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Rapid formation of exoplanetesimals revealed by white dwarfs

Abstract

The timing of formation of the first planetesimals determines the mode of planetary accretion and their geophysical and compositional evolution. Astronomical observations of circumstellar disks and Solar System geochronology provide evidence for planetesimal formation during molecular cloud collapse, much earlier than previously estimated. Here we present distinct observational evidence from white dwarf planetary systems for planetesimal formation occurring during the first few hundred thousand years after cloud collapse in exoplanetary systems. A substantial fraction of white dwarfs have accreted planetary material rich in iron core or mantle material. For the exo-asteroids accreted by white dwarfs to form iron cores, substantial heating is required. By simulating planetesimal evolution and collisional evolution, we show that the most likely heat source is short-lived radioactive nuclides such as 26Al (which has a half-life of ~0.7 Myr). Core-rich materials in the atmospheres of white dwarfs, therefore, provide independent evidence for rapid planetesimal formation, concurrent with star formation.

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Fig. 1: Enrichment in Fe, Ni and Cr relative to Ca, Mg and Si of planetary materials accreted by white dwarfs suggests the accretion of core- or mantle-rich material.
Fig. 2: Almost all planetesimals that undergo core–mantle differentiation form within the Class 0/I collapse phase in exoplanetary systems with plausible levels of 26Al enrichment.
Fig. 3: The core- or mantle-rich materials in the atmospheres of white dwarfs are the collision fragments of planetesimals that formed earlier than ~1 Myr, when large-scale melting was fuelled by the decay of 26Al.
Fig. 4: Plutos can be the source of core-rich planetesimal debris only in rare (<1%) white dwarf systems with massive, close-in planetesimal belts.

Data availability

The data used to create Figs. 1–4 are available in the Supplementary Information; the white dwarf data (Sample Two) are provided in Supplementary Tables 1–3, while data for Sample One can be found in ref. 26. Source data are provided with this paper.

Code availability

The code used to create Figs. 1–4 and the collisional evolution code is available at https://github.com/abonsor/collcascade and models for Fig. 2 are available at https://github.com/timlichtenberg/2stage_scripts_data.

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Acknowledgements

A.B. acknowledges support from a Royal Society Dorothy Hodgkin Research Fellowship (grant number DH150130) and a Royal Society University Research Fellowship (grant number URF\R1\211421). T.L. was supported by a grant from the Simons Foundation (SCOL award number 611576). J.D. acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme under grant agreement number 714769. A.M.B. acknowledges support from a Royal Society funded PhD studentship (grant number RGFEA180174). We acknowledge fruitful discussions with M. Brouwers, L. Rogers, E. Lynch, A. Curry, T. Birnstiel, M. Wyatt and R. J. Parker.

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The idea for the study came from discussions between A.B., J.D. and T.L. The analysis of the white dwarf data was performed by A.M.B. and T.L. supplied the thermal evolution models used for Fig. 2. All authors contributed to writing the manuscript.

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Correspondence to Amy Bonsor.

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Bonsor, A., Lichtenberg, T., Dra̧żkowska, J. et al. Rapid formation of exoplanetesimals revealed by white dwarfs. Nat Astron (2022). https://doi.org/10.1038/s41550-022-01815-8

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