Gradual desiccation of rocky protoplanets from aluminum-26-heating

Tuesday 3 July, 14:00

The formation and distribution of Earth-like planets remain poorly constrained. However, the intrinsic variety of exoplanet compositions and limited water solubility in rocky planet interiors argue for numerous low-mass planets covered in impenetrable volatile layers that shut-off geochemical cycling through active interior-atmosphere exchange. So far neglected in this picture is the thermo-chemical imprint of the short-lived radionuclide aluminum-26, which provided a powerful heat-source in the early solar system, driving planetesimal melting, chemical differentiation, and volatile exsolution during accretion.

Here we show that bulk water fractions and radii of rocky planets, in general, are negatively correlated with aluminum-26 abundance in the host system as a result of their accretion history. We present models of water delivery to planets around G and M stars, considering the dehydration of icy planetesimals from radioactive heating during planetary formation. Planetary systems with greater than ~ solar aluminum-26 levels are significantly water-depleted relative to systems with negligible aluminum-26. For comparable accretion dynamics, the grade of water depletion is primarily controlled by aluminum-26-heating. 

Combined with interior structure models, our results demonstrate the sensitivity of primary (mass, radius) and secondary (composition) exoplanet observables on primordial aluminum-26 abundances. These inferences may enable near-future transit surveys to statistically distinguish enriched (= volatile-poor) from not-enriched (= volatile-rich) systems through the mean radii of low-mass planets. In addition, they provide a direct link between the star-forming environment and planet composition after accretion, and offer an explanation for the system-wide scarcity of water in the TRAPPIST-1 planets.

Submitted by