Precise mass determination for the keystone sub-Neptune planet transiting the mid-type M dwarf G 9-40

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. <BR /> 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. <BR /> 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. <BR /> 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<SUB>b</SUB> = 1.900 ± 0.065 R<SUB>⊕</SUB>) and determine its mass with a precision of 16% (M<SUB>b</SUB> = 4.00 ± 0.63 M<SUB>⊕</SUB>). 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. <BR /> 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<SUB>eq</SUB> ~ 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.