Validation and atmospheric exploration of the sub-Neptune TOI-2136b around a nearby M3 dwarf

¹ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
e-mail: kiyoe2.1222@gmail.com
² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
³ Komaba Institute for Science, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
⁴ JST, PRESTO, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
⁵ Astrobiology Center, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁶ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁷ Department of Multi-Disciplinary Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
⁸ Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
⁹ Okayama Observatory, Kyoto University, 3037-5 Honjo, Kamogatacho, Asakuchi, Okayama 719-0232, Japan
¹⁰ Department of Astronomy, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
¹¹ NASA Ames Research Center, Moffett Field, CA 94035, US
¹² European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
¹³ Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
¹⁴ Department of Space, Earth and Environment, Astronomy and Plasma Physics, Chalmers University of Technology, 412 96 Gothenburg, Sweden
¹⁵ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹⁶ Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan
¹⁷ Department of Astronomy, School of Science, The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo, Japan
¹⁸ Institute of Astronomy and Astrophysics, Academia Sinica, P.O. Box 23-141, Taipei 10617, Taiwan, ROC
¹⁹ Department of Astrophysics, National Taiwan University, Taipei 10617, Taiwan, ROC
²⁰ NASA Exoplanet Science Institute, Caltech/IPAC, Mail Code 100-22 1200 E. California Blvd., Pasadena, CA 91125, USA
²¹ Department of Astronomy, University of Florida, Gainesville, FL 32611, USA
²² Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²³ Subaru Telescope, 650 N. Aohoku Place, Hilo, HI 96720, USA
²⁴ University of Hawaii, Institute for Astronomy, 640 N. Aohoku Place, Hilo, HI 96720, USA
²⁵ Division of Science, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
²⁶ Faculty of Science and Technology, Oita University, 700 Dannoharu, Oita 870-1192, Japan
²⁷ Institute of Engineering, Tokyo University of Agriculture and Technology, 2-24-16, Naka-cho, Koganei, Tokyo 184-8588, Japan

Received: 21 February 2022
Accepted: 14 June 2022

Abstract

Context. The NASA space telescope TESS is currently in the extended mission of its all-sky search for new transiting planets. Of the thousands of candidates that TESS is expected to deliver, transiting planets orbiting nearby M dwarfs are particularly interesting targets since they provide a great opportunity to characterize their atmospheres by transmission spectroscopy.

Aims. We aim to validate and characterize the new sub-Neptune-sized planet candidate TOI-2136.01 orbiting a nearby M dwarf (d = 33.36 ± 0.02pc, T_eff = 3373 ± 108 K) with an orbital period of 7.852 days.

Methods. We use TESS data, ground-based multicolor photometry, and radial velocity measurements with the InfraRed Doppler (IRD) instrument on the Subaru Telescope to validate the planetary nature of TOI-2136.01, and estimate the stellar and planetary parameters. We also conduct high-resolution transmission spectroscopy to search for helium in its atmosphere.

Results. We confirm that TOI-2136.01 (now named TOI-2136b) is a bona fide planet with a planetary radius of R_p = 2.20 ± 0.07R_⊕ and a mass of M_p = 4.7_−2.6^+3.1 M_⊕. We also search for helium 10830 Å absorption lines and place an upper limit on the equivalent width of <7.8 mÅ and on the absorption signal of <1.44% with 95% confidence.

Conclusions. TOI-2136b is a sub-Neptune transiting a nearby and bright star (J = 10.8 mag), and is a potentially hycean planet, which is a new class of habitable planets with large oceans under a H₂-rich atmosphere, making it an excellent target for atmospheric studies to understand the formation, evolution, and habitability of the small planets.