Survival Function Analysis of Planet Size Distribution

TL;DR

This study uses survival function analysis on Kepler exoplanet data to identify natural divisions in planet sizes, proposing a new classification scheme based on radius thresholds linked to planetary composition and formation models.

Contribution

It introduces a novel application of survival function analysis to classify exoplanets into four size-based categories with implications for understanding planet formation.

Findings

Identified natural divisions at 2, 4, and 10 Earth radii.

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

Proposed a new four-category classification scheme for exoplanets.

Abstract

Applying the survival function analysis to the planet radius distribution of the Kepler exoplanet candidates, 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. Thus, 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.