years, and that population matches the Kepler demographic at the 500 Myr to 1 Gyr timescale. Our young planet population study helps to break the decade-old degeneracy between gas-rich and water-rich super-Earths and sub-Neptune interior models. Our results are most consistent with small close-in planets around FGK stars being formed gas-rich with puffy hydrogen-helium envelopes, juxtaposing ocean-world models that are largely steady-state in radius over this same timeframe. For the <200 Myr population, we measure an occurrence rate for mini- and super-Neptunes of 22%(+8.6/-6.8) and 13%(+4.9/-3.9) respectively, an overabundance compared to the mature Kepler distribution at the >2.5 sigma level. This overabundance disappears at the ~500 Myr timescale (<1 sigma level) as the planet population evolves to match the observed demographics of the Kepler yield.