We report the detection of a new planetary system orbiting the nearby M2.5V star GJ 357 , using precision radial-velocities from three separate echelle spectrographs , HARPS , HiRES , and UVES .
Three small planets have been confirmed in the system , with periods of 9.125 \pm 0.001 , 3.9306 \pm 0.0003 , and 55.70 \pm 0.05 days , and minimum masses of 3.33 \pm 0.48 , 2.09 \pm 0.32 , and 6.72 \pm 0.94 M _ { \oplus } , respectively .
The second planet in our system , GJ 357c , was recently shown to transit by the Transiting Exoplanet Survey Satellite \citep [ TESS ; ] [ ] luque19 , but we could find no transit signatures for the other two planets .
Dynamical analysis reveals the system is likely to be close to coplanar , is stable on Myrs timescales , and places strong upper limits on the masses of the two non-transiting planets b and d of 4.25 and 11.20 M _ { \oplus } , respectively .
Therefore , we confirm the system contains at least two super-Earths , and either a third super-Earth or mini-Neptune planet .
GJ 357b & c are found to be close to a 7:3 mean motion resonance , however no libration of the orbital parameters was found in our simulations .
Analysis of the photometric lightcurve of the star from the TESS , when combined with our radial-velocities , reveal GJ 357c has an absolute mass , radius , and density of 2.248 ^ { +0.117 } _ { -0.120 } M _ { \oplus } , 1.167 ^ { +0.037 } _ { -0.036 } R _ { \oplus } , and 7.757 ^ { +0.889 } _ { -0.789 } gcm ^ { -3 } , respectively .
Comparison to super-Earth structure models reveals the planet is likely an iron dominated world .
The GJ 357 system adds to the small sample of low-mass planetary systems with well constrained masses , and further observational and dynamical follow-up is warranted to better understand the overall population of small multi-planet systems in the solar neighbourhood .