Masses and radii for the three super-Earths orbiting GJ 9827, and implications for the composition of small exoplanets
Bouchy, F.; Vanderburg, A.; Poretti, E.; Sozzetti, A.; Ségransan, D.; Piotto, G.; Collier Cameron, A.; Lovis, C.; Nascimbeni, V.; Udry, S.; Pepe, F.; Micela, G.; Figueira, P.; Mayor, M.; Damasso, M.; Bonomo, A. S.; Zeng, L.; Affer, L.; Latham, D. W.; Malavolta, L.; Mortier, A.; Haywood, R. D.; Dumusque, X.; Rice, K.; Buchhave, L. A.; Martinez Fiorenzano, A. F.; Molinari, E.; Phillips, D. F.; Sasselov, D.; Watson, C.; Charbonneau, D.; Lopez, E.; Coffinet, A.; Lopez-Morales, M.; Mayo, A.; Johnson, J. Asher; Nava, C.
United Kingdom, Italy, United States, Denmark, Spain, Switzerland, Chile, Portugal
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 pre-dominantly rocky ones that have radii below ∼1.5R⊕, and ones that still retain a substantial atmosphere and/or volatile components, and have radii above ∼2R⊕. 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.