Exoplanets, -moons, -comets

An Earthlike Density For The Temperate Earth-sized Planet GJ 12 b

By Keith Cowing

Status Report

The Astronomical Journal

November 20, 2025

Filed under atmosphere, exoplanet, GJ 12 b, habitability, Habitable Zone, imaging, JWST, M dwarf, MAROON-X, Spectroscopy, The Astronomical Journal, TRAPPIST-1

An Earthlike Density For The Temperate Earth-sized Planet GJ 12 b — The mass and radius of our two models for GJ 12 b (circular and eccentric orbits) compared to various composition models from L. Zeng et al. (2019), as well as a steam atmosphere model from M. Turbet et al. (2020) and an H/He atmospheric model from M. C. Nixon & N. Madhusudhan (2021) and M. C. Nixon et al. (2024). The star corresponding to the eccentric-orbit model has been shifted slightly upward so that the errors of the two points are distinguishable. We note that the 50% water model, which is from L. Zeng et al. (2019), assumes a planet temperature of 300 K, which is very close to the actual equilibrium temperature of this planet. — The Astronomical Journal

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 12 b is a newly discovered temperate (T_eq ≈ 300 K), Earth-sized (R_p = 0.96 ± 0.05 R⊕) 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 0.70 ± 0.10 M⊕ for GJ 12 b.

We also recover a planetary eccentricity of 0.24, but there is no strong evidence that the planet has an eccentric orbit. GJ 12 b’s mass results in a planetary density comparable to or less dense than that of Earth, possibly indicating the presence of water or a low bulk iron mass fraction.

With its low mass, GJ 12 b 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.

An Earthlike Density for the Temperate Earth-sized Planet GJ 12 b, The Astronomical Journal (open access)
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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