An Earth-like Density for the Temperate Earth-sized Planet GJ 12b

TL;DR

GJ 12b is a temperate, Earth-sized exoplanet with a density similar to Earth's, making it a promising target for atmospheric studies with JWST, despite potential challenges in retaining its primary atmosphere.

Contribution

This study provides the first mass measurement of GJ 12b, revealing its Earth-like density and eccentric orbit, and discusses its suitability for atmospheric characterization.

Findings

GJ 12b has a mass of 0.71 +/- 0.12 Earth masses.

The planet's density is comparable to or less than Earth's.

GJ 12b may have a moderate eccentricity (~0.16).

Abstract

While JWST has provided us with the opportunity to probe the atmospheres of potentially-habitable planets, observations of the TRAPPIST-1 system have shown us that active stars severely complicate efforts at studying their planets. GJ 12b is a newly-discovered temperate (Teq ~ 300 K), Earth-sized (Rp = 0.96 +/- 0.05 Earth radii) planet orbiting an inactive M dwarf that might be a good alternate to the TRAPPIST-1 planets for atmospheric characterization. In this paper, we use MAROON-X radial velocities to measure a mass of 0.71 +/- 0.12 Earth masses for GJ 12b. We also find moderate evidence that the planet has an eccentric (e ~ 0.16) orbit. GJ 12b's mass results in a planetary density comparable to or less dense than Earth, possibly indicating the presence of water or a low bulk iron mass fraction. With its low mass, GJ 12b is likely within reach of JWST transmission spectroscopy observations, making it an excellent target for determining the location of the cosmic shoreline. Its low mass may mean that the planet could have trouble retaining its primary atmosphere during the star's active pre-main-sequence phase. However, if it has a heightened eccentricity, it may be able to sustain a secondary atmosphere through tidally-induced volcanism.