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 formation1,2 and order-of-magnitude deviations in exoplanet volatile contents3 suggest that rocky worlds engulfed in thick volatile ice layers4,5 are the dominant family of terrestrial analogues6,7 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 correlation9,10 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.

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Fig. 1: Dehydration of icy planetesimals from 26Al-heating and resulting influence on planet water abundance.
Fig. 2: Gradual desiccation of protoplanets as a function of 26Al0 for planets with \(f_{{\mathrm{H}}_{\mathrm{2}}{\mathrm{O}}} > 0\).
Fig. 3: Qualitative sketch of the effects of 26Al enrichment on planetary accretion.
Fig. 4: Shift in mean transit radii between planetary populations with varying 26Al enrichment.

Code and data availability

The data that support the plots within this paper, precompiled versions of the custom computer codes used, and other findings of this study are available from the corresponding author upon reasonable request.


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The authors thank R. Parker for helping to initiate this project, E. Kite, T. Roger and C. Dorn for comments and discussions, and M. Bizzarro for comments that helped to improve the manuscript. T.L. was supported by ETH Zürich Research Grant ETH-17 13-1 and acknowledges partial financial support from the Swiss Society for Astrophysics and Astronomy through a MERAC travel grant. Y.A. acknowledges support from the Swiss National Science Foundation (SNSF). C.M. acknowledges support from the SNSF under grant BSSGI0_155816 ‘PlanetsInTime’. The numerical simulations in this work were partially performed on the EULER computing cluster of ETH Zürich. Parts of this work have been carried out within the framework of the National Center of Competence in Research PlanetS supported by the SNSF.

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T.L., G.J.G., M.R.M. and Y.A. initiated the project. T.L. and R.B. performed the theoretical calculations. T.V.G., Y.A. and C.M. led the development of the computer codes used. T.L. wrote the manuscript. All authors contributed to the discussion and the interpretation of the results.

Correspondence to Tim Lichtenberg.

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Lichtenberg, T., Golabek, G.J., Burn, R. et al. A water budget dichotomy of rocky protoplanets from 26Al-heating. Nat Astron 3, 307–313 (2019). https://doi.org/10.1038/s41550-018-0688-5

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