Possible Outcomes of Coplanar High-eccentricity Migration: Hot Jupiters, Close-in Super-Earths, and Counter-orbiting Planets

TL;DR

This paper explores the formation of close-in planets through coplanar high-eccentricity migration in hierarchical triple systems, considering finite mass effects, short-range forces, and tidal effects, and compares results with observed planetary systems.

Contribution

It generalizes previous CHEM models by including finite mass effects and short-range forces, and extends the analysis to super-Earths and observed systems.

Findings

Most systems end in tidal disruption or produce prograde hot Jupiters.

CHEM can explain some hierarchical triple systems but struggles with counter-orbiting hot Jupiters.

Formation of close-in super-Earths via CHEM is also possible.

Abstract

We investigate the formation of close-in planets in near-coplanar eccentric hierarchical triple systems via the secular interaction between an inner planet and an outer perturber (Coplanar High-eccentricity Migration, CHEM). We generalize the previous work on the analytical condition for successful CHEM for point masses interacting only through gravity by taking into account the finite mass effect of the inner planet. We find that efficient CHEM requires that the systems should have m_1<<m_0 and m_1<<m_2. In addition to the gravity for point masses, we examine the importance of the short-range forces, and provide an analytical estimate of the migration time scale. We perform a series of numerical simulations in CHEM for systems consisting of a sun-like central star, giant gas inner planet and planetary outer perturber, including the short-range forces and stellar and planetary dissipative tides. We find that most of such systems end up with a tidal disruption, a small fraction of the systems produce prograde hot Jupiters (HJs), but no retrograde one. In addition, we extend CHEM to super-Earth mass range, and show that the formation of close-in super-Earths in prograde orbits is also possible. Finally, we carry out CHEM simulation for the observed hierarchical triple and counter-orbiting HJ systems. We find that CHEM can explain a part of the former systems, but it is generally very difficult to reproduce counter-orbiting HJ systems.