Implications of the TTV-Detection of Close-In Terrestrial Planets Around M Stars for Their Origin and Dynamical Evolution

TL;DR

This study investigates how close-in terrestrial planets around M stars, detectable via TTV signals, likely form at larger distances and migrate inward with giant planets, rather than forming in-situ.

Contribution

It provides simulation-based evidence supporting the idea that close-in terrestrial planets around M stars are captured during giant planet migration, challenging in-situ formation scenarios.

Findings

Slow migration (less than 1E-7 AU/year) may allow in-situ formation.

Most close-in terrestrial planets are likely formed at larger distances.

Terrestrial planets are probably captured into resonance during giant planet migration.

Abstract

It has been shown that an Earth-size planet or a super-Earth, in resonance with a transiting Jupiter-like body in a short-period orbit around an M star, can create detectable TTV signals (Kirste \& Haghighipour, 2011). Given the low masses of M stars and their circumstellar disks, it is expected that such a transiting giant planet to have formed at large distances and migrated to its close-in orbit. That implies, if such systems are discovered around M stars, the terrestrial planet had to form during the migration of the giant planet. The formation of this object may be either in-situ (in a close-in orbit) followed by its capture in resonance, or the object is formed at larger distances where it was subsequently captured in a resonance with the migrating giant planet. We have investigated these two scenarios by simulating the dynamics of a disk of protoplanetary embryos and the formation of terrestrial planets during the migration of a Jupiter-like planet around an M star. Results suggest that unless the migration of the giant planet is very slow (slower than 1E-7 AU/year), it is unlikely that the close-in terrestrial planet is formed in-situ. If a terrestrial planet is detected in a mean-motion resonance with a close-in giant planet around an M star, the terrestrial planet was most likley formed at large distances and carried to its close-in resonant orbit by the migrating giant body.