Outcomes and Duration of Tidal Evolution in a Star-Planet-Moon System

TL;DR

This paper develops a model for the tidal decay lifetimes of moons orbiting extrasolar planets, accounting for lunar tides, and finds that moons are more stable further from stars, around heavier planets, or lighter stars.

Contribution

It introduces a new formulation that includes lunar tides, enabling analysis of larger moons around Neptune-mass and super-Earth planets, extending previous models.

Findings

Moons are more stable further from their stars.

Including lunar tides extends moon lifetime estimates.

First exomoon around a G-type star likely at 0.4-0.6 AU.

Abstract

We formulated tidal decay lifetimes for hypothetical moons orbiting extrasolar planets with both lunar and stellar tides. Previous work neglected the effect of lunar tides on planet rotation, and are therefore applicable only to systems in which the moon's mass is much less than that of the planet. This work, in contrast, can be applied to the relatively large moons that might be detected around newly-discovered Neptune-mass and super-Earth planets. We conclude that moons are more stable when the planet/moon systems are further from the parent star, the planets are heavier, or the parent stars are lighter. Inclusion of lunar tides allows for significantly longer lifetimes for a massive moon relative to prior formulations. We expect that the semi-major axis of the planet hosting the first detected exomoon around a G-type star is 0.4-0.6 AU and is 0.2-0.4 AU for an M-type star.