干旱区生态环境知识资源中心

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