# The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets

## TL;DR

This study uses precise measurements from the California-Kepler Survey to identify a significant gap in the size distribution of small, close-in exoplanets, revealing two distinct populations separated by a size gap.

## Contribution

It provides the first detailed detection of a size gap at 1.5-2.0 R⊕ in small planets, supporting a two-regime model of planet composition and formation.

## Key findings

- A factor of ≥2 deficit at 1.5-2.0 R⊕ in planet occurrence.
- Two size regimes: <1.5 R⊕ and 2.0-3.0 R⊕, with few planets in between.
- Similar intrinsic frequencies for both regimes after detection efficiency correction.

## 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.