The HARPS search for southern extra-solar planets XLII. A system of Earth-mass planets around the nearby M dwarf YZ Ceti

TL;DR

This study reports the discovery of at least three Earth-mass planets orbiting the nearby M dwarf YZ Ceti, with evidence suggesting a possible fourth, using radial velocity measurements and dynamical modeling.

Contribution

First detection of multiple low-mass planets around the closest M dwarf, demonstrating the effectiveness of radial velocity methods for such faint stars.

Findings

At least three planets with masses near Earth’s in compact orbits.

Inner two planets near a 2:3 mean-motion resonance.

Potential fourth planet with even lower mass.

Abstract

Exoplanet surveys have shown that systems with multiple low-mass planets on compact orbits are common. Except for few cases, however, the masses of these planets are generally unknown. At the very end of the main sequence, host stars have the lowest mass and hence offer the largest reflect motion for a given planet. In that context, we monitored the low-mass (0.13Msun) M dwarf YZ Cet (GJ 54.1, HIP 5643) intensively and obtained both radial velocities and stellar-activity indicators derived from both spectroscopy and photometry. We find strong evidence that it is orbited by at least three planets in compact orbits (P=1.97, 3.06, 4.66 days), with the inner two near a 2:3 mean-motion resonance. The minimum masses are comparable to that of Earth (Msini=0.75+-0.13, 0.98+-0.14, and 1.14+-0.17 Mearth) and also the lowest masses measured by radial velocity so far. We note the possibility for an even lower-mass, fourth planet with Msini=0.472+-0.096 Mearth at P=1.04 days. An n-body dynamical model is used to put further constraints on the system parameters. At 3.6 parsecs, YZ Cet is the nearest multi-planet system detected to date.