Long-term tidal evolution of short-period planets with companions

TL;DR

This paper investigates how undetected low-mass companions can sustain the eccentricity of short-period exoplanets, influencing their radii and potential habitability, by analyzing tidal interactions and orbital decay timescales.

Contribution

It quantifies the conditions under which low-mass companions can maintain eccentricity in short-period planets over system lifetimes, highlighting their role in planetary inflation and habitability.

Findings

Low-mass companions as small as a fraction of Earth mass can sustain eccentricity.

Eccentricity decay timescales depend on the interior structure of the planet.

Systems with inflated planets are promising targets for terrestrial planet searches.

Abstract

Of the fourteen transiting extrasolar planetary systems for which radii have been measured, at least three appear to be considerably larger than theoretical estimates suggest. It has been proposed by Bodenheimer, Lin & Mardling that undetected companions acting to excite the orbital eccentricity are responsible for these oversized planets, as they find new equilibrium radii in response to being tidally heated. In the case of HD 209458, this hypothesis has been rejected by some authors because there is no sign of such a companion at the 5 m/s level, and because it is difficult to say conclusively that the eccentricity is non-zero. Transit timing analysis [...]. Whether or not a companion is responsible for the large radius of HD 209458b, almost certainly some short-period systems have companions which force their eccentricities to nonzero values. This paper is dedicated to quantifying this effect. The eccentricity of a short-period planet will only be excited as long as its (non-resonant) companion's eccentricity is non-zero. Here we show that the latter decays on a timescale which depends on the structure of the interior planet, a timescale which is often shorter than the lifetime of the system. This includes Earth-mass planets in the habitable zones of some stars. We determine which configurations are capable of sustaining significant eccentricity for at least the age of the system, and show that these include systems with companion masses as low as a fraction of an Earth mass. The orbital parameters of such companions are consistent with recent calculations which show that the migration process can induce the formation of low mass planets external to the orbits of hot Jupiters. Systems with inflated planets are therefore good targets in the search for terrestrial planets.