1 Super-Earths and Neptune-mass planets around solar-type stars2 The HARPS and CORALIE planet-search programmes2.1 Stellar samples2.1.1 The CORALIE survey2.1.2 The HARPS survey2.1.3 A volume-limited sample for the analysis2.1.4 Detection limits2.1.5 Completeness of the survey and planetary rate3 Planetary systems in the combined sample4 Statistical analysis of the sample4.1 The mass-period distribution4.2 Occurrence rate of planets as a function of planetary masses4.3 The mass distribution4.4 The period distribution of Super-Earth and Neptune-mass planets4.5 Orbital eccentricities of Super-Earth and Neptune-type planets4.6 Fraction of multiplanetary systems with low mass planets5 Host star metallicities as a function of planetary masses6 Planets in the habitable zone of solar-type stars7 Conclusions8 Acknowledgments9 ReferencesAstronomy & Astrophysics manuscript no. HARPS˙statc ESO 2011September 13, 2011The HARPS search for southern extra-solar planetsXXXIV. Occurrence, mass distribution and orbital properties of super-Earths andNeptune-mass planets?M. Mayor1, M. Marmier1, C. Lovis1, S. Udry1, D. S´egransan1, F. Pepe1, W. Benz2, J.-L. Bertaux3, F. Bouchy4,X. Dumusque1, G. LoCurto5, C. Mordasini6, D. Queloz1, and N.C. Santos7,81Observatoire Astronomique de l’Universit´e de Gen`eve, 51 ch. des Maillettes - Sauverny, CH-1290 Versoix, Switzerland2Physikalisches Institut, Universit¨at Bern, Silderstrasse 5, CH-3012 Bern, Switzerland3LATMOS, CNRS/UVSQ Universit´e de Versailles, Saint Quentin, France4Observatoire de Haute-Provence, 04870 Saint-Michel l’Observatoire, France5European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching bei M¨unchen, Germany6Max-Planck-Institut f¨ur Astronomie, K¨onigsstuhl 17, D-69117 Heidelberg, Germany7Centro de Astrof´ısica , Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal8Departamento de F´ısica e Astronomia, Faculdade de Ciˆencias, Universidade do Porto, PortugalReceived September 2011 / Accepted 2011ABSTRACTAims. We report on the results of an 8-year survey carried out at the La Silla Observatory with the HARPS spectrograph to detect andcharacterize 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 sufficiently largenumber of low-mass planets to study the statistical properties of their orbital elements, the correlation of the host-star metallicity withthe 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 ofany mass and with period up to 100 days. Different properties are observed for the population of planets less massive than about30 M⊕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 15 M⊕. The SEN occurence rate does not exhibit a preference for metal rich stars. Most of theSEN 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-starmetallicity. About 14 % of solar-type stars have a planetary companion more massive than 50 M⊕on an orbit with a period shorterthan 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-typestars. 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.Key words. Stars: late-type – Planetary systems – Techniques: radial velocities – Techniques: spectroscopy – Methods: statisticalanalysis1. Super-Earths and Neptune-mass planets aroundsolar-type starsThanks to the increasing sensitivity of instruments optimized forthe measurement of stellar radial velocities, planets with massessmaller than 2 M⊕have been detected. Over the past few yearsan impressive progress has been made for the detection of close-in super-Earths (planets with masses between 1 and 10 M⊕) andNeptune-mass planets. At present, the number of low-mass plan-ets discovered is sufficient to study the statistical properties ofSend offprint requests to: M. Mayor, e-mail:[email protected]?Based on observations made with the HARPS instrument on ESO’s3.6 m telescope at the La Silla Observatory in the frame of the HARPS-GTO Program ID 072.C-0488, the large program for the search of plan-ets around solar-type stars ID 183.C-0972 and the HARPS-UpgradeGTO program ID 69.A-0123this rich sub-population and to estimate their occurrence fre-quency around solar-type stars.The first hint to low-mass planets orbiting solar-type starson tight orbits was provided by the discoveries of the short pe-riod companions hosted by 55 Cnc (McArthur et al. 2004) andµ Ara (Santos et al. 2004a). Both discoveries were the result ofintensive and special-interest Doppler monitoring: the study ofthe complex dynamics for the first system and an asteroseismol-ogy campaign for the second. Orbital solutions and masses forthese two low-mass planets have been subsequently revised asmore measurements were gathered and new approaches appliedfor the data analysis. For 55 Cnc e, Dawson & Fabrycky (2010)analyzed the impact of observation aliases on the solution de-termination and proposed a revised value for the period passingfrom 2.7 down to P = 0.736 days corresponding to a new mini-mum mass m2sin i = 8.3 M⊕) for the planet. In the case of µ AraPepe et al. (2007) revised the mass of the planet at P = 9.6 daysto m2sin i = 10.5 M⊕in the context of a new orbital solution1arXiv:1109.2497v1 [astro-ph.EP] 12 Sep 2011M. Mayor et al.: The HARPS search for southern extra-solar planetsimplying a total of four planets. These first detections inducedimportant changes in our observing strategy, leading now to thediscovery of a wealth of low-mass planets with short orbital pe-riods. Measurements were made long enough to diminish thestellar acoustic noise, as well as series of measurements overconsecutive nights to
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