  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.
