Small flow rate can supply inwardly migrating shortest-period planets

TL;DR

This paper proposes that a small but continuous inward migration of planets can explain the presence of very short-period exoplanets, challenging previous assumptions of weak tidal dissipation.

Contribution

It demonstrates that a low rate of inward planet migration can account for the observed distribution of short-period exoplanets without requiring extremely weak tidal dissipation.

Findings

A small inward flow of planets can explain the short-period planet population.

Tidal dissipation in stars may be similar to that observed in binary stars.

Depletion of the three-day pileup is consistent with ongoing inward migration.

Abstract

The number of exoplanets found with periods as short as one day and less was surprising given how fast these planets had been expected to migrate into the star due to the tides raised on the star by planets at such close distances. It has been seen as improbable that we would find planets in such a small final fraction of their lives. The favored solution has been that the tidal dissipation is much weaker than expected, which would mean that the final infall would be a larger fraction of the planets' life. We find no reason to exclude the explanation that a small number of planets are continuously sent migrating inwards such that these planets indeed are in the last fraction of their lives. Following the observation that the distribution of medium planets disfavors tidal dissipation being significantly weaker than has been found from observations of binary stars, we now show that the numbers of planets in such a "flow" of excess planets migrating inwards is low enough that even depletion of the three-day pileup is a plausible source. Then the shortest period occurrence distribution would be shaped by planets continuously being sent into the star, which may explain the depletion of the pileup in the Kepler field relative to the solar neighborhood (Howard et al. 2012, hereafter H12). Because Kepler observes above the galactic plane, H12 suggested the Kepler field may include an older population of stars. The tidal dissipation strength in stars due to giant planets may be not greatly weaker than it is in binary stars.