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

Luque, R.; Nowak, G.; Hirano, T.; Kossakowski, D.; Pallé, E.; Nixon, M. C.; Morello, G.; Amado, P. J.; Albrecht, S. H.; Caballero, J. A.; Cifuentes, C.; Cochran, W. D.; Deeg, H. J.; Dreizler, S.; Esparza-Borges, E.; Fukui, A.; Gandolfi, D.; Goffo, E.; Guenther, E. W.; Hatzes, A. P.; Henning, T.; Kabath, P.; Kawauchi, K.; Korth, J.; Kotani, T.; Kudo, T.; Kuzuhara, M.; Lafarga, M.; Lam, K. W. F.; Livingston, J.; Morales, J. C.; Muresan, A.; Murgas, F.; Narita, N.; Osborne, H. L. M.; Parviainen, H.; Passegger, V. M.; Persson, C. M.; Quirrenbach, A.; Redfield, S.; Reffert, S.; Reiners, A.; Ribas, I.; Serrano, L. M.; Tamura, M.; Van Eylen, V.; Watanabe, N.; Zapatero Osorio, M. R.
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Astronomy and Astrophysics

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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% (Rb = 1.900 ± 0.065 R⊕) and determine its mass with a precision of 16% (Mb = 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 (Teq ~ 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.
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