Another Mercury, but as large as Earth
2018 March 26

Artistic concept of an exoplanet orbiting close to its parent star. Credits: ESO/L. CalçadaDiagram showing the interior structure of Mercury. The metallic core extends from the center to a large fraction of the planetary radius. Credits: Nicolle Rager Fuller, National Science Foundation(Click on the image to enlarge).
It is very similar to Earth in size, but has two and half times the mass of our planet, which makes it after all much denser and, in its global composition, more resembling Mercury. A planet discovered at a distance of 340 light-years might clarify the peculiarities of the Sun’s nearest planet, according to a paper1 published today in Nature Astronomy and authored by an international team2 including nine researchers from the Instituto de Astrofísica e Ciências do Espaço (IA3).

Planet K2-229 b grabbed the attention of the team for its size very similar to Earth’s. However, its metallic core should make up 68% of the mass, comparing to less than one third in the case of our planet. This result wouldn’t be expected considering the chemical composition of the parent star, says Vardan Adibekyan (IA and Universidade do Porto), one of the authors of this study and who contributed to the chemical characterisation of the star K2-229.

This star is slightly younger and less massive than our Sun, and has a slightly smaller proportion of chemical elements heavier than hydrogen and helium. This disagreement between star and planet is the first detected in an extrasolar system, but even on our doorstep, in Mercury, we see something similar.

Earth, Mars and Venus, as well as the Sun, share the same relative amounts of certain chemical elements, like iron, magnesium or silicon, says Vardan Adibekyan. “Mercury is different and it is believed that some external process may have significantly altered its composition. Now we found a planet that shows the same peculiarity as Mercury, of having a different composition than what one would expect from the composition of its host star.”

The team hopes that the discovery of other planets of the same kind might help better understand how planets like Mercury formed and evolved. It might even complement data from missions to this body at the inner edge of the Solar System, like Messenger and the forthcoming BepiColombo, to be launched this year.

To Susana Barros (IA and Universidade do Porto), co-author of the paper and who contributed to the detection and characterisation of planet K2-229 b, this one belongs to a very interesting class of planets. “It is of terrestrial type but orbits really close to its star, something surprisingly, since they aren’t found in the Solar System.”

In fact, the similarities with Mercury end here. K2-229 b orbits much closer to its parent star, completing a full turn in only 14 hours (one year in Mercury lasts 88 terrestrial days). Moreover, its day temperature is more than four times higher than the one on the day side of the smallest planet of the Solar System, capable of attaining 2000 degrees Celsius, enough to melt iron.

In the K2-229 system, two other planets were identified, K2-229 c e K2-229 d. This planetary system was detected using data from Kepler space telescope, of NASA, and confirmed and characterised with the HARPS spectrograph, of ESO.


1.The article “An Earth-sized exoplanet with a Mercury-like composition” was published in Nature Astronomy on the 26th of March, 2018 (DOI: 10.1038/s41550-018-0420-5).

2.The team comprises 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 and A. Vigan

3.The Instituto de Astrofísica e Ciências do Espaço (Institute of Astrophysics and Space Sciences – IA) is the reference Portuguese research unit in space sciences and integrates researchers from the University of Lisbon and the University of Porto. The institute encompasses most of the field’s national scientific output and it was evaluated as Excellent in the last evaluation Fundação para a Ciência e Tecnologia (FCT) commissioned from the European Science Foundation (ESF). IA's activity is funded by national and international funds, including Fundação para a Ciência e a Tecnologia (UID/FIS/04434/2013), POPH/FSE and FEDER through COMPETE 2020.

Vardan Adibekyan 
Susana Barros

Science communication Group
Sérgio Pereira
Ricardo Cardoso Reis
João Retrê (Coordination, Lisboa)
Daniel Folha (Coordination, Porto)

Faculdade de Ciências da Universidade de Lisboa Universidade do Porto Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia