An Earth-sized exoplanet with a Mercury-like composition

A. Santerne, B. Brugger, D. J. Armstrong, V. Adibekyan, J. Lillo-Box, H. Gosselin, A. Aguichine, J.-M. Almenara, D. Barrado, S. C. C. Barros, D. Bayliss, I. Boisse, A. S. Bonomo, F. Bouchy, D. J. A. Brown, M. Deleuil, E. Delgado Mena, O. Demangeon, R. F. Díaz, A. Doyle, X. Dumusque, F. Faedi, J. P. Faria, P. Figueira, E. Foxell, H. Giles, G. Hébrard, S. Hojjatpanah, M. Hobson, J. Jackman, G. King, J. Kirk, K. W. F. Lam, R. Ligi, C. Lovis, T. Louden, J. McCormac, O. Mousis, J. J. Neal, H. P. Osborn, F. Pepe, D. Pollacco, N. C. Santos, S. G. Sousa, S. Udry, A. Vigan
(Submitted on 22 May 2018)

The Earth, Venus, Mars, and some extrasolar terrestrial planets have a mass and radius that is consistent with a mass fraction of about 30% metallic core and 70% silicate mantle. At the inner frontier of the solar system, Mercury has a completely different composition, with a mass fraction of about 70% metallic core and 30% silicate mantle. Several formation or evolution scenarios are proposed to explain this metal-rich composition, such as a giant impact, mantle evaporation, or the depletion of silicate at the inner-edge of the proto-planetary disk. These scenarios are still strongly debated. Here we report the discovery of a multiple transiting planetary system (K2-229), in which the inner planet has a radius of 1.165+/-0.066 Rearth and a mass of 2.59+/-0.43 Mearth. This Earth-sized planet thus has a core-mass fraction that is compatible with that of Mercury, while it was expected to be similar to that of the Earth based on host-star chemistry. This larger Mercury analogue either formed with a very peculiar composition or it has evolved since, e.g. by losing part of its mantle. Further characterisation of Mercury-like exoplanets like K2-229 b will help putting the detailed in-situ observations of Mercury (with Messenger and BepiColombo) into the global context of the formation and evolution of solar and extrasolar terrestrial planets.

Comments:    Accepted preprint in Nature Astronomy. Publisher-edited version available at this http URL Supplement materials available at this https URL
Subjects:    Earth and Planetary Astrophysics (astro-ph.EP)
Journal reference:    Nature Astronomy volume 2, pages 393-400 (2018)
DOI:    10.1038/s41550-018-0420-5
Cite as:    arXiv:1805.08405 [astro-ph.EP] (or arXiv:1805.08405v1 [astro-ph.EP] for this version)
Submission history
From: Alexandre Santerne 
[v1] Tue, 22 May 2018 05:38:39 GMT (1674kb,D)
https://arxiv.org/abs/1805.08405