The HARPS search for southern extra-solar planets. VI. A Neptune-mass planet around the nearby M dwarf Gl 581

TL;DR

This paper reports the discovery of a Neptune-mass exoplanet around the nearby M dwarf Gl 581 using HARPS Doppler measurements, highlighting the prevalence of low-mass planets in such systems.

Contribution

First detection of a Neptune-mass planet around an M dwarf using high-precision Doppler spectroscopy, expanding knowledge of planetary systems around low-mass stars.

Findings

The planet has a 5.366-day circular orbit.

The planet's minimum mass is approximately 16.6 Earth masses.

This system is one of the few known around M dwarfs.

Abstract

We report the discovery of a Neptune-mass planet around Gl 581 (M3V, M = 0.31 Msol), based on precise Doppler measurements with the HARPS spectrograph at La Silla Observatory. The radial velocities reveal a circular orbit of period P = 5.366 days and semi-amplitude K1 = 13.2 m/s. The resulting minimum mass of the planet (m2 sin i) is only 0.052 Mjup = 0.97 Mnep = 16.6 Mearth making Gl 581b one of the lightest extra-solar planet known to date. The Gl 581 planetary system is only the third centered on an M dwarf, joining the Gl 876 three-planet system and the lone planet around Gl 436. Its discovery reinforces the emerging tendency of such planets to be of low mass, and found at short orbital periods. The statistical properties of the planets orbiting M dwarfs do not seem to match a simple mass scaling of their counterparts around solar-type stars.