Survival function analysis of planet size distribution with Gaia Data Release 2 updates

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 (R_⊕) and 10 R_⊕. These divisions place constraints on planet formation and interior structure model. The division at 4 R_⊕ separates small exoplanets from large exoplanets above. When combined with the recently discovered radius gap at 2 R_⊕, it supports the treatment of planets in between 2 and 4 R_⊕ as a separate group, likely water worlds. For planets around solar-type FGK main-sequence stars, we argue that 2 R_⊕ is the separation between water-poor and water-rich planets, and 4 R_⊕ is the separation between gas-poor and gas-rich planets. We confirm the slope of the survival function in between 4 and 10 R_⊕ to be shallower compared to either ends, indicating a relative paucity of planets in between 4 and 10 R_⊕, 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 R_⊕) rocky planets, (2-4 R_⊕) water worlds, (4-10 R_⊕) transitional planets, and (>10 R_⊕) gas giants.