EDP Sciences
Free access
Volume 447, Number 1, February III 2006
Page(s) 361 - 367
Section Planets and planetary systems
DOI http://dx.doi.org/10.1051/0004-6361:20054084
A&A 447, 361-367 (2006)
DOI: 10.1051/0004-6361:20054084

The HARPS search for southern extra-solar planets

V. A 14 Earth-masses planet orbiting HD 4308
S. Udry1, M. Mayor1, W. Benz2, J.-L. Bertaux3, F. Bouchy4, C. Lovis1, C. Mordasini2, F. Pepe1, D. Queloz1 and J.-P. Sivan4

1  Observatoire de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
    e-mail: stephane.udry@obs.unige.ch
2  Physikalisches Institut Universität Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
3  Service d'Aéronomie du CNRS, BP 3, 91371 Verrières-le-Buisson, France
4  Laboratoire d'Astrophysique de Marseille, Traverse du Siphon, 13013 Marseille, France

(Received 22 August 2005 / Accepted 28 September 2005 )

We present here the discovery and characterisation of a very light planet around HD 4308. The planet orbits its star in 15.56 days. The circular radial-velocity variation presents a tiny semi-amplitude of 4.1 m s-1 that corresponds to a planetary minimum mass $m_2\sin{i}$ = 14.1  $M_{\oplus}$ (Earth masses). The planet was unveiled by high-precision radial-velocity measurements obtained with the HARPS spectrograph on the ESO 3.6-m telescope. The radial-velocity residuals around the Keplerian solution are 1.3 m s-1, demonstrating the very high quality of the HARPS measurements. Activity and bisector indicators exclude any significant perturbations of stellar intrinsic origin, which supports the planetary interpretation. Contrary to most planet-host stars, HD 4308 has a marked sub-solar metallicity ([Fe/H] = -0.31), raising the possibility that very light planet occurrence might show a different coupling with the parent star's metallicity than do giant gaseous extra-solar planets. Together with Neptune-mass planets close to their parent stars, the new planet occupies a position in the mass-separation parameter space that is constraining for planet-formation and evolution theories. The question of whether they can be considered as residuals of evaporated gaseous giant planets, ice giants, or super-earth planets is discussed in the context of the latest core-accretion models.

Key words: stars: individual: HD 4308 -- methods: observational -- techniques: radial velocities -- techniques: spectroscopic -- instrumentation: spectrographs

SIMBAD Objects

© ESO 2006