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The HARPS Search for Southern Extra-Solar Planets. XIII. a Planetary

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The HARPS Search for Southern Extra-Solar Planets. XIII. a Planetary Astronomy & Astrophysics manuscript no. November 29, 2018 (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, and D. Segransan1 1 Observatoire de Geneve,` Universite´ de Geneve,` 51 ch. des Maillettes, 1290 Sauverny, Switzerland e-mail: [email protected] 2 Physikalisches Institut, Universitat¨ Bern, Silderstrasse 5, CH-3012 Bern, Switzerland 3 Service d’Aeronomie´ du CNRS/IPSL, Universite´ de Versailles Saint-Quentin, BP3, 91371 Verrieres-le-Buisson,` France 4 Institut d’Astrophysique de Paris, CNRS, Universite´ Pierre et Marie Curie, 98bis Bd Arago, 75014 Paris, France Received ; accepted To be inserted later Abstract. This paper reports on the detection of a planetary system with three Super-Earths orbiting HD 40307. HD 40307 is a 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 periods of 4.3, 9.6 and 20.5 days. The planet with the shortest period is the lightest planet detected to-date orbiting a main sequence star. The detection of the correspondingly low amplitudes of the induced radial-velocity variations is completely secured by the 135 very high-quality HARPS observations illustrated by the radial-velocity residuals around the 3-Keplerian solution of only 0.85 ms−1. Activity and bisector indicators exclude any significant perturbations of stellar intrinsic origin, which supports the 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 on host star’s metallicity compared to the population of gas giant planets. In addition to the 3 planets close to the central star, a small drift of the radial-velocity residuals reveals the presence of another companion in the system the nature of which is still unknown. Key words. stars: individual: HD 40307, stars: planetary systems – techniques: radial velocities – techniques: spectroscopy 1. Introduction and context & Santos 2007; Marcy et al. 2005). These statistical properties provide constraints to complex physical scenarios of planetary The planet-search programme conducted at high precision with system formation. One of the most obvious example of that di- the HARPS spectrograph on the ESO 3.6-m telescope at La alogue between planetary formation theory and observations is Silla aims at the detection of very low-mass planets in a sample illustrated by the comparison of the planetary mass vs semi- of solar-type stars already screened for giant planets at a lower major axis (m − a) diagram (Ida & Lin 2004a; Mordasini et al. precision with CORALIE on the 1.2-m Swiss telescope on the 2 2008). Comparison can be made for specific categories of host same site. About 50 % of the HARPS GTO time is dedicated to stars by selecting different primary masses (m ) or metallici- this survey. After 4.5 year of the programme, we are starting to 1 ties ([Fe/H]). In all cases, global features of planet formation see a large population of Neptune-mass and super-Earth planets directly affect the overall topology of the (m − a) diagram. emerging from the data, including the system presented here. 2 arXiv:0806.4587v1 [astro-ph] 27 Jun 2008 In particular, the location of the large population of very low- Several reasons motivate our interest to search for very low- mass planets predicted by the models (Mordasini et al. 2008; mass planets, with masses in the range of the Neptunes or the Ida & Lin 2008) depends upon the extend of migration under- so-called Super-Earths (∼ 2 M ≤ m sin i ≤ 10 M ). ⊕ 2 ⊕ gone by the planets during their formation. The detection of a i) Over the past decade, several statistical distributions of large sample of planets with masses less than roughly 25 M⊕ the orbital elements of gaseous giant planets have emerged at relatively close distances is therefore an important indicator from the nearly 300 detected planetary systems (see e.g. Udry of the efficiency of type I migration (assuming that the planets Send offprint requests to: M. Mayor are not too close so that evaporation can be neglected). Despite ? Based on observations made with the HARPS instrument on the the still very limited number of planets detected in the range of ESO 3.6 m telescope at La Silla Observatory under the GTO pro- Neptune masses, already a few interesting characteristics are gramme ID 072.C-0488. emerging (Mayor & Udry 2008): 2 M. Mayor et al.: The HARPS search for southern extra-solar planets - The distribution of planetary masses appears as bimodal. and HD 40307 are on the other hand both K dwarfs. The detec- A new population of light planets, although more difficult to tion of planetary systems around bright stars by Doppler spec- detect, is