Limits on forming coreless terrestrial worlds in the TRAPPIST-1 system
Year: 2026
Type: article
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 oxidized. 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 oxidize 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 $\sim$4 M$_{\oplus }$. 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.
Authors: Dongyang Huang, Caroline Dorn
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $3,046
OpenAlex ID: W4416435403