Discovery of an exoplanet which helps us to understand the formation of Mercury

The planet K2-229b has a similar composition to Mercury, because the vicinity to its sun blows its mantle, forming an atmosphere of evaporated silicates. This finding is partly based on data obtained from Calar Alto.

An international team has found a planetary system around a star (named K2-229) similar to our Sun, which helps us to understand the formation and properties of the planet Mercury. The planet K2-229b, lying in this system, has a composition very similar to that of Mercury. The finding, published in Nature Astronomy, is the result of an international cooperation led by Alexandre Santerne, from Centre National de la Recherche Scientifique (CNRS, France).

“This discovery brings essential information to understand the properties and formation of Mercury and, overall, contributes to understand the processes that created the rocky planets in the Solar System”, says David Barrado, researcher at Centro de Astrobiologia (CSIC-INTA, Madrid). Mercury is the target of BepiColombo, the European space probe to be launched this Fall, led by the European Spatial Agency and the Japan Aerospace Exploration agency.

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The Earth, Venus, Mars and some extrasolar planets have masses composed of 30% for the metallic core and 70% for the silicate mantle. Instead, Mercury, located on the inner limit of the Solar System, has a completely different composition, with 70% for the metallic core and 30% for the silicate mantle. This richness in metals was tentatively explained by various formation or evolution scenarios, like a giant collision, an evaporation of the mantle or a depletion of silicates in the inner limit of the protoplanetary disc of the Solar System.

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“The proximity of the planet K2-229b to its star possibly causes its mantle, the most external part of the planet, to volatilize, forming a silicate vapor atmosphere”, points out Barrado. “Part of this material might have got lost in space, which would explain the singular composition of the planet. An alternative explanation would be an impact against large asteroids, similarly to what happened to the Earth when the Moon was formed”, he adds.

“This new planetary system presents various peculiarities which make it extraordinarily interesting, e.g., it was detected by the Kepler spatial observatory through the planetary transit method (occultations similar to eclipses)”, Barrado details. “The three planets have very short periods (14 hours only for K2-229b, the closest to the star) and indeed the most external transit only was observed once” adds the researcher.

The ground-based observations made use of the HARPS spectrograph of the European Southern Observatory, located in La silla (Chile), and the high- resolution camera AstraLux installed on the 2.2-m telescope at Calar Alto.

AstraLux has provided very sharp images which rule out the presence of a possible companion star in the vicinity of K2-229, confirming the planetary nature of the three objects. All of them have radii somewhat larger than the Earth (between 1.16 and 2.65 Earth radii), and the superficial temperature of the two innermost planets (b and c) would be around 2000 to 700 celsius.

The current data only allow us to determine the mass of the innermost one (b), which would be 2.65 Earth masses. Its density is significantly higher than the terrestrial one. The models suggest it has an extraordinary large core of heavy elements. The comparison to the Solar System rocky planets indicates that it looks much more similar to Mercury rather than Venus, Mars or our own Earth.

“Before discovering the first planet outside the Solar System, we were limited by our anthropocentric preconceptions and were searching for systems like ours”, argues Barrado. “During the last 20+ years we have been surprised by the large diversity and now, exoplanets tell us things about our Solar System. Thus, the flow of information has been reversed.”, he concludes.

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. An Earth-sized exoplanet with a Mercury-like composition. Nature Astronomy. DOI: 10.1038/s41550-018-0420-5


The German-Spanish Calar Alto Observatory is located at Sierra de los Filabres, north of Almería (Andalucía, Spain). It is jointly operated by the Instituto Max Planck de Astronomía in Heidelberg, Germany, and the Instituto de Astrofísica de Andalucía (CSIC) in Granada, Spain. Calar Alto has three telescopes with apertures of 1.23m, 2.2m and 3.5m. A 1.5m aperture telescope, also located at the mountain, is operated under control of the Observatorio de Madrid. 

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