Volume 447, Number 1, February III 2006
|Page(s)||361 - 367|
|Section||Planets and planetary systems|
|Published online||27 January 2006|
The HARPS search for southern extra-solar planets
V. A 14 Earth-masses planet orbiting HD 4308
Observatoire de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland e-mail: email@example.com
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
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 = 14.1 (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
© ESO, 2006
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