A water budget dichotomy of rocky protoplanets from 26Al-heating

In contrast to the water-poor planets of the inner Solar System, stochasticity during planetary formation^1,2 and order-of-magnitude deviations in exoplanet volatile contents³ suggest that rocky worlds engulfed in thick volatile ice layers^4,5 are the dominant family of terrestrial analogues^6,7 among the extrasolar planet population. However, the distribution of compositionally Earth-like planets remains insufficiently constrained³, and it is not clear whether the Solar System is a statistical outlier or can be explained by more general planetary formation processes. Here we use numerical models of planet formation, evolution and interior structure to show that a planet's bulk water fraction and radius are anti-correlated with initial ²⁶Al levels in the planetesimal-based accretion framework. The heat generated by this short-lived radionuclide rapidly dehydrates planetesimals⁸ before their accretion onto larger protoplanets and yields a system-wide correlation^9,10 of planetary bulk water abundances, which, for instance, can explain the lack of a clear orbital trend in the water budgets of the TRAPPIST-1 planets¹¹. Qualitatively, our models suggest two main scenarios for the formation of planetary systems: high-²⁶Al systems, like our Solar System, form small, water-depleted planets, whereas those devoid of ²⁶Al predominantly form ocean worlds. For planets of similar mass, the mean planetary transit radii of the ocean planet population can be up to about 10% larger than for planets from the ²⁶Al-rich formation scenario.