The HARPS search for southern extra-solar planets

V. A 14 Earth-masses planet orbiting HD 4308

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

Abstract

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.