Characterization of the L 98-59 multiplanetary system with HARPS. Mass characterization of a hot super-Earth, a sub-Neptune, and a mass upper limit on the third planet

Abstract

Aims.L 98-59 (TIC 307210830, TOI-175) is a nearby M3 dwarf around which TESS revealed three small transiting planets (0.80,1.35, 1.57 Earth radii) in a compact configuration with orbital periods shorter than 7.5 days. Here we aim to measure the masses of theknown transiting planets in this system using precise radial velocity (RV) measurements taken with the HARPS spectrograph.Methods.We considered both trained and untrained Gaussian process regression models of stellar activity, which are modeled simul-taneously with the planetary signals. Our RV analysis was then supplemented with dynamical simulations to provide strong constraintson the planets’ orbital eccentricities by requiring long-term stability.Results.We measure the planet masses of the two outermost planets to be2.42±0.35and2.31±0.46Earth masses, which confirmsthe bulk terrestrial composition of the former and eludes to a significant radius fraction in an extended gaseous envelope for the latter.We are able to place an upper limit on the mass of the smallest, innermost planet of<1.01Earth masses with 95% confidence. OurRV plus dynamical stability analysis places strong constraints on the orbital eccentricities and reveals that each planet’s orbit likely hase<0.1.Conclusions.L 98-59 is likely a compact system of two rocky planets plus a third outer planet with a lower bulk density possiblyindicative of the planet having retained a modest atmosphere. The system offers a unique laboratory for studies of planet formation,dynamical stability, and comparative atmospheric planetology as the two outer planets are attractive targets for atmospheric character-ization through transmission spectroscopy. Continued RV monitoring will help refine the characterization of the innermost planet andpotentially reveal additional planets in the system at wider separations.