Limits on forming coreless terrestrial worlds in the TRAPPIST-1 system

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ID: 319053
2026
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Abstract
Abstract 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 core-free scenario where the metallic core is fully oxidised. Here, we test the validity of the core-free scenario given thermodynamic constraints. In particular, we update a metal–silicate partitioning model within the equilibrium differentiation framework. We show that during core–mantle differentiation, oxygen becomes more siderophile (iron-loving) with increasing pressure, implying larger planet radii. For the seven TRAPPIST-1 planets, however, we find that they are not sufficiently massive to oxidise all the iron in the core, if they differentiate from an Earth-like composition. Oxygen partitioning in rocky worlds thermodynamically precludes coreless planets up to ∼4 M⊕. 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.
Reference Key
openalex_W4416435403 Use this key to autocite in the manuscript while using SciMatic Manuscript Manager or Thesis Manager
Authors Dongyang Huang, Caroline Dorn
Journal monthly notices of the royal astronomical society
Year 2026
DOI
10.1093/mnras/stag1234
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