科研成果 by Year: 2026

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 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.

Earth’s core has long been speculated to be the largest reservoir of hydrogen (H) on the planet. However, current estimates of its H content involve substantial uncertainties, due to the challenge of quantifying H under extreme conditions. Here, we perform superliquidus metal-silicate partitioning experiments on H using laser-heated diamond anvil cells, and combine it with atom probe tomography. The direct observation of H at silicon- and oxygen-rich nanostructures in the iron alloy indicates coupled sequestration of silicon, oxygen and hydrogen into Earth’s core during its formation. With the observed molar Si/H ratio close to unity, Earth’s core is estimated to contain 0.07-0.36 wt.% H, equivalent to 9-45 oceans of water. Such an amount would require the Earth to obtain the majority of its water from the main stages of terrestrial accretion, instead of through comets during late addition.

Volatile elements are essential for life development and planetary evolution. However, the timing of their delivery to terrestrial planets remains unclear. Sulfur, selenium, and tellurium are volatiles, but also siderophile elements. Their abundances in Earth’s mantle can be used to determine whether volatile elements were delivered to Earth during or after the segregation of the core. Here, we experimentally measured their partition coefficients between core-forming metal and mantle silicate under pressure, temperature, and oxygen fugacity conditions relevant to a deep magma ocean. Our results show that these elements exhibit similar partitioning behaviors, indicating that core-mantle equilibrium preserves their chondritic relative abundances. If a volatile-rich late veneer has been delivered to Earth after core segregation, it must have been limited in mass, making up a maximum of 0.1% Earth’s mass. This suggests that volatile elements, including water, were accreted continuously during Earth’s growth rather than being delivered predominantly by a late veneer of volatile-rich material such as carbonaceous chondrites.