The California-Kepler Survey V. Peas in a Pod: Planets in a Kepler Multi-planet System are Similar in Size and Regularly Spaced

The California-Kepler Survey V. Peas in a Pod: Planets in a Kepler Multi-planet System are Similar in Size and Regularly Spaced

Authors:

Weiss et al

Abstract:
We have established precise planet radii, semi-major axes, incident stellar fluxes, and stellar masses for 909 planets in 355 multi-planet systems discovered by Kepler. We find that planets within a single multi-planet system tend to be closer in size than planets drawn randomly from the collection of multi-planet systems. This is true even when considering systems with similar host stars; we find, at most, a weak correlation between planet radius and stellar mass. Evidently, it is not the stellar mass but some other property or process that enforces the similarity of planet sizes. When adjacent planets in a multi-planet system are not similar in size, the inner planet is smaller in 65±6\% of cases. The tendency for the inner planet to be smaller is especially pronounced when the inner planet has a short period (≲10 days) or equivalently, high radiation flux (≳150 times the Earth's insolation). This could be the result of photoevaporation. We also find that adjacent planets within a given system tend to be spaced in a regular geometric progression, with a typical semi-major axis ratio of 1.5. Using empirical mass-radius relationships, we estimate the mutual Hill separations of planet pairs. We find that 93\% of the planet pairs are at least 10 mutual Hill radii apart, and that a spacing of 10-30 mutual Hill radii is most common