HD 97658 and its super-Earth

¹ Institut d’Astrophysique et de Géophysique, Université de Liège, Belgium
² Department of Physical and Environmental Sciences, University of Toronto, Canada
³ Institute of Astronomy, University of Cambridge, UK
⁴ Las Cumbres Observatory Global Telescope Network, USA
⁵ Department of Physics, Broida Hall, UC Santa Barbara, USA
⁶ Department of Astrophysical Sciences, Princeton University, USA

^a e-mail: valerie.vangrootel@ulg.ac.be

Published online: 23 September 2015

Abstract

Super-Earths transiting nearby bright stars are key objects that simultaneously allow for accurate measurements of both their mass and radius, providing essential constraints on their internal composition. We present the confirmation, based on Spitzer observations, that the super-Earth HD 97658 b transits its host star. HD 97658 is a low-mass (M_* = 0.77 ± 0.05 M_⊙) K1 dwarf, as determined from the Hipparcos parallax and stellar evolution modeling. To constrain the planet parameters, we carry out Bayesian global analyses of Keck-HIRES radial velocities, and MOST and Spitzer photometry. HD 97658 b is a massive (M_P = 7.55_−0.79^+0.83 M_⊕) and large (R_P = 2.247_−0.095^+0.098 R_⊕ at 4.5 μm super-Earth. We investigate the possible internal compositions for HD 97658 b. Our results indicate a large rocky component, by at least 60% by mass, and very little H-He components, at most 2% by mass. We also discuss how future asteroseismic observations can improve the knowledge of the HD 97658 system, in particular by constraining its age.