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 ( 0.13M _ { \odot } ) M dwarf YZ Cet ( GJ 54.1 , HIP 5643 ) intensively and obtained radial velocities and stellar-activity indicators derived from spectroscopy and photometry , 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 \pm 0.13 , 0.98 \pm 0.14 , and 1.14 \pm 0.17 M _ { \oplus } ) , 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 M \sin i=0.472 \pm 0.096 M _ { \oplus } 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 .