Title: The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets,
Abstract: The size of a planet is an observable property directly connected to the
physics of its formation and evolution. We used precise radius measurements
from the California-Kepler Survey (CKS) to study the size distribution of 2025
$\textit{Kepler}$ planets in fine detail. We detect a factor of $\geq$2 deficit
in the occurrence rate distribution at 1.5-2.0 R$_{\oplus}$. This gap splits
the population of close-in ($P$ < 100 d) small planets into two size regimes:
R$_P$ < 1.5 R$_{\oplus}$ and R$_P$ = 2.0-3.0 R$_{\oplus}$, with few planets in
between. Planets in these two regimes have nearly the same intrinsic frequency
based on occurrence measurements that account for planet detection
efficiencies. The paucity of planets between 1.5 and 2.0 R$_{\oplus}$ supports
the emerging picture that close-in planets smaller than Neptune are composed of
rocky cores measuring 1.5 R$_{\oplus}$ or smaller with varying amounts of
low-density gas that determine their total sizes.