The HARPS search for southern extra-solar planets

XLIII. A compact system of four super-Earth planets orbiting HD 215152^★

¹ Observatoire de l’Université de Genève, 51 chemin des Maillettes, 1290 Sauverny, Switzerland
e-mail: jean-baptiste.delisle@unige.ch
² ASD, IMCCE, Observatoire de Paris - PSL Research University, UPMC Univ. Paris 6, CNRS, 77 avenue Denfert-Rochereau, 75014 Paris, France
³ Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Buenos Aires, Argentina
⁴ CONICET - Universidad de Buenos Aires, Instituto de Astronomía y Física del Espacio (IAFE), Buenos Aires, Argentina
⁵ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
⁶ Dpto. de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁷ Physikalisches Institut, Universitat Bern, Silderstrasse 5, 3012 Bern, Switzerland
⁸ School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK
⁹ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
¹⁰ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
¹¹ Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 17, Bat. B5C, 4000 Liège, Belgium
¹² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
¹³ Canada France Hawaii Telescope Corporation, Kamuela 96743, USA
¹⁴ Department of Physics, University of Warwick, Coventry 4 7AL, UK
¹⁵ Cavendish Laboratory, J J Thomson Avenue, Cambridge 3 0HE, UK
¹⁶ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre 4169-007, Porto, Portugal

Received: 22 December 2017
Accepted: 8 February 2018

Abstract

We report the discovery of four super-Earth planets around HD 215152, with orbital periods of 5.76, 7.28, 10.86, and 25.2 d, and minimum masses of 1.8, 1.7, 2.8, and 2.9 M_⊕ respectively. This discovery is based on 373 high-quality radial velocity measurements taken by HARPS over 13 yr. Given the low masses of the planets, the signal-to-noise ratio is not sufficient to constrain the planet eccentricities. However, a preliminary dynamical analysis suggests that eccentricities should be typically lower than about 0.03 for the system to remain stable. With two pairs of planets with a period ratio lower than 1.5, with short orbital periods, low masses, and low eccentricities, HD 215152 is similar to the very compact multi-planet systems found by Kepler, which is very rare in radial-velocity surveys. This discovery proves that these systems can be reached with the radial-velocity technique, but characterizing them requires a huge amount of observations.