Gregor Golabek — 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 and order of-magnitude deviations
in exoplanet volatile contents suggest that rocky worlds engulfed in thick
volatile ice layers are the dominant family of terrestrial analogues among
the extrasolar planet population.
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 26Al levels in the planetesimal-based
accretion framework. The heat generated by this short-lived radionuclide
rapidly dehydrates planetesimals before their accretion onto larger
protoplanets and yields a system-wide correlation 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.
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.