Introduction and contextChallenges to high-precision radial velocitiesStar sample and observational strategyPrecision improvements through software developmentsStellar characteristics of HD40307The HD40307 planetary systemRadial velocity observationsSummary and discussionAstronomy & Astrophysics manuscript no. December 26, 2008(DOI: will be inserted by hand later)The HARPS search for southern extra-solar planets?XIII. A planetary system with 3 Super-Earths (4.2, 6.9, & 9.2 M⊕)M. Mayor1, S. Udry1, C. Lovis1, F. Pepe1, D. Queloz1, W. Benz2, J.-L. Bertaux3, F. Bouchy4, C. Mordasini2, andD. Segransan11Observatoire de Gen`eve, Universit´e de Gen`eve, 51 ch. des Maillettes, 1290 Sauverny, Switzerland e-mail:[email protected] Institut, Universit¨at Bern, Silderstrasse 5, CH-3012 Bern, Switzerland3Service d’A´eronomie du CNRS/IPSL, Universit´e de Versailles Saint-Quentin, BP3, 91371 Verri`eres-le-Buisson, France4Institut d’Astrophysique de Paris, CNRS, Universit´e Pierre et Marie Curie, 98bis Bd Arago, 75014 Paris, FranceReceived ; accepted To be inserted laterAbstract. This paper reports on the detection of a planetary system with three Super-Earths orbiting HD 40307. HD 40307 isa K2 V metal-deficient star at a distance of only 13 parsec, part of the HARPS GTO high-precision planet-search programme.The three planets on circular orbits have very low minimum masses of respectively 4.2, 6.9 and 9.2 Earth masses and periodsof 4.3, 9.6 and 20.5 days. The planet with the shortest period is the lightest planet detected to-date orbiting a main sequencestar. The detection of the correspondingly low amplitudes of the induced radial-velocity variations is completely secured bythe 135 very high-quality HARPS observations illustrated by the radial-velocity residuals around the 3-Keplerian solution ofonly 0.85 ms−1. Activity and bisector indicators exclude any significant perturbations of stellar intrinsic origin, which supportsthe planetary interpretation. Contrary to most planet-host stars, HD 40307 has a marked sub-solar metallicity ([Fe/H] = −0.31),further supporting the already raised possibility that the occurrence of very light planets might show a different dependence onhost star’s metallicity compared to the population of gas giant planets. In addition to the 3 planets close to the central star, asmall drift of the radial-velocity residuals reveals the presence of another companion in the system the nature of which is stillunknown.Key words. stars: individual: HD 40307, stars: planetary systems – techniques: radial velocities – techniques: spectroscopy1. Introduction and contextThe planet-search programme conducted at high precision withthe HARPS spectrograph on the ESO 3.6-m telescope at LaSilla aims at the detection of very low-mass planets in a sampleof solar-type stars already screened for giant planets at a lowerprecision with CORALIE on the 1.2-m Swiss telescope on thesame site. About 50 % of the HARPS GTO time is dedicated tothis survey. After 4.5 year of the programme, we are starting tosee a large population of Neptune-mass and super-Earth planetsemerging from the data, including the system presented here.Several reasons motivate our interest to search for very low-mass planets, with masses in the range of the Neptunes or theso-called Super-Earths (∼ 2 M⊕≤ m2sin i ≤ 10 M⊕).i) Over the past decade, several statistical distributions ofthe orbital elements of gaseous giant planets have emergedfrom the nearly 300 detected planetary systems (see e.g. UdrySend offprint requests to: M. Mayor?Based on observations made with the HARPS instrument on theESO 3.6 m telescope at La Silla Observatory under the GTO pro-gramme ID 072.C-0488.& Santos 2007; Marcy et al. 2005). These statistical propertiesprovide constraints to complex physical scenarios of planetarysystem formation. One of the most obvious example of that di-alogue between planetary formation theory and observations isillustrated by the comparison of the planetary mass vs semi-major axis (m2− a) diagram (Ida & Lin 2004a; Mordasini et al.2008). Comparison can be made for specific categories of hoststars by selecting different primary masses (m1) or metallici-ties ([Fe/H]). In all cases, global features of planet formationdirectly affect the overall topology of the (m2− a) diagram.In particular, the location of the large population of very low-mass planets predicted by the models (Mordasini et al. 2008;Ida & Lin 2008) depends upon the extend of migration under-gone by the planets during their formation. The detection of alarge sample of planets with masses less than roughly 25 M⊕at relatively close distances is therefore an important indicatorof the efficiency of type I migration (assuming that the planetsare not too close so that evaporation can be neglected). Despitethe still very limited number of planets detected in the range ofNeptune masses, already a few interesting characteristics areemerging (Mayor & Udry 2008):arXiv:0806.4587v1 [astro-ph] 27 Jun 20082 M. Mayor et al.: The HARPS search for southern extra-solar planets- The distribution of planetary masses appears as bimodal.A new population of light planets, although more difficult todetect, is differentiating itself from the distribution of giant-planet masses. The Neptune- and super-Earth mass distributionseems not to be the extrapolation towards lower masses of thedistribution for gaseous giant planets.- The very strong correlation observed between the host starmetallicity and occurrence frequency of giant planets (Santoset al. 2001, 2004; Fischer & Valenti 2005) seems to be vanish-ing or at least to be reduced (Udry et al. 2006).- Neptune-mass planets and Super-Earths are found mostof the time in multiplanetary systems (> 80 % of the knowncandidates).ii) Simulations of planetary formation do not only providethe statistical distributions of masses and semi-major axes. Forevery planet, we have, in addition, a prediction of its internalstructure. The internal composition of the planet at the end ofthe formation/migration process carries a fossil signature of thesystem history. The end-state diversity is broad: rocky planets,icy planets, ocean planets, evaporated gaseous giant planets, orpossibly objects with variable percentages of these ingredients.The predicted distributions of the planetary internal composi-tion, as a function of the different significant parameters (m1,m2, a, [Fe/H]) can be observed in the
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