Does the innermost occurrence distribution measure tidal dissipation, reveal a flow of giant planets, or both?

TL;DR

This paper investigates whether the distribution of short-period giant exoplanets reflects tidal dissipation, planetary migration, or both, by comparing observed distributions with theoretical models to constrain tidal friction levels.

Contribution

It introduces a method to constrain tidal dissipation using occurrence distributions and star-planet age data, revealing unexpectedly low tidal dissipation for giant planets.

Findings

Short-period giant planets show a low level of tidal dissipation.

The distribution suggests a continuous resupply of migrating giant planets.

Giant planets are more likely to undergo gradual inward migration.

Abstract

The occurrence distribution of the shortest period giant exoplanets as found by Kepler show a drop-off that is a remarkable match to the drop-off expected by taking migration due to tides in the star. We present a comparison that can show the level of tidal dissipation (friction) as a function of the distribution of the ages of the star and planet system, with known dependencies on basic star and planet parameters. Use of this relation enables constraints to be put on the value of the tidal dissipation, constraints that will be improved as the distribution of the ages are determined. For the giant planets, this leads to an unexpectedly low value of tidal dissipation. This over-abundance of short period giant planets may be due to a continuing resupply of longer period giant planets migrating into a shorter period pileup, disrupting the presence of smaller planets along the way. Perhaps the occurrence distribution of close Neptune sized planets will better measure the tidal friction, while the distribution of Jupiter sized planets reveals that giant planets are more likely to complete a gradual migration into the star.