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

Exoplanet surveys have shown that systems with multiple low-mass planets on compact orbits are common. Except for a 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 this context, we monitored the low-mass (0.13 M-circle dot) M dwarf YZ Cet (GJ 54.1, HIP 5643) intensively and obtained radial velocities and stellar-activity indicators derived from spectroscopy and p\hotometry, respectively. We find strong evidence that it is orbited by at least three planets in compact orbits (P-Orb = 1.97, 3.06, 4.66 days), with the inner two near a 2: 3 mean-motion resonance. The minimum masses are comparable to the mass of Earth (M sin i = 0.75 +/- 0.13, 0.98 +/- 0.14, and 1.14 +/- 0.17 M-circle plus), and they are also the lowest masses measured by radial velocity so far. We note the possibility for a fourth planet with an even lower mass of Msin i = 0.472 +/- 0.96 M-circle plus at P-Orb = 1 : 04 days. An n-body dynamical model is used to place further constraints on the system parameters. At 3.6 parsecs, YZ Cet is the nearest multi-planet system detected to date.