The mass-period distribution of close-in exoplanets

TL;DR

This study investigates the distribution of close-in exoplanets' masses and periods, proposing that stellar tides and disk interactions explain the observed lower period limit and the mass-dependent distribution of these planets.

Contribution

The paper combines hydrodynamical simulations and tidal evolution analysis to explain the observed mass-period distribution of close-in exoplanets, highlighting the role of disk edges and stellar tides.

Findings

Most close-in exoplanets are consistent with a disk inner edge at P>2 days.

Planets more massive than Jupiter likely undergo type II migration and halt near the 2/1 resonance.

Smaller planets remain trapped at the disk cavity edge, with CoRoT-7b requiring additional evolutionary explanations.

Abstract

The lower limit to the distribution of orbital periods P for the current population of close-in exoplanets shows a distinctive discontinuity located at approximately one Jovian mass. Most smaller planets have orbital periods longer than P~2.5 days, while higher masses are found down to P~1 day. We analyze whether this observed mass-period distribution could be explained in terms of the combined effects of stellar tides and the interactions of planets with an inner cavity in the gaseous disk. We performed a series of hydrodynamical simulations of the evolution of single-planet systems in a gaseous disk with an inner cavity mimicking the inner boundary of the disk. The subsequent tidal evolution is analyzed assuming that orbital eccentricities are small and stellar tides are dominant. We find that most of the close-in exoplanet population is consistent with an inner edge of the protoplanetary disk being located at approximately P>2 days for solar-type stars, in addition to orbital decay having been caused by stellar tides with a specific tidal parameter on the order of Q'*=10^7. The data is broadly consistent with planets more massive than one Jupiter mass undergoing type II migration, crossing the gap, and finally halting at the interior 2/1 mean-motion resonance with the disk edge. Smaller planets do not open a gap in the disk and remain trapped in the cavity edge. CoRoT-7b appears detached from the remaining exoplanet population, apparently requiring additional evolutionary effects to explain its current mass and semimajor axis.