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Temperate Earth-sized exoplanets around late-M dwarfs offer a rare opportunity to explore under which conditions planets can develop hospitable climate conditions. The small stellar radius amplifies the atmospheric transit signature, making even compact secondary atmospheres dominated by N 2 or CO 2 amenable to characterization with existing instrumentation 1 . Yet, despite large planet search efforts 2 , detection of low-temperature Earth-sized planets around late-M dwarfs has remained rare and the TRAPPIST-1 system, a resonance chain of rocky planets with seemingly identical compositions, has not yet shown any evidence of volatiles in the system 3 . Here we report the discovery of a temperate Earth-sized planet orbiting the cool M6 dwarf LP 791-18. The newly discovered planet, LP 791-18d, has a radius of 1.03 ± 0.04  R ⊕ and an equilibrium temperature of 300–400 K, with the permanent night side plausibly allowing for water condensation. LP 791-18d is part of a coplanar system 4 and provides a so-far unique opportunity to investigate a temperate exo-Earth in a system with a sub-Neptune that retained its gas or volatile envelope. On the basis of observations of transit timing variations, we find a mass of 7.1 ± 0.7  M ⊕ for the sub-Neptune LP 791-18c and a mass of 0.9 − 0.4 + 0.5 M ⊕ for the exo-Earth LP 791-18d. The gravitational interaction with the sub-Neptune prevents the complete circularization of LP 791-18d’s orbit, resulting in continued tidal heating of LP 791-18d’s interior and probably strong volcanic activity at the surface 5 , 6 . The authors report on a temperate Earth-sized planet orbiting the cool M6 dwarf LP 791-18 with a radius of 1.03 ± 0.04  R ⊕ and an equilibrium temperature of 300–400 K, with the permanent night side plausibly allowing for water condensation.

We present ground and space-based photometric observations of TOI-270 (L231-32), a system of three transiting planets consisting of one super-Earth and two sub-Neptunes discovered by TESS around a bright (K-mag=8.25) M3V dwarf. The planets orbit near low-order mean-motion resonances (5:3 and 2:1), and are thus expected to exhibit large transit timing variations (TTVs). Following an extensive observing campaign using 8 different observatories between 2018 and 2020, we now report a clear detection of TTVs for planets c and d, with amplitudes of \\(\\sim\\)10 minutes and a super-period of \\(\\sim\\)3 years, as well as significantly refined estimates of the radii and mean orbital periods of all three planets. Dynamical modeling of the TTVs alone puts strong constraints on the mass ratio of planets c and d and on their eccentricities. When incorporating recently published constraints from radial velocity observations, we obtain masses of \\(M_{\\mathrm{b}}=1.48\\pm0.18\\,M_\\oplus\\), \\(M_{c}=6.20\\pm0.31\\,M_\\oplus\\) and \\(M_{\\mathrm{d}}=4.20\\pm0.16\\,M_\\oplus\\) for planets b, c and d, respectively. We also detect small, but significant eccentricities for all three planets : \\(e_\\mathrm{b} =0.0167\\pm0.0084\\), \\(e_{c} =0.0044\\pm0.0006\\) and \\(e_{d} = 0.0066\\pm0.0020\\). Our findings imply an Earth-like rocky composition for the inner planet, and Earth-like cores with an additional He/H\\(_2\\)O atmosphere for the outer two. TOI-270 is now one of the best-constrained systems of small transiting planets, and it remains an excellent target for atmospheric characterization.

We report the discovery of two TESS sub-Neptunes orbiting the early M dwarf TOI-904 (TIC 261257684). Both exoplanets, TOI-904 b and c, were initially observed in TESS sector 12 with twin sizes of 2.49R\\(_\\oplus\\) and 2.31R\\(_\\oplus\\), respectively. Through observations in five additional sectors in the TESS primary mission and the first and second extended missions, the orbital periods of both planets were measured to be 10.887\\(\\pm\\)0.001 and 83.999\\(\\pm\\)0.001 days, respectively. Reconnaissance radial velocity measurements (taken with EULER/CORALIE) and high resolution speckle imaging with adaptive optics (obtained from SOAR/HRCAM and Gemini South/ZORRO) show no evidence of an eclipsing binary or a nearby companion, which together with the low false positive probabilities calculated with the statistical validation software TRICERATOPS establish the planetary nature of these candidates. The outer planet, TOI-904 c, is the longest-period M dwarf exoplanet found by TESS, with an estimated equilibrium temperature of 217K. As the three other validated planets with comparable host stars and orbital periods were observed by Kepler around much dimmer stars (J\\(_{mag}\\) \\(>\\) 12), TOI-904 c, orbiting a brighter star (J\\(_{mag}\\) \\(=\\) 9.6), is the coldest M dwarf planet easily accessible for atmospheric follow-up. Future mass measurements and transmission spectroscopy of the similar sized planets in this system could determine whether they are also similar in density and composition, suggesting a common formation pathway, or whether they have distinct origins.