# Homogeneous Analysis of Hot Earths: Masses, Sizes, and Compositions

## TL;DR

This study conducts a uniform analysis of 11 hot Earths, revealing their compositions and masses, and suggesting an 8 Earth-mass threshold for runaway gas accretion, with most planets being Earth-like in composition.

## Contribution

It provides the first homogeneous analysis of hot Earths, combining all available data and Gaia information to refine their mass and composition estimates.

## Key findings

- Most hot Earths are consistent with Earth-like composition.
- Two planets show higher iron fractions, one has low-density volatiles.
- All planets are less massive than 8 Earth masses, indicating a critical runaway accretion threshold.

## Abstract

Terrestrial planets have been found orbiting Sun-like stars with extremely short periods --- some as short as 4 hours. These "ultra-short-period planets" or "hot Earths" are so strongly irradiated that any initial H/He atmosphere has probably been lost to photoevaporation. As such, the sample of hot Earths may give us a glimpse at the rocky cores that are often enshrouded by thick H/He envelopes on wider-orbiting planets. However, the mass and radius measurements of hot Earths have been derived from a hodgepodge of different modeling approaches, and include several cases of contradictory results. Here, we perform a homogeneous analysis of the complete sample of 11 known hot Earths with an insolation exceeding 650 times that of the Earth. We combine all available data for each planet, incorporate parallax information from {\it Gaia} to improve the stellar and planetary parameters, and use Gaussian Process regression to account for correlated noise in the radial-velocity data. The homogeneous analysis leads to a smaller dispersion in the apparent composition of hot Earths, although there does still appear to be some intrinsic dispersion. Most of the planets are consistent with an Earth-like composition (35\% iron and 65\% rock), but two planets (K2-141b and K2-229b) show evidence for a higher iron fraction, and one planet (55\,Cnc\,e) has either a very low iron fraction or an envelope of low-density volatiles. All of the planets are less massive than 8\,$M_\oplus$, despite the selection bias towards more massive planets, suggesting that 8\,$M_\oplus$ is the critical mass for runaway accretion.