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We report the detection of a new planetary system orbiting the nearby M2.5Vstar GJ357, using precision radial-velocities from three separate echellespectrographs, HARPS, HiRES, and UVES. Three small planets have been confirmedin the system, with periods of 9.125+/-0.001, 3.9306+/-0.0003, and 55.70+/-0.05days, and minimum masses of 3.33+/-0.48, 2.09+/-0.32, and 6.72+/-0.94 Me,respectively. The second planet in our system, GJ357c, was recently shown totransit by the Transiting Exoplanet Survey Satellite (TESS; Luque et al. 2019),but we could find no transit signatures for the other two planets. Dynamicalanalysis reveals the system is likely to be close to coplanar, is stable onMyrs timescales, and places strong upper limits on the masses of the twonon-transiting planets b and d of 4.25 and 11.20 Me, respectively. Therefore,we confirm the system contains at least two super-Earths, and either a thirdsuper-Earth or mini-Neptune planet. GJ357b & c are found to be close to a 7:3mean motion resonance, however no libration of the orbital parameters was foundin our simulations. Analysis of the photometric lightcurve of the star from theTESS, when combined with our radial-velocities, reveal GJ357c has an absolutemass, radius, and density of 2.248+0.117-0.120 Me, 1.167+0.037-0.036 Re, and7.757+0.889-0.789 g/cm3, respectively. Comparison to super-Earth structuremodels reveals the planet is likely an iron dominated world. The GJ357 systemadds to the small sample of low-mass planetary systems with well constrainedmasses, and further observational and dynamical follow-up is warranted tobetter understand the overall population of small multi-planet systems in thesolar neighbourhood.

GJ357: A low-mass planetary system uncovered by precision radial-velocities and dynamical simulations