TRAPPIST-1

Limits On Forming Coreless Terrestrial Worlds In The TRAPPIST-1 System

By Keith Cowing
Status Report
astro-ph.EP
November 4, 2025
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Limits On Forming Coreless Terrestrial Worlds In The TRAPPIST-1 System
Core–mantle partition coefficient of oxygen as a function of pressure and temperature. Left: Experimental data for metal–silicate partitioning of O obtained at high pressures up to 104 GPa. Horizontal dashed line located at DO=0.83 indicates full oxidation of the core (see the main text), and therefore distinguishes ‘terrestrial’ from ‘coreless’ planets. It is clear that, within the investigated pressure range, DO asymptotically approaches the dashed line, but never reaches the core-free regime, albeit higher pressures and temperatures promote the sequestration of O into the metallic core, i.e. larger DO. Right: The same dataset for O partitioning plotted as a function of reciprocal temperature. Dashed lines are fitted results (Eq. 7) showing the dependence of O partitioning on temperature at a given pressure or vice versa. — astro-ph.EP

With seven temperate Earth-sized planets revolving around an ultracool red dwarf, the nearby TRAPPIST-1 system offers a unique opportunity to verify models of exoplanet composition, differentiation, and interior structure.

In particular, the low bulk densities of the TRAPPIST-1 planets, compared to terrestrial planets in our solar system, require either substantial amount of volatiles to be present or a corefree scenario where the metallic core is fully oxidised. Here, using an updated metal-silicate partitioning model, we show that during core-mantle differentiation oxygen becomes more siderophile (iron-loving) implying larger planet radii.

arge to oxidise all the iron in the core, if they differentiate from an Earth-like composition. Oxygen partitioning in rocky worlds precludes coreless planets up to about 4 Earth masses. The observed density deficit in the TRAPPIST-1 planets, and more generally in M dwarf systems if confirmed by future observations, may be explained by system-dependent element budgets during planet formation, which are intrinsically linked to their stellar metallicity.

Dongyang Huang, Caroline Dorn

Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2511.01231 [astro-ph.EP] (or arXiv:2511.01231v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2511.01231
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Submission history
From: Dongyang Huang
[v1] Mon, 3 Nov 2025 05:14:50 UTC (1,607 KB)
https://arxiv.org/abs/2511.01231

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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