Barnard's Star

The Barnard’s Star Planetary System: Stability, Composition, and Evolution of Four Sub-Earth Exoplanets

By Keith Cowing
Status Report
astro-ph.EP
June 24, 2026
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The Barnard’s Star Planetary System: Stability, Composition, and Evolution of Four Sub-Earth Exoplanets
Atmosphere loss of Barnard’s Star planets, under various assumptions of planet masses, bulk densities, and initial atmospheric mass fraction. The mass range tested for a given planet is between its minimum mass (based on RV data; B25) and 2.5 times the minimum mass (see Section 2). The bulk densities 𝜌, used to calculate planet radius in the evolution models, span a liberal range: the Solar System rocky planets have densities between 3.93 g cm−3 (Mars) and 5.51 g cm−3 (Earth). No matter what assumptions are made, the planets are completely stripped of their atmospheres within 2 Gyr. The atmospheres survive longer if the planets are more massive, denser, farther from the star, and begin with a larger atmospheric mass fraction. — astro-ph.EP

Barnard’s Star is the nearest single star to the Sun (1.8 pc), and hosts four recently-discovered planets.

The star also has well-characterized stellar abundances of important rock-forming elements, including Fe, Mg, and Si. For refractory elements like these, the planets have likely inherited similar bulk elemental abundance ratios to the star, facilitating modelling of their interior structures.

We present here an analysis of the Barnard’s Star planetary system on several fronts. We perform a detailed stability analysis of the system, ascertaining that all four planets likely have masses between 0.19 and 0.84 M⊕, and are likely tidally locked, whereas a 4:3 mean-motion resonance chain for the inner three planets cannot be ruled out.

Using atmospheric evolution models, we show that the prospect of extant primary atmospheres is highly unlikely on any of the planets. Barnard’s Star’s abnormally high Mg/Si ratio and low Th/Mg ratio imply planetary mantles which (a) are rich in (Mg,Fe)O ferropericlase; (b) have less than half the water capacity as Earth; (c) generate about half of the radiogenic heating as Earth; and (d) are cool and unlikely to have outgassed secondary atmospheres.

Our analysis of this system presents an accessible set of first steps for the study of other nearby exoplanetary systems, as well as sub-Earth planets which will be increasingly discovered over the coming years.

Xander Byrne, Claire Marie Guimond, Amy Bonsor, Haiyang S. Wang, Sophia R. Vaughan, James G. Rogers

Comments: Accepted for publication in MNRAS 16 Jun 2026. 17 pages, 7 figures, 6 tables
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2606.23310 [astro-ph.EP](or arXiv:2606.23310v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2606.23310
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Submission history
From: Xander Byrne
[v1] Mon, 22 Jun 2026 13:24:05 UTC (2,718 KB)
https://arxiv.org/abs/2606.23310
Astrobiology, exoplanet,

Keith Cowing

Biologist, Explorers Club Fellow, ex-NASA Space Biologist and Payload integrator, Editor of NASAWatch.com and Astrobiology.com, Lapsed climber, Explorer, Synaesthete, Former Challenger Center board member 🖖🏻

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