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

Lichtenberg, Tim; Golabek, Gregor J.; Burn, Remo; Meyer, Michael R.; Alibert, Yann; Gerya, Taras V.; Mordasini, Christoph (2019). A water budget dichotomy of rocky protoplanets from ²⁶Al-heating. Nature astronomy, 3(4), pp. 307-313. Nature Publishing Group 10.1038/s41550-018-0688-5

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In contrast to the water-poor inner solar system planets, stochasticity during planetary formation (Alibert & Benz 2017; Raymond& Izidoro 2017) and order of magnitude deviations in exoplanet volatile contents (Kaltenegger 2017) suggest that rocky worldsengulfed in thick volatile ice layers (Kuchner 2003; Léger et al. 2004) are the dominant family of terrestrial analogues (Tian & Ida 2015; Ramirez & Levi 2018) among the extrasolar planet population. However, the distribution of compositionally Earth-like planetsremains insufficiently constrained, and it is not clear whether the solar system is a statistical outlier or can be explained by moregeneral planetary formation processes. Here we employ numerical models of planet formation, evolution, and interior structure, toshow that a planet’s bulk water fraction and radius are anti-correlated with initial ²⁶Al levels in the planetesimal-based accretionframework. The heat generated by this short-lived radionuclide rapidly dehydrates planetesimals (Grimm & McSween 1993) priorto accretion onto larger protoplanets and yields a system-wide correlation (Millholland et al. 2017; Weiss et al. 2018) of planet bulkabundances, which, for instance, can explain the lack of a clear orbital trend in the water budgets of the TRAPPIST-1 planets (Dornet al. 2018). Qualitatively, our models suggest two main scenarios of planetary systems’ formation: high-²⁶Al systems, like our solarsystem, form small, water-depleted planets, whereas those devoid of ²⁶Al predominantly form ocean worlds, where the mean planetradii between both scenarios deviate by up to≈10%.

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