Detailed Architecture Of The L 98-59 System And Confirmation Of A Fifth Planet In The Habitable Zone

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, PTTV=396 days) constrain the eccentricities of all planets to near-circular orbits (e≲0.04).

An internal structure analysis of the transiting planets reveals increasing water-mass fractions (fH2O) with orbital distance, reaching f_H2O≈0.16 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.

Top: Above view of the L 98-59 planetary system on BJD = 2 460 000 assuming circular orbits. The line of sight towards Earth is to the right. The Habitable Zone defined in Kopparapu et al. 2013 is shown in green for runaway/maximum greenhouse (conservative) and pale green for early recent Venus/early Mars (optimistic). Bottom: Comparative exoplanetology of M-dwarf multiplanetary systems ordered by stellar effective temperature and planetary instellation. An arbitrary shift of +100 K in T_eff has been applied to TOI-700 and Teegarden’s star for clarity. In both panels, transiting (non-transiting) exoplanets are shown as filled (open) circles, with circle size proportional to planetary radius. For planets with only a minimum mass constraint, the radius is set to the 95th percentile of the prediction from spright (Parviainen et al. 2024). — astro-ph.EP

Charles Cadieux, Alexandrine L’Heureux, Caroline Piaulet-Ghorayeb, René Doyon, Étienne Artigau, Neil J. Cook, Louis-Philippe Coulombe, Pierre-Alexis Roy, David Lafrenière, Pierrot Lamontagne, Michael Radica, Björn Benneke, Eva-Maria Ahrer, Drew Weisserman, Ryan Cloutier

Comments: 32 pages, 12 figures, 8 tables, accepted for publication in AJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2507.09343 [astro-ph.EP] (or arXiv:2507.09343v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2507.09343
Focus to learn more
Submission history
From: Charles Cadieux
[v1] Sat, 12 Jul 2025 16:43:07 UTC (9,235 KB)
https://arxiv.org/abs/2507.09343
Astrobiology,