The HARPS search for southern extra-solar planets V. A 14 Earth-masses planet orbiting HD 4308

TL;DR

This paper reports the discovery of a 14 Earth-mass planet orbiting HD 4308 using high-precision radial velocity measurements, providing insights into low-mass planet occurrence and formation theories.

Contribution

First detection of a very light planet around HD 4308 with high-precision HARPS data, challenging existing correlations between stellar metallicity and planet mass.

Findings

Planet has a 15.56-day orbit with a 4.1 m/s radial velocity semi-amplitude.

HD 4308 has sub-solar metallicity, suggesting different formation pathways.

Results constrain models of planet formation and evolution for low-mass planets.

Abstract

We present here the discovery and characterisation of a very light planet around HD4308. The planet orbits its star in 15.56 days. The circular radial-velocity variation presents a tiny semi-amplitude of 4.1 m/s that corresponds to a planetary minimum mass m2sin(i)=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, 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, HD4308 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.