Testing In Situ Assembly with the Kepler Planet Candidate Sample

TL;DR

This paper develops a Monte Carlo model for in situ planetary assembly, comparing its predictions with Kepler observations, and finds good agreement in some properties but discrepancies in the number of single-planet systems.

Contribution

It introduces a new Monte Carlo simulation framework for planetary system formation via in situ assembly, incorporating observational selection effects.

Findings

Model matches observed multi-planet system frequencies

Model reproduces mutual inclination and period distributions

Underpredicts the number of single-planet systems

Abstract

We present a Monte Carlo model for the structure of low mass (total mass < 25 earth mass) planetary systems that form by the in situ gravitational assembly of planetary embryos into final planets. Our model includes distributions of mass, eccentricity, inclination and period spacing that are based on the simulation of a disk of 20 earth masses, forming planets around a solar mass star, and assuming a power law surface density distribution $\propto a^{-1.5}$. The output of the Monte Carlo model is then subjected to the selection effects that mimic the observations of a transiting planet search such as that performed by the Kepler satellite. The resulting comparison of the output to the properties of the observed sample yields an encouraging agreement in terms of the relative frequencies of multiple planet systems and the distribution of the mutual inclinations, when moderate tidal circularisation is taken into account. The broad features of the period distribution and radius distribution can also be matched within this framework, although the model underpredicts the distribution of small period ratios. This likely indicates that some dissipation is still required in the formation process. The most striking deviation between model and observations is in the ratio of single to multiple systems, in that there are roughly 50% more single planet candidates observed than are produced in any model population. This suggests that some systems must suffer additional attrition to reduce the number of planets or increase the range of inclinations.