RESEARCH
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The HARPS search for southern extra-solar planets
XXXIV. Occurrence, mass distribution and orbital properties of super-Earths and Neptune-mass planets

M. Mayor, M. Marmier, C. Lovis, S. Udry, D. Sťgransan, F. Pepe, W. Benz, J.-L. Bertaux, F. Bouchy, X. Dumusque, G. Lo Curto, C. Mordasini, D. Queloz, N. C. Santos

Abstract
Aims. We report on the results of an 8-year survey carried out at the La Silla Observatory with the HARPS spectrograph to detect and characterize planets in the super-Earth and Neptune mass regime.
Methods. The size of our star sample and the precision achieved with HARPS have allowed the detection of a suffciently large number of low-mass planets to study the statistical properties of their orbital elements, the correlation of the host-star metallicity with the planet masses, as well as the occurrence rate of planetary systems around solar-type stars.
Results. A robust estimate of the frequency of systems shows that more than 50% of solar-type stars harbor at least one planet of any mass and with period up to 100 days. Different properties are observed for the population of planets less massive than about 30M compared to the population of gaseous giant planets. The mass distribution of Super-Earths and Neptune-mass planets (SEN) is strongly increasing between 30 and 15M. The SEN occurence rate does not exhibit a preference for metal rich stars. Most of the SEN planets belong to multi-planetary systems. The orbital eccentricities of the SEN planets seems limited to 0.45. At the opposite, the occurence rate of gaseous giant planets is growing with the logarithm of the period, and is strongly increasing with the host-star metallicity. About 14% of solar-type stars have a planetary companion more massive than 50M on an orbit with a period shorter than 10 years. Orbital eccentricities of giant planets are observed up to 0.9 and beyond.
Conclusions. The precision of HARPS-type spectrographs opens the possibility to detect planets in the habitable zone of solar-type stars. Identification of a significant number of super-Earths orbiting solar-type of the Sun vicinity is achieved by Doppler spectroscopy. 41 newly discovered planets with HARPS are announced in the Appendix of this paper, among which 16 Super-Earths.

Keywords
Stars: late-type – Planetary systems – Techniques: radial velocities – Techniques: spectroscopy – Methods: statistical analysis

Notes
Based on observations made with the HARPS instrument on ESO’s 3.6m telescope at the La Silla Observatory in the frame of the HARPSGTO Program ID 072.C-0488, the large program for the search of planets around solar-type stars ID 183.C-0972 and the HARPS-Upgrade GTO program ID 69.A-0123

Astronomy and Astrophysics
Submitted

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