Gliese 12 b: A Temperate Earth-sized Planet at 12 pc Ideal for Atmospheric Transmission Spectroscopy
Authors: Kuzuhara Masayuki , Fukui Akihiko , Livingston John H. , Caballero José A. , de Leon Jerome P. , Hirano Teruyuki , Kasagi Yui , Murgas Felipe , Narita Norio , Omiya Masashi , Orell-Miquel Jaume , Palle Enric , Changeat Quentin , Esparza-Borges Emma , Harakawa Hiroki , Hellier Coel , Hori Yasunori , Ikuta Kai , Ishikawa Hiroyuki Tako , Kodama Takanori , Kotani Takayuki , Kudo Tomoyuki , Morales Juan C. , Mori Mayuko , Nagel Evangelos , Parviainen Hannu , Perdelwitz Volker , Reiners Ansgar , Ribas Ignasi , Sanz-Forcada Jorge , Sato Bun’ei , Schweitzer Andreas , Tabernero Hugo M. , Takarada Takuya , Uyama Taichi , Watanabe Noriharu , Zechmeister Mathias , García Néstor Abreu , Aoki Wako , Beichman Charles , Béjar Víctor J. S. , Brandt Timothy D. , Calatayud-Borras Yéssica , Carleo Ilaria , Charbonneau David , Collins Karen A. , Currie Thayne , Doty John P. , Dreizler Stefan , Fernández-Rodríguez Gareb , Fukuda Izuru , Galán Daniel , Geraldía-González Samuel , González-Rodríguez Josafat , Hayashi Yuya , Hedges Christina , Henning Thomas , Hodapp Klaus , Ikoma Masahiro , Isogai Keisuke , Jacobson Shane , Janson Markus , Jenkins Jon M. , Kagetani Taiki , Kambe Eiji , Kawai Yugo , Kawauchi Kiyoe , Kokubo Eiichiro , Konishi Mihoko , Korth Judith , Krishnamurthy Vigneshwaran , Kurokawa Takashi , Kusakabe Nobuhiko , Kwon Jungmi , Laza-Ramos Andrés , Libotte Florence , Luque Rafael , Madrigal-Aguado Alberto , Matsumoto Yuji , Mawet Dimitri , McElwain Michael W. , Meni Gallardo Pedro Pablo , Morello Giuseppe , Muñoz Torres Sara , Nishikawa Jun , Nugroho Stevanus K. , Ogihara Masahiro , Peláez-Torres Alberto , Rapetti David , Sánchez-Benavente Manuel , Schlecker Martin , Seager Sara , Serabyn Eugene , Serizawa Takuma , Stangret Monika , Takahashi Aoi , Teng Huan-Yu , Tamura Motohide , Terada Yuka , Ueda Akitoshi , Usuda Tomonori , Vanderspek Roland , Vievard Sébastien , Watanabe David , Winn Joshua N. , Zapatero Osorio Maria Rosa 
Published: May 2024

Recent discoveries of Earth-sized planets transiting nearby M dwarfs have made it possible to characterize the atmospheres of terrestrial planets via follow-up spectroscopic observations. However, the number of such planets receiving low insolation is still small, limiting our ability to understand the diversity of the atmospheric composition and climates of temperate terrestrial planets. We report the discovery of an Earth-sized planet transiting the nearby (12 pc) inactive M3.0 dwarf Gliese 12 (TOI-6251) with an orbital period (P
orb) of 12.76 days. The planet, Gliese 12 b, was initially identified as a candidate with an ambiguous P
orb from TESS data. We confirmed the transit signal and P
orb using ground-based photometry with MuSCAT2 and MuSCAT3, and validated the planetary nature of the signal using high-resolution images from Gemini/NIRI and Keck/NIRC2 as well as radial velocity (RV) measurements from the InfraRed Doppler instrument on the Subaru 8.2 m telescope and from CARMENES on the CAHA 3.5 m telescope. X-ray observations with XMM-Newton showed the host star is inactive, with an X-ray-to-bolometric luminosity ratio of

log

L

X

/

L

bol

≈
−
5.7

. Joint analysis of the light curves and RV measurements revealed that Gliese 12 b has a radius of 0.96 ± 0.05 R
⊕, a 3σ mass upper limit of 3.9 M
⊕, and an equilibrium temperature of 315 ± 6 K assuming zero albedo. The transmission spectroscopy metric (TSM) value of Gliese 12 b is close to the TSM values of the TRAPPIST-1 planets, adding Gliese 12 b to the small list of potentially terrestrial, temperate planets amenable to atmospheric characterization with JWST.