An Earth-sized exoplanet with a Mercury-like composition

BRUGGER, B.; LILLO-BOX, J.; ALMENARA, J.-M.; BAYLISS, D.; BOUCHY, F.; DELGADO MENA, E.; DOYLE, A.; FARIA, J.P.; GILES, H.; HOBSON, M.; KIRK, J.; LOVIS, C.; MOUSIS, O.; PEPE, F.; SOUSA, S.G.; SANTERNE, A.; ADIBEKYAN, V.; AGUICHINE, A.; BARROS, S.C.C.; BONOMO, A.S.; DELEUIL, M.; DÍAZ, R.F.; FAEDI, F.; FOXELL, E.; ARMSTRONG, D.J.; HOJJATPANAH, S.; GOSSELIN, H.; KING, G.; BARRADO, D.; LIGI, R.; BOISSE, I.; MCCORMAC, J.; BROWN, D.J.A.; OSBORN, H.P.; DEMANGEON, O.; SANTOS, N.C.; DUMUSQUE, X.; VIGAN, A.; FIGUEIRA, P.; HÉBRARD, G.; JACKMAN, J.; LAM, K.W.F.; LOUDEN, T.; NEAL, J.J.; POLLACCO, D.; UDRY, S.

Nature Astronomy

Nature Publishing Group

Año: 2018 vol. 2 p. 393 - 400

Earth, Venus, Mars and some extrasolar terrestrial planets 1 have a mass and radius that is consistent with a mass fraction of about 30% metallic core and 70% silicate mantle 2 . 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 3 . Several formation or evolution scenarios are proposed to explain this metal-rich composition, such as a giant impact 4 , mantle evaporation 5 or the depletion of silicate at the inner edge of the protoplanetary disk 6 . 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 Earth radii and a mass of 2.59 ± 0.43 Earth masses. This Earth-sized planet thus has a core-mass fraction that is compatible with that of Mercury, although it was expected to be similar to that of Earth based on host-star chemistry 7 . This larger Mercury analogue either formed with a very peculiar composition or has evolved, for example, by losing part of its mantle. Further characterization of Mercury-like exoplanets such as K2-229 b will help to put the detailed in situ observations of Mercury (with MESSENGER and BepiColombo 8 ) into the global context of the formation and evolution of solar and extrasolar terrestrial planets.