Exoplanets, -moons, -comets

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

By Keith Cowing

Status Report

astro-ph.EP

June 27, 2025

Filed under astro-ph.EP, astronomy, Earth-analog, exoplanet, GJ 12b, habitability, Habitable Zone, JWST, M dwarf, Spectroscopy, TRAPPIST-1

An Earth-like Density For The Temperate Earth-sized Planet GJ 12b — [L] 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 a H/He atmosphere model from M. C. Nixon & N. Madhusudhan (2021) and M. C. Nixon et al. (2024). We note that the 50% water model, which is from L. Zeng et al. (2019), both assumes a planet temperature of 300 K, which is very close to the actual equilibrium temperature of this planet. — astro-ph.EP [R] The cosmic shoreline formulation from E. K. Pass et al. (2025) for small planets orbiting mid-to-late M dwarfs within 50 pc, showing our eccentric model for GJ 12 b (black star) in escape velocity versus XUV radiation space compared to the solar system planets (various colors), TRAPPIST-1 planets (green), and other planets with Rp < 1.8 R⊕ (gray). The location of the tentative shoreline is placed based on what we know from the solar system, with Mars on the shoreline while Mercury is airless. -- astro-ph.EP

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 (T_eq ~ 300 K), Earth-sized (R_p = 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.

Madison Brady, Jacob Bean, Ritvik Basant, Nina Brown, Tanya Das, Matthew Nixon, Rafael Luque, Caroline Piaulet-Ghorayeb, Michael Radica, Andreas Seifahrt, Julian Stürmer, Lily Zhao

Comments: 23 pages, 14 figures, submitted to AJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2506.20561 [astro-ph.EP] (or arXiv:2506.20561v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2506.20561
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Submission history
From: Madison Brady
[v1] Wed, 25 Jun 2025 15:58:07 UTC (4,548 KB)
https://arxiv.org/abs/2506.20561
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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