Context. The NASA space telescope TESS is currently in the extended mission of its all-sky search for new transiting planets. Of the thousands of candidates that TESS is expected to deliver, transiting planets orbiting nearby M dwarfs are particularly interesting targets since they provide a great opportunity to characterize their atmospheres by transmission spectroscopy. Aims. We aim to validate and characterize the new sub-Neptune sized planet candidate TOI-2136.01 orbiting a nearby M dwarf (d = 33.36 ± 0.02 pc, T e f f = 3373 ± 108 K) with an orbital period of 7.852 days. Methods. We use TESS data, ground-based multi-color photometry, and radial velocity measurements with the InfraRed Doppler (IRD) instrument on the Subaru Telescope to validate the planetary nature of TOI-2136.01 and estimate the stellar and planetary parameters. We also conduct highresolution transmission spectroscopy to search for helium in its atmosphere. Results. We confirmed that TOI-2136.01 (now named as TOI-2136b) is a bona fide planet with a planetary radius of R p = 2.2 ± 0.07 R ⊕ and a mass of M p = 4.7 +3.1 −2.6 M ⊕ . We also search for helium 10830 Å absorption lines and place an upper limit on the equivalent width of < 7.8 mÅ (95% confidence) and on the absorption signal of < 1.44 % (95% confidence). Conclusions. TOI-2136b is a sub-Neptune transiting a nearby and bright star (J=10.8) and is a potential hycean planet, making it an excellent target for atmospheric studies to understand the formation, evolution, and habitability of the small planets.

Context. Despite being a prominent subset of the exoplanet population discovered in the past three decades, the nature and provenance of sub-Neptune-sized planets is still one of the open questions in exoplanet science. Aims. For planets orbiting bright stars, precisely measuring the orbital and planet parameters of the system is the best approach to distinguish between competing theories regarding their formation and evolution. Methods. We obtained 69 new radial velocity observations of the mid-M dwarf G 9-40 with the CARMENES instrument to measure for the first time the mass of its transiting sub-Neptune planet, G 9-40 b, discovered in data from the K2 mission. Results. Combined with new observations from the TESS mission during Sectors 44, 45, and 46, we are able to measure the radius of the planet to an uncertainty of 3.4% (R b = 1.900 ± 0.065 R ⊕ ) and determine its mass with a precision of 16% (M b = 4.00 ± 0.63 M ⊕ ). The resulting bulk density of the planet is inconsistent with a terrestrial composition and suggests the presence of either a water-rich core or a significant hydrogen-rich envelope. Conclusions. G 9-40 b is referred to as a keystone planet due to its location in period-radius space within the radius valley. Several theories offer explanations for the origin and properties of this population and this planet is a valuable target for testing the dependence of those models on stellar host mass. By virtue of its brightness and small size of the host, it joins L 98-59 d as one of the two best warm (T eq ∼ 400 K) sub-Neptunes for atmospheric characterization with JWST, which will probe cloud formation in sub-Neptune-sized planets and break the degeneracies of internal composition models.