TOI-4438 b: a transiting mini-Neptune amenable to atmospheric characterization

E. Goffo, P. Chaturvedi, F. Murgas, G. Morello, J. Orell-Miquel, L. Acuña, L. Peña-Moñino, E. Pallé, A. P. Hatzes, S. Geraldía-González, F. J. Pozuelos, A. F. Lanza, D. Gandolfi, J. A. Caballero, M. Schlecker, M. Pérez-Torres, N. Lodieu, A. Schweitzer, C. Hellier, S. V. JeffersC. Duque-Arribas, C. Cifuentes, V. J.S. Béjar, M. Daspute, F. Dubois, S. Dufoer, E. Esparza-Borges, A. Fukui, Y. Hayashi, E. Herrero, M. Mori, N. Narita, H. Parviainen, L. Tal-Or, S. Vanaverbeke, I. Hermelo, P. J. Amado, S. Dreizler, Th Henning, J. Lillo-Box, R. Luque, M. Mallorquín, E. Nagel, A. Quirrenbach, S. Reffert, A. Reiners, I. Ribas, P. Schöfer, H. M. Tabernero, M. Zechmeister

Research output: Contribution to journalArticlepeer-review


We report the confirmation and mass determination of a mini-Neptune transiting the M3.5 V star TOI-4438 (G 182-34) every 7.44 days. A transit signal was detected with NASA's TESS space mission in the sectors 40, 52, and 53. In order to validate the planet TOI-4438 b and to determine the system properties, we combined TESS data with high-precision radial velocity measurements from the CARMENES spectrograph, spanning almost one year, and ground-based transit photometry. We found that TOI-4438 b has a radius of Rb = 2.52 ± 0.13 R (5% precision), which together with a mass of Mb = 5.4 ± 1.1 M (20% precision), results in a bulk density of ρb = 1.85-0.44+0.51 g cm-3 (~28% precision), aligning the discovery with a volatile-rich planet. Our interior structure retrieval with a pure water envelope yields aminimum water mass fraction of 46% (1σ). TOI-4438 b is a volatile-rich mini-Neptune with likely H/He mixed with molecules, such as water, CO2, and CH4. The primary star has a J-band magnitude of 9.7, and the planet has a high transmission spectroscopy metric (TSM) of 136 ± 13. Taking into account the relatively warm equilibrium temperature of Teq = 435 ± 15 K, and the low activity level of its host star, TOI-4438 b is one of the most promising mini-Neptunes around an M dwarf for transmission spectroscopy studies.

Original languageEnglish
Article numberA147
JournalAstronomy and Astrophysics
StatePublished - 1 May 2024


  • Planets and satellites: composition
  • Planets and satellites: detection
  • Techniques: photometric
  • Techniques: radial velocities


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