On the origin of dynamically isolated hot Earths

TL;DR

This paper proposes that hot Earths are remnants of giant planets stripped of their gaseous envelopes after high-eccentricity migration near their Roche limits, explaining their distribution and occurrence rates.

Contribution

It introduces a new model linking hot Earths to high-eccentricity migration and tidal stripping of giant planets, providing a unified explanation for their properties and distribution.

Findings

Hot Earths likely originate from tidally stripped giant planets.

The model explains the spatial distribution of hot Earths.

It accounts for the similarity in occurrence rates between hot Earths and hot Jupiters.

Abstract

A distinct population of planetary systems that contain dynamically isolated, Earth-size planets with orbital periods $P_\mathrm{orb}\sim 1\,$day was recently identified in an analysis of data from the Kepler planet candidate catalog. We argue that these objects could represent the remnant rocky cores of giant planets that arrived at the stellar vicinity on high-eccentricity orbits and were rapidly stripped of their gaseous envelopes after crossing their respective Roche limits (RLs) $a_\mathrm{R,p}$. In this picture, objects with $P_\mathrm{orb}\gtrsim1\,$day are mostly "early" cores that originated in planets with an initial periastron distance $a_\mathrm{per,0}\le a_\mathrm{R,p}$; they had high initial eccentricities but their orbits underwent fast tidal circularization after the cores were exposed. Objects with $P_\mathrm{orb}\lesssim 1\,$day are, by contrast, mostly "late" cores that originated in planets with $a_\mathrm{per,0}>a_\mathrm{R,p}$; these planets underwent orbital circularization to a radius $>a_\mathrm{per,0}$ but eventually reached $a_\mathrm{R,p}$ through tidal orbital decay. This picture naturally accounts for the spatial distribution of hot Earths and for the similarity of their inferred occurrence rate to that of hot Jupiters, and it fits well with the interpretation of the so-called sub-Jovian desert in the orbital-period--planetary-mass plane in terms of high-eccentricity planet migration to the vicinity of the RL.