The Discovery and Follow-up of Four Transiting Short-period Sub-Neptunes Orbiting M Dwarfs
Authors: Hori Yasunori , Fukui Akihiko , Hirano Teruyuki , Narita Norio , de Leon Jerome P. , Ishikawa Hiroyuki Tako , Hartman Joel D. , Morello Giuseppe , García Nestor Abreu , Hernández Leticia Álvarez , Béjar Víctor J. S. , Calatayud-Borras Yéssica , Carleo Ilaria , Enoc Gareb , Esparza-Borges Emma , Fukuda Izuru , Galán Daniel , Geraldía-González Samuel , Hayashi Yuya , Ikoma Masahiro , Ikuta Kai , Isogai Keisuke , Kagetani Taiki , Kawai Yugo , Kawauchi Kiyoe , Kimura Tadahiro , Kodama Takanori , Korth Judith , Kusakabe Nobuhiko , Laza-Ramos Andrés , Livingston John H. , Luque Rafael , Miyakawa Kohei , Mori Mayuko , Murgas Felipe , Orell-Miquel Jaume , Palle Enric , Parviainen Hannu , Peláez-Torres Alberto , Puig-Subirà Marta , Sánchez-Benavente Manuel , Sosa-Guillén Paula , Stangret Monika , Terada Yuka , Muñoz Torres Sara , Watanabe Noriharu , Bakos Gaspar Á. , Barkaoui Khalid , Beichman Charles , Benkhaldoun Zouhair , Boyle Andrew W. , Ciardi David R. , Clark Catherine A. , Collins Karen A. , Collins Kevin I. , Conti Dennis M. , Crossfield Ian J.M. , Everett Mark E. , Furlan Elise , Ghachoui Mourad , Gillon Michaël , Gonzales Erica J. , Higuera Jesus , Horne Keith , Howell Steve B. , Jehin Emmanuël , Lester Kathryn V. , Lund Michael B. , Matson Rachel , Matthews Elisabeth C. , Pozuelos Francisco J. , Safonov Boris S. , Schlieder Joshua E. , Schwarz Richard P. , Sefako Ramotholo , Srdoc Gregor , Strakhov Ivan A. , Timmermans Mathilde , Waalkes William C. , Ziegler Carl , Charbonneau David , Essack Zahra , Guerrero Natalia M. , Harakawa Hiroki , Hedges Christina , Ishizuka Masato , Jenkins Jon M. , Konishi Mihoko , Kotani Takayuki , Kudo Tomoyuki , Kurokawa Takashi , Kuzuhara Masayuki , Nishikawa Jun , Omiya Masashi , Ricker George R. , Seager Sara , Serizawa Takuma , Striegel Stephanie , Tamura Motohide , Ueda Akitoshi , Vanderspek Roland , Vievard Sébastien , Winn Joshua N. 
Published: May 2024

Abstract
Sub-Neptunes with radii of 2–3 R
⊕ are intermediate in size between rocky planets and Neptune-sized planets. The orbital properties and bulk compositions of transiting sub-Neptunes provide clues to the formation and evolution of close-in small planets. In this paper, we present the discovery and follow-up of four sub-Neptunes orbiting M dwarfs (TOI-782, TOI-1448, TOI-2120, and TOI-2406), three of which were newly validated by ground-based follow-up observations and statistical analyses. TOI-782 b, TOI-1448 b, TOI-2120 b, and TOI-2406 b have radii of

R

p

=

2.740

−
0.079

+
0.082

R

⊕

,

2.769

−
0.068

+
0.073

R

⊕

, 2.120 ± 0.067 R
⊕, and

2.830

−
0.066

+
0.068

R

⊕

and orbital periods of P = 8.02, 8.11, 5.80, and 3.08 days, respectively. Doppler monitoring with the Subaru/InfraRed Doppler instrument led to 2σ upper limits on the masses of <19.1 M
⊕, <19.5 M
⊕, <6.8 M
⊕, and <15.6 M
⊕ for TOI-782 b, TOI-1448 b, TOI-2120 b, and TOI-2406 b, respectively. The mass–radius relationship of these four sub-Neptunes testifies to the existence of volatile material in their interiors. These four sub-Neptunes, which are located above the so-called “radius valley,” are likely to retain a significant atmosphere and/or an icy mantle on the core, such as a water world. We find that at least three of the four sub-Neptunes (TOI-782 b, TOI-2120 b, and TOI-2406 b), orbiting M dwarfs older than 1 Gyr, are likely to have eccentricities of e ∼ 0.2–0.3. The fact that tidal circularization of their orbits is not achieved over 1 Gyr suggests inefficient tidal dissipation in their interiors.