# Characterization of the L 98-59 multi-planetary system with HARPS: two confirmed terrestrial planets and a mass upper limit on the third

## TL;DR

This study characterizes the L 98-59 multi-planet system using HARPS radial velocity data, confirming two terrestrial planets' masses, constraining the third's upper mass limit, and analyzing orbital stability and eccentricities.

## Contribution

It provides the first precise mass measurements for two planets in the system and sets an upper limit for the third, enhancing understanding of their compositions and orbital dynamics.

## Key findings

- Two outer planets have masses of approximately 2.5 and 2.3 Earth masses.
- The innermost planet's mass is constrained to be less than 0.98 Earth masses.
- Orbital eccentricities are likely below 0.1 for stability.

## Abstract

L 98-59 (TIC 307210830, TOI-175) is a nearby M3 dwarf around which TESS revealed three terrestrial-sized 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 the known transiting planets in this system using precise radial velocity (RV) measurements taken with the HARPS spectrograph. We consider both trained and untrained Gaussian process regression models of stellar activity to simultaneously model the RV data with the planetary signals. Our RV analysis is then supplemented with dynamical simulations to provide strong constraints on the planets' orbital eccentricities by requiring long-term stability. We measure the planet masses of the two outermost planets to be $2.46\pm 0.31$ and $2.26\pm 0.50$ Earth masses which confirms their bulk terrestrial compositions. We are able to place an upper limit on the mass of the smallest, innermost planet of $<0.98$ Earth masses with 95% confidence. Our RV plus dynamical stability analysis places strong constraints on the orbital eccentricities and reveals that each planet's orbit likely has $e<0.1$ to ensure a dynamically stable system. The L 98-59 compact system of three likely rocky planets offers a unique laboratory for studies of planet formation, dynamical stability, and comparative atmospheric planetology. Continued RV monitoring will help refine the characterization of the innermost planet and potentially reveal additional planets in the system at wider separations.