differentiating itself from the distribution of giant- troscopy is very important because it concerns mostly nearby planet masses. The Neptune- and super-Earth mass distribution stars for which we can obtain interesting further information seems not to be the extrapolation towards lower masses of the (e.g. on their orbital, planetary and stellar properties), open- distribution for gaseous giant planets. ing the possibility of rich complementary studies. This is even - The very strong correlation observed between the host star more true when the planet is transiting in front of its parent star metallicity and occurrence frequency of giant planets (Santos (see for illustration the wealth of studies concerning GJ 436). et al. 2001, 2004; Fischer & Valenti 2005) seems to be vanish- In this paper, we characterize the orbits of three new super- ing or at least to be reduced (Udry et al. 2006). Earth planets on short-period trajectories orbiting the main se- - Neptune-mass planets and Super-Earths are found most quence star HD 40307. Section 2 will briefly describe the im- of the time in multiplanetary systems (> 80 % of the known provement of the HARPS radial velocities in term of observa- candidates). tional strategy and software developments. The stellar charac- ii) Simulations of planetary formation do not only provide teristics of HD 40307 are presented in Sect. 3. Section 4 deals the statistical distributions of masses and semi-major axes. For with the derived orbital solution while summary and conclu- every planet, we have, in addition, a prediction of its internal sion are given in Sect. 5. structure. The internal composition of the planet at the end of the formation/migration process carries a fossil signature of the system history. The end-state diversity is broad: rocky planets, 2. Challenges to high-precision radial velocities icy planets, ocean planets, evaporated gaseous giant planets, or 2.1. Star sample and observational strategy possibly objects with variable percentages of these ingredients. The predicted distributions of the planetary internal composi- About 50 % of the HARPS GTO time (guaranteed time ob- tion, as a function of the different significant parameters (m1, servation) is devoted to the search for very low-mass planets m2, a, [Fe/H]) can be observed in the corresponding radius- orbiting G and K dwarfs of the solar vicinity, in the southern mass (m2 − R) diagrams. These predicted (m2 − R) distributions sky. A companion programme is focused on the same goal but can then be compared to the observed distributions derived for planets around M-dwarf stars. HARPS is a vacuum high- from combined radial-velocity and transit searches. The com- resolution spectrograph fiber-fed by the ESO 3.6-m telescope ing soon results from space missions searching for planetary at La Silla Observatory (Mayor et al. 2003). HD 40307 is part transits combined with ground-based, high-precision, radial- of our G–K survey. The stars in this programme were selected velocity follow-up, will provide rich observational constraints from the volume-limited sample (1650 stars) measured since to the planetary formation theory via the m2 − R distributions 1998 with the CORALIE spectrograph on the EULER tele- of low-mass planets. scope, also at La Silla (Udry et al. 2000). In a first step we iii) In a more distant future, space missions will be de- have selected some 400 G and K dwarfs with v sin i less than veloped to search for life signatures in the atmosphere of 3 kms−1 and removing the known spectroscopic binaries. In a terrestrial-type planets. Before the detailed design of such am- second step, using the high quality HARPS spectra, we have bitious missions, it would be wise to have first insights in the concentrated our efforts on the less active stars of the sample 0 occurrence frequency of terrestrial planets and on the statistical (log RHK < −4:8). The remaining ∼ 200 stars with rather low properties of their orbits. Still more valuable is the detection of chromospheric activity constitute the core of the programme to planets in the habitable zone of our closest neighbours, initi- search for Neptune-mass and super-Earth planets. ating the preparation of an ”input catalogue” for these future The planetary minimum mass estimated from Doppler missions. measurements is directly proportional to the amplitude of the Already a few planets have been detected with masses be- reflex motion of the primary star. The measure of very precise tween 3 and 10 M⊕. In 1992, from precise timing, (Wolszczan radial velocities requires that all steps along the light path, from & Frail 1992) have discovered two planets with masses of 2.8 the star to the detector, are well understood.
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