@article {41284,
	title = {Limits on forming coreless terrestrial worlds in the TRAPPIST-1 system},
	journal = {Monthly Notices of the Royal Astronomical Society},
	volume = {550},
	year = {2026},
	pages = {stag1234},
	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{\textendash}silicate partitioning model within the equilibrium differentiation framework. We show that during core{\textendash}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 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.},
	url = {https://academic.oup.com/mnras/article/550/2/stag1234/8719665},
	author = {Huang, Dongyang and Caroline Dorn}
}
