Detailed Architecture of the L 98-59 System and Confirmation of a Fifth Planet in the Habitable Zone
arXiv:2507.09343
Abstract
The L 98-59 system, identified by TESS in 2019, features three transiting exoplanets in compact orbits of 2.253, 3.691, and 7.451 days around an M3V star, with an outer 12.83-day non-transiting planet confirmed in 2021 using ESPRESSO. The planets exhibit a diverse range of sizes (0.8-1.6 R), masses (0.5-3 M), and likely compositions (Earth-like to possibly water-rich), prompting atmospheric characterization studies with HST and JWST. Here, we analyze 16 new TESS sectors and improve radial velocity (RV) precision of archival ESPRESSO and HARPS data using a line-by-line framework, enabling stellar activity detrending via a novel differential temperature indicator. We refine the radii of L 98-59 b, c, and d to 0.837 0.019 R, 1.329 0.029 R, 1.627 0.041 R, respectively. Combining RVs with transit timing variations (TTV) of L 98-59 c and d from TESS and JWST provides unprecedented constraints on the masses and eccentricities of the planets. We report updated masses of 0.46 0.11 M for b, 2.00 0.13 M for c, and 1.64 0.07 M for d, and a minimum mass of 2.82 0.19 M for e. We additionally confirm L 98-59\,f, a non-transiting super-Earth with a minimal mass of 2.80 0.30 M on a 23.06-day orbit inside the Habitable Zone. The TTVs of L 98-59 c and d (<3 min, days) constrain the eccentricities of all planets to near-circular orbits (). An internal structure analysis of the transiting planets reveals increasing water-mass fractions () with orbital distance, reaching for L 98-59\ d. We predict eccentricity-induced tidal heating in L 98-59 b with heat fluxes comparable to those of Io, potentially driving volcanic activity.
32 pages, 12 figures, 8 tables, accepted for publication in AJ. [v2] Corrected a minor formatting issue in the caption of Figure 5