A Thermochemical Origin for the Diversity of Ice-to-Rock Ratios in the LHS 1903 System

The recently discovered four-planet system around LHS 1903 exhibits a remarkable diversity in inferred water mass fractions and bulk densities, suggesting strongly contrasting ice-to-rock ratios among its planets [1]. Interior structure models indicate that the innermost planet, LHS 1903 b, is predominantly rocky and water-poor, while planets c and d appear substantially volatile-rich and may contain up to ~25-33 wt% water. The outermost planet, LHS 1903 e, appears mostly rocky but may still contain a moderate water fraction (down to 7%). Such diversity raises important questions regarding the origin and transport of water-bearing material during the formation of the system.

Here we investigate whether the diversity of ice-to-rock ratios inferred in the LHS 1903 system can be explained using a formation framework analogous to those recently proposed for the Galilean moons and the TRAPPIST-1 system [2, 3]. In these scenarios, hydrated phyllosilicate-rich particles drift inward through the protoplanetary disk and dehydrate once they cross the phyllosilicate dehydration line (PDL), which is at 500-600 K in the protoplanetary disk. The released water vapor diffuses both inward and outward before recondensing beyond the snow line, thereby enriching solids in water ice.

We adapt this framework to the physical conditions expected in the LHS 1903 protoplanetary disk and investigate whether the inferred planetary compositions can be reproduced through a combination of thermal dehydration, vapor redistribution, ice recondensation, and planetary migration. Preliminary calculations suggest that the volatile-rich planets c and d accreted near the snow line from solids enriched by outwardly diffusing water vapor, whereas planet b likely formed interior to the PDL from dry or partially dehydrated material. In contrast, planet e may have formed beyond the snow line and accreted primarily from hydrated but only weakly processed minerals. This interpretation is consistent with its inferred water mass fraction (~7–28%), which overlaps the typical ∼5–20 wt% water contents expected for hydrated silicates and aqueously altered materials. The resulting formation sequence naturally explains the large diversity in water mass fractions observed within the compact architecture of the LHS 1903 system.

This work suggests that the diversity of ice-to-rock ratios observed in compact exoplanetary systems may arise from local thermochemical processing of hydrated rocks within protoplanetary disks rather than requiring the direct inward transport of primordial icy planetesimals from distant cold reservoirs. The LHS 1903 system may therefore represent another exoplanetary analog of the compositional gradients observed among the Galilean satellites and TRAPPIST-1 planets.

References

[1] Wilson, T.G., et al. 2026. Science, DOI: 10.1126/science.adl2348

[2] Mousis O., Schneeberger A., Lunine J.I., Glein C.R., Bouquet A., Vance S.D., 2023, ApJL, 944, L37. doi:10.3847/2041-8213/acb5a4

[3] Schneeberger A., Mousis O., Deleuil M., Lunine J.I., 2024, A&A, 682, L10. doi:10.1051/0004-6361/202348309