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

Publication date: 
Main author: 
Luque, R.
IAA authors: 
Luque, R.;Amado, P. J.
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.
Astronomy and Astrophysics
Publication type: 
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.
ADS Bibcode: 
planetary systems;techniques: photometric;techniques: radial velocities;stars: low-mass;stars: individual: G 9-40;Astrophysics - Earth and Planetary Astrophysics