Survival Function Analysis of Planet Size Distribution with GAIA Data Release 2 Updates

TL;DR

This study uses survival function analysis on Kepler data to identify natural divisions in planet sizes, revealing insights into planet formation, structure, and classification, including the identification of a sub-Saturnian desert.

Contribution

It introduces a new classification scheme for exoplanets based on size divisions supported by survival function analysis of Kepler data.

Findings

Identified two natural divisions at 4 and 10 Earth radii.

Supported the separation of water worlds and gas-rich planets.

Confirmed a paucity of planets between 4 and 10 Earth radii.

Abstract

Applying the survival function analysis to the planet radius distribution of the Kepler confirmed/candidate planets, we have identified two natural divisions of planet radius at 4 Earth radii and 10 Earth radii. These divisions place constraints on planet formation and interior structure model. The division at 4 Earth radii separates small exoplanets from large exoplanets above. When combined with the recently-discovered radius gap at 2 Earth radii, it supports the treatment of planets 2-4 Earth radii as a separate group, likely water worlds. For planets around solar-type FGK main-sequence stars, we argue that 2 Earth radii is the separation between water-poor and water-rich planets, and 4 Earth radii is the separation between gas-poor and gas-rich planets. We confirm that the slope of survival function in between 4 and 10 Earth radii to be shallower compared to either ends, indicating a relative paucity of planets in between 4-10 Earth radii, namely, the sub-Saturnian desert there. We name them transitional planets, as they form a bridge between the gas-poor small planets and gas giants. Accordingly, we propose the following classification scheme: (<2 Earth radii) rocky planets, (2-4 Earth radii) water worlds, (4-10 Earth radii) transitional planets, and (>10 Earth radii) gas giants.