A large sub-Neptune transiting the thick-disk M4 v TOI-2406

R. D. Wells, B. V. Rackham, N. Schanche, R. Petrucci, Y. Gómez Maqueo Chew, B. O. Demory, A. J. Burgasser, R. Burn, F. J. Pozuelos, M. N. Günther, L. Sabin, U. Schroffenegger, M. A. Gómez-Muñoz, K. G. Stassun, V. Van Grootel, S. B. Howell, D. Sebastian, A. H.M.J. Triaud, D. Apai, I. Plauchu-FraynC. A. Guerrero, P. F. Guillén, A. Landa, G. Melgoza, F. Montalvo, H. Serrano, H. Riesgo, K. Barkaoui, A. Bixel, A. Burdanov, W. P. Chen, P. Chinchilla, K. A. Collins, T. Daylan, J. De Wit, L. Delrez, M. Dévora-Pajares, J. Dietrich, G. Dransfield, E. Ducrot, M. Fausnaugh, E. Furlan, P. Gabor, T. Gan, L. Garcia, M. Ghachoui, S. Giacalone, A. B. Gibbs, M. Gillon, C. Gnilka, R. Gore, N. Guerrero, T. Henning, K. Hesse, E. Jehin, J. M. Jenkins, D. W. Latham, K. Lester, J. McCormac, C. A. Murray, P. Niraula, P. P. Pedersen, D. Queloz, G. Ricker, D. R. Rodriguez, A. Schroeder, R. P. Schwarz, N. Scott, S. Seager, C. A. Theissen, S. Thompson, M. Timmermans, J. D. Twicken, J. N. Winn

Research output: Contribution to journalArticlepeer-review


Context. Large sub-Neptunes are uncommon around the coolest stars in the Galaxy and are rarer still around those that are metal-poor. However, owing to the large planet-to-star radius ratio, these planets are highly suitable for atmospheric study via transmission spectroscopy in the infrared, such as with JWST. Aims. Here we report the discovery and validation of a sub-Neptune orbiting the thick-disk, mid-M dwarf star TOI-2406. The star's low metallicity and the relatively large size and short period of the planet make TOI-2406 b an unusual outcome of planet formation, and its characterisation provides an important observational constraint for formation models. Methods. We first infer properties of the host star by analysing the star's near-infrared spectrum, spectral energy distribution, and Gaia parallax. We use multi-band photometry to confirm that the transit event is on-target and achromatic, and we statistically validate the TESS signal as a transiting exoplanet. We then determine physical properties of the planet through global transit modelling of the TESS and ground-based time-series data. Results. We determine the host to be a metal-poor M4 V star, located at a distance of 56 pc, with properties Teff = 3100 ± 75 K, M∗ = 0.162 ± 0.008Mo˙, R∗ = 0.202 ± 0.011Ro˙, and [Fe∕ H] = -0.38 ± 0.07, and a member of the thick disk. The planet is a relatively large sub-Neptune for the M-dwarf planet population, with Rp = 2.94 ± 0.17R⊕ and P= 3.077 d, producing transits of 2% depth. We note the orbit has a non-zero eccentricity to 3σ, prompting questions about the dynamical history of the system. Conclusions. This system is an interesting outcome of planet formation and presents a benchmark for large-planet formation around metal-poor, low-mass stars. The system warrants further study, in particular radial velocity follow-up to determine the planet mass and constrain possible bound companions. Furthermore, TOI-2406 b is a good target for future atmospheric study through transmission spectroscopy. Although the planet's mass remains to be constrained, we estimate the S/N using amass-radius relationship, ranking the system fifth in the population of large sub-Neptunes, with TOI-2406 b having a much lower equilibrium temperature than other spectroscopically accessible members of this population.

Original languageEnglish (US)
Article numberA97
JournalAstronomy and Astrophysics
StatePublished - Sep 1 2021

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Planets and satellites: detection
  • Stars: individual: TOI-2406
  • Techniques: photometric


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