A large sub-Neptune transiting the thick-disk M4 V TOI-2406
Authors: Wells R. D. , Rackham B. V. , Schanche N. , Petrucci R. , Gómez Maqueo Chew Y. , Demory B.-O. , Burgasser A. J. , Burn R. , Pozuelos F. J. , Günther M. N. , Sabin L. , Schroffenegger U. , Gómez-Muñoz M. A. , Stassun K. G. , Van Grootel V. , Howell S. B. , Sebastian D. , Triaud A. H. M. J. , Apai D. , Plauchu-Frayn I. , Guerrero C. A. , Guillén P. F. , Landa A. , Melgoza G. , Montalvo F. , Serrano H. , Riesgo H. , Barkaoui K. , Bixel A. , Burdanov A. , Chen W. P. , Chinchilla P. , Collins K. A. , Daylan T. , de Wit J. , Delrez L. , Dévora-Pajares M. , Dietrich J. , Dransfield G. , Ducrot E. , Fausnaugh M. , Furlan E. , Gabor P. , Gan T. , Garcia L. , Ghachoui M. , Giacalone S. , Gibbs A. B. , Gillon M. , Gnilka C. , Gore R. , Guerrero N. , Henning T. , Hesse K. , Jehin E. , Jenkins J. M. , Latham D. W. , Lester K. , McCormac J. , Murray C. A. , Niraula P. , Pedersen P. P. , Queloz D. , Ricker G. , Rodriguez D. R. , Schroeder A. , Schwarz R. P. , Scott N. , Seager S. , Theissen C. A. , Thompson S. , Timmermans M. , Twicken J. D. , Winn J. N. 
Published: July 2021

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, andGaiaparallax. 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 propertiesTeff= 3100 ± 75 K,M*= 0.162 ± 0.008M⊙,R*= 0.202 ± 0.011R⊙, 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, withRp= 2.94 ± 0.17R⊕andP= 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.