Exoplanet characterisation in the longest known resonant chain: the K2-138 system seen by HARPS^★

¹ Aix-Marseille University, CNRS, CNES, LAM, Marseille, France
e-mail: theo.lopez@lam.fr
² Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
³ Observatoire Astronomique de l’Université de Genève, 51 Chemin des Maillettes, 1290 Versoix, Switzerland
⁴ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
⁵ Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁶ Cornell Center for Astrophysics and Planetary Sciences, Department of Astronomy, Cornell University, Ithaca, NY, USA
⁷ Depto. de Astrofísica, Centro de Astrobiología (CSIC-INTA), ESAC campus 28692 Villanueva de la Cañada (Madrid), Spain
⁸ INAF – Osservatorio Astrofisico di Torino, Strada Osservatorio 20, 10025 Pino Torinese (TO), Italy
⁹ University of Glasgow, Glasgow, G12 8QQ, UK
¹⁰ Facultad de Ciencias Exactas y Naturales. Buenos Aires, Universidad de Buenos Aires, Argentina
¹¹ CONICET – Universidad de Buenos Aires, Instituto de Astronomía y Física del Espacio (IAFE), Buenos Aires, Argentina
¹² Departamento de Física e Astronomia, Faculdade de Ciencias, Universidade do Porto, Rua Campo Alegre, 4169-007 Porto, Portugal
¹³ Institut d’Astrophysique de Paris, UMR7095 CNRS, Universite Pierre & Marie Curie, 98bis boulevard Arago, 75014 Paris, France
¹⁴ Aix-Marseille Univ., CNRS, OHP, Observatoire de Haute Provence, Saint Michel l’Observatoire, France
¹⁵ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁶ CAP – Centre for Applied Photonics, INESC TEC, Porto, Portugal
¹⁷ European Southern Observatory (ESO), Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile

Received: 8 July 2019
Accepted: 20 September 2019

Abstract

The detection of low-mass transiting exoplanets in multiple systems brings new constraints to planetary formation and evolution processes and challenges the current planet formation theories. Nevertheless, only a mere fraction of the small planets detected by Kepler and K2 have precise mass measurements, which are mandatory to constrain their composition. We aim to characterise the planets that orbit the relatively bright star K2-138. This system is dynamically particular as it presents the longest chain known to date of planets close to the 3:2 resonance. We obtained 215 HARPS spectra from which we derived the radial-velocity variations of K2-138. Via a joint Bayesian analysis of both the K2 photometry and HARPS radial-velocities (RVs), we constrained the parameters of the six planets in orbit. The masses of the four inner planets, from b to e, are 3.1, 6.3, 7.9, and 13.0 M_⊕ with a precision of 34, 20, 18, and 15%, respectively. The bulk densities are 4.9, 2.8, 3.2, and 1.8 g cm⁻³, ranging from Earth to Neptune-like values. For planets f and g, we report upper limits. Finally, we predict transit timing variations of the order two to six minutes from the masses derived. Given its peculiar dynamics, K2-138 is an ideal target for transit timing variation (TTV) measurements from space with the upcoming CHaracterizing ExOPlanet Satellite (CHEOPS) to study this highly-packed system and compare TTV and RV masses.