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 orderof- magnitude deviations in exoplanet volatile contents(3) 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 constrained(3), 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 anticorrelated with initial Al-26 levels in the planetesimal-based accretion framework. The heat generated by this short-lived radionuclide rapidly dehydrates planetesimals(8) 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 planets(11). Qualitatively, our models suggest two main scenarios for the formation of planetary systems: high-Al-26 systems, like our Solar System, form small, water-depleted planets, whereas those devoid of Al-26 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 Al-26-rich formation scenario.