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

TL;DR

This study investigates the possibility of forming coreless terrestrial planets in the TRAPPIST-1 system, concluding that planets up to about 4 Earth masses cannot be fully oxidized to be coreless based on current models.

Contribution

It provides updated insights into oxygen partitioning during planetary differentiation, challenging the feasibility of coreless planets in the TRAPPIST-1 system based on their sizes.

Findings

TRAPPIST-1 planets are too small to be fully oxidized and form coreless planets.

Oxygen partitioning limits coreless planet formation to planets less than about 4 Earth masses.

Density deficits may be explained by element budgets linked to stellar metallicity.

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 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. For the seven TRAPPIST-1 planets, however, we find that they are not sufficiently large 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.