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Masses and radii for the three super-Earths orbiting GJ 9827, and implications for the composition of small exoplanets

K. Rice, L. Malavolta, A. W. Mayo, A. Mortier, L. Affer, A. Vanderburg, E. Poretti, L. Zeng, A. C. Cameron, M. Damasso, A. Coffinet, D. W. Latham, A. S. Bonomo, F. Bouchy, D. Charbonneau, X. Dumusque, P. Figueira, A. F. Martinez Fiorenziano, R. D. Haywood, J. A. Johnson, E. Lopez, C. Lovis, M. Mayor, G. Micela, E. Molinari, V. Nascimbeni, D. F. Phillips, G. Piotto, D. Sasselov, D. Sťgransan, A. Sozzetti, S. Udry, C. A. Watson

Abstract

Super-Earths belong to a class of planet not found in the Solar system, but which appear common in the Galaxy. Given that some super-Earths are rocky, while others retain substantial atmospheres, their study can provide clues as to the formation of both rocky and gaseous planets, and - in particular - they can help to constrain the role of photoevaporation in sculpting the exoplanet population. GJ 9827 is a system already known to host three super Earths with orbital periods of 1.2, 3.6, and 6.2 d. Here, we use new HARPS-N radial velocity measurements, together with previously published radial velocities, to better constrain the properties of the GJ 9827 planets. Our analysis cannot place a strong constraint on the mass of GJ 9827 c, but does indicate that GJ 9827 b is rocky with a composition that is probably similar to that of the Earth, while GJ 9827 d almost certainly retains a volatile envelope. Therefore, GJ 9827 hosts planets on either side of the radius gap that appears to divide super-Earths into predominantly rocky ones that have radii below similar to 1.5R(circle plus), and ones that still retain a substantial atmosphere and/or volatile components, and have radii above similar to 2R(circle plus). That the less heavily irradiated of the three planets still retains an atmosphere, may indicate that photoevaporation has played a key role in the evolution of the planets in this system.

Keywords
techniques: radial velocities; planets and satellites: composition; planets and satellites: detection; planets and satellites: fundamental parameters; planets and satellites: general; Stars: individual: GJ 9827 (2MASS J23270480-0117108; EPIC 246389858; HI

Monthly Notices of the Royal Astronomical Society
Volume 484, Issue 3, Page 3731
2019 April

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Instituto de Astrof√≠sica e Ci√™ncias do Espa√ßo Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia