Validation and atmospheric exploration of the sub-Neptune TOI-2136b around a nearby M3 dwarf
Kuzuhara, M.; Tamura, M.; Kudo, T.; Konishi, M.; Hirano, T.; Korth, J.; Narita, N.; Palle, E.; Kimura, T.; Stangret, M.; Sánchez-Benavente, M.; Luque, R.; Kodama, T.; Morello, G.; Murgas, F.; Fukui, A.; Crouzet, N.; Parviainen, H.; Watanabe, N.; Kusakabe, N.; Mori, M.; Terada, Y.; de Leon, J. P.; Nishiumi, T.; Orell-Miquel, J.; Gnilka, C. L.; Zou, Y.; Guerrero, N.; Howell, S. B.; Isogai, K.; Laza-Ramos, A.; Livingston, J. H.; Kawauchi, K.; Ishikawa, H. T.; Esparaza-Borges, E.; Krishnamurthy, V.; Kagetani, T.; Kurita, S.; Madrigal-Aguado, A.; Miyakawa, K.; Rodríguez, G. E. F.; Teng, H.; Harakawa, H.; Hodapp, K.; Hori, Y.; Ikoma, M.; Jacobson, S.; Kotani, T.; Kurokowa, T.; Nishikawa, J.; Omiya, M.; Serizawa, T.; Ueda, A.; Vievard, S.
Spain, Japan, United States, Netherlands, Sweden, Taiwan
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, Teff = 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 Rp = 2.20 ± 0.07R⊕ and a mass of Mp = 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.