Detailed Architecture of the L 98-59 System and Confirmation of a Fifth Planet in the Habitable Zone

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 nontransiting 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 Hubble Space Telescope 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 ⊕ , and 1.627 ± 0.041 R ⊕ , respectively. Combining RVs with transit timing variations (TTVs) 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 nontransiting 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 (&lt;3 minutes, P TTV = 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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>f</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> <mml:mi mathvariant="normal">O</mml:mi> </mml:mrow> </mml:msub> </mml:math> ) with orbital distance, reaching <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>f</mml:mi> <mml:mrow> <mml:msub> <mml:mi mathvariant="normal">H</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:mi mathvariant="normal">O</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≈</mml:mo> <mml:mn>0.16</mml:mn> </mml:math> 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.