The statistics of multi-planet systems

TL;DR

This paper develops statistical methods to measure the distribution of the number of planets around stars using transit and radial-velocity data, accounting for observational biases and system orientations.

Contribution

It introduces a framework for relating different survey data and correcting for selection effects to better understand exoplanet system multiplicities.

Findings

Kepler data alone cannot tightly constrain the mean inclination of multi-planet systems.

Comparison suggests the mean inclination is between 0-5 degrees.

Current data do not precisely determine the multiplicity function of Kepler planets.

Abstract

We describe statistical methods for measuring the exoplanet multiplicity function - the fraction of host stars containing a given number of planets - from transit and radial-velocity surveys. The analysis is based on the approximation of separability - that the distribution of planetary parameters in an n-planet system is the product of identical 1-planet distributions. We review the evidence that separability is a valid approximation for exoplanets. We show how to relate the observable multiplicity function in surveys with similar host-star populations but different sensitivities. We also show how to correct for geometrical selection effects to derive the multiplicity function from transit surveys if the distribution of relative inclinations is known. Applying these tools to the Kepler transit survey and radial-velocity surveys, we find that (i) the Kepler data alone do not constrain the mean inclination of multi-planet systems; even spherical distributions are allowed by the data but only if a small fraction of host stars contain large planet populations (> 30); (ii) comparing the Kepler and radial-velocity surveys shows that the mean inclination of multi-planet systems lies in the range 0-5 degrees; (iii) the multiplicity function of the Kepler planets is not well-determined by the present data.