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

¹ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
e-mail: robert.wells@csh.unibe.ch
² Department of Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
³ Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁴ Universidad Nacional de Córdoba – Observatorio Astronómico de Córdoba, Laprida 854, X5000BGR, Córdoba, Argentina
⁵ Consejo Nacional de Investigaciones Cientf́icas y Técnicas (CONICET), Buenos Aires, Argentina
⁶ Universidad Nacional Autónoma de México, Instituto de Astronomía, AP 70-264, CDMX 04510, México
⁷ Center for Astrophysics and Space Science, University of California San Diego, La Jolla, CA 92093, USA
⁸ Max Planck Institute for Astronomy (MPIA), Koenigstuhl 17, 69117 Heidelberg, Germany
⁹ Astrobiology Research Unit, Université de Liège, Allée du 6 Août 19C, 4000 Liège, Belgium
¹⁰ Space Sciences, Technologies and Astrophysics Research (STAR) Institute, Universitďe Liège, Allée du 6 Août 19C, 4000 Liège, Belgium
¹¹ Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹² Universidad Nacional Autónoma de México, Instituto de Astronomía, AP 106, Ensenada 22800, BC, México
¹³ Department of Physics & Astronomy, Vanderbilt University, 6301 Stevenson Center Ln., Nashville, TN 37235, USA
¹⁴ NASA Ames Research Center, Moffett Field, CA 94035, USA
¹⁵ School of Physics & Astronomy, University of Birmingham, Edgbaston, Birmimgham B15 2TT, UK
¹⁶ Steward Observatory, The University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA
¹⁷ Lunar and Planetary Laboratory, The University of Arizona, 1639 E. University Boulevard, Tucson, AZ 85721, USA
¹⁸ Oukaimeden Observatory, High Energy Physics and Astrophysics Laboratory, Cadi Ayyad University, Marrakech, Morocco
¹⁹ Graduate Institute of Astronomy, National Central University, 300 Jhongda Road, Zhongli, Taoyuan 32001, Taiwan
²⁰ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
²¹ Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA, 02138, USA
²² Dpto. Física Teórica y del Cosmos, Universidad de Granada, 18071, Granada, Spain
²³ NASA Exoplanet Science Institute, Caltech/IPAC, Mail Code 100-22, 1200 E. California Blvd., Pasadena, CA 91125, USA
²⁴ Vatican Observatory Research Group, University of Arizona, 933 N Cherry Ave., Tucson AZ, 85721-0065, USA
²⁵ Department of Astronomy, Tsinghua University, Beijing 100084, PR China
²⁶ Department of Astronomy, 501 Campbell Hall, University of California at Berkeley, Berkeley, CA, 94720, USA
²⁷ Department of Physics & Astronomy, University of California, Los Angeles, Los Angeles, CA 90095, USA
²⁸ Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
²⁹ Cavendish Laboratory, JJ Thomson Avenue, Cambridge, CB3 0HE, UK
³⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218, USA
³¹ Patashnick Voorheesville Observatory, Voorheesville, NY 12186, USA
³² Department of Aeronautics and Astronautics, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
³³ SETI Institute, Mountain View, CA 94043, USA
³⁴ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08544, USA

Received: 10 May 2021
Accepted: 23 July 2021

Abstract

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 T_eff = 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, with R_p = 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.