Confirmation of a non-transiting planet in the habitable zone of the nearby M dwarf L 98-59

Abstract

Only 40 exoplanetary systems with five or more planets are currently known. These systems are crucial for our understanding of planet formation and planet-planet interaction. The M dwarf L 98-59 has previously been found to show evidence of five planets, three of which are transiting. Our aim is to confirm the fifth planet in this system and to refine the system characteristics namely minimum masses, radii and the orbital parameters of the planets around L 98-59. We reanalysed RV and activity data from HARPS and ESPRESSO alongside TESS and HST transit data using a joint model. The parameter space was sampled using the dynesty nested sampler. We confirm the previously known fifth planet in the system's habitable zone with an orbital period of $23.07\pm0.08\,d$, a minimum mass of $3.0\pm0.5\,M_{\oplus}$ and an effective temperature of 289 K. We find an additional planet candidate in the RV data with an orbital period of $1.7361^{+0.0007}_{-0.0008}\,d$ and a minimum mass of $0.58\pm0.12\,M_{\oplus}$. This candidate (L 98-59.06) has a statistical significance between $2.9σ$ and $4.2σ$, details depending on the modelling of stellar variability. Moreover, we present evidence for a stellar rotation period of $76\pm4\,d$.