# Masses and radii for the three super-Earths orbiting GJ 9827, and implications for the composition of small exoplanets

## TL;DR

This study refines the masses and radii of the three super-Earths orbiting GJ 9827, revealing diverse compositions that shed light on planetary formation and atmospheric retention, especially regarding the role of photo-evaporation.

## Contribution

It provides improved constraints on the masses and compositions of GJ 9827's planets, highlighting their positions relative to the radius gap and implications for planetary evolution.

## Key findings

- GJ 9827 b is likely rocky with Earth-like composition.
- GJ 9827 d probably retains a substantial volatile envelope.
- Photo-evaporation may significantly influence planetary atmospheres in this system.

## Abstract

Super-Earths belong to a class of planet not found in the Solar System, but which appear common in the Galaxy. Given that some super-Earths are rocky, while others retain substantial atmospheres, their study can provide clues as to the formation of both rocky planets and gaseous planets, and - in particular - they can help to constrain the role of photo-evaporation in sculpting the exoplanet population. GJ 9827 is a system already known to host 3 super-Earths with orbital periods of 1.2, 3.6 and 6.2 days. Here we use new HARPS-N radial velocity measurements, together with previously published radial velocities, to better constrain the properties of the GJ 9827 planets. Our analysis can't place a strong constraint on the mass of GJ 9827 c, but does indicate that GJ 9827 b is rocky with a composition that is probably similar to that of the Earth, while GJ 9827 d almost certainly retains a volatile envelope. Therefore, GJ 9827 hosts planets on either side of the radius gap that appears to divide super-Earths into pre-dominantly rocky ones that have radii below $\sim 1.5 R_\oplus$, and ones that still retain a substantial atmosphere and/or volatile components, and have radii above $\sim 2 R_\oplus$. That the less heavily irradiated of the 3 planets still retains an atmosphere, may indicate that photoevaporation has played a key role in the evolution of the planets in this system.