A Close-in Puffy Neptune with Hidden Friends: The Enigma of TOI 620

Full list of authors: Reefe, Michael A.; Luque, Rafael; Gaidos, Eric; Beard, Corey; Plavchan, Peter P.; Cointepas, Marion; Cale, Bryson L.; Palle, Enric; Parviainen, Hannu; Feliz, Dax L.; Eastman, Jason; Stassun, Keivan; Gagné, Jonathan; Jenkins, Jon M.; Boyd, Patricia T.; Kidwell, Richard C.; McDermott, Scott; Collins, Karen A.; Fong, William; Guerrero, Natalia; Almenara-Villa, Jose-Manuel; Bean, Jacob; Beichman, Charles A.; Berberian, John; Bieryla, Allyson; Bonfils, Xavier; Bouchy, François; Brady, Madison; Bryant, Edward M.; Cacciapuoti, Luca; Cañas, Caleb I.; Ciardi, David R.; Collins, Kevin I.; Crossfield, Ian J. M.; Dressing, Courtney D.; Eigmüller, Philipp; El Mufti, Mohammed; Esparza-Borges, Emma; Fukui, Akihiko; Gao, Peter; Geneser, Claire; Gnilka, Crystal L.; Gonzales, Erica; Gupta, Arvind F.; Halverson, Sam; Hearty, Fred; Howell, Steve B.; Irwin, Jonathan; Kanodia, Shubham; Kasper, David; Kodama, Takanori; Kostov, Veselin; Latham, David W.; Lendl, Monika; Lin, Andrea; Livingston, John H.; Lubin, Jack; Mahadevan, Suvrath; Matson, Rachel; Matthews, Elisabeth; Murgas, Felipe; Narita, Norio; Newman, Patrick; Ninan, Joe; Osborn, Ares; Quinn, Samuel N.; Robertson, Paul; Roy, Arpita; Schlieder, Joshua; Schwab, Christian; Seifahrt, Andreas; Smith, Gareth D.; Sohani, Ahmad; Stefánsson, Guðmundur; Stevens, Daniel; Stürmer, Julian; Tanner, Angelle; Terrien, Ryan; Teske, Johanna; Vermilion, David; Wang, Sharon X.; Wittrock, Justin; Wright, Jason T.; Zechmeister, Mathias; Zohrabi, Farzaneh.--This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.
We present the validation of a transiting low-density exoplanet orbiting the M2.5 dwarf TOI 620 discovered by the NASA Transiting Exoplanet Survey Satellite (TESS) mission. We utilize photometric data from both TESS and ground-based follow-up observations to validate the ephemerides of the 5.09 day transiting signal and vet false-positive scenarios. High-contrast imaging data are used to resolve the stellar host and exclude stellar companions at separations ≳0farcs2. We obtain follow-up spectroscopy and corresponding precise radial velocities (RVs) with multiple precision radial velocity (PRV) spectrographs to confirm the planetary nature of the transiting exoplanet. We calculate a 5σ upper limit of MP < 7.1 M⊕ and ρP < 0.74 g cm−3, and we identify a nontransiting 17.7 day candidate. We also find evidence for a substellar (1–20 MJ) companion with a projected separation ≲20 au from a combined analysis of Gaia, adaptive optics imaging, and RVs. With the discovery of this outer companion, we carry out a detailed exploration of the possibilities that TOI 620 b might instead be a circum-secondary planet or a pair of eclipsing binary stars orbiting the host in a hierarchical triple system. We find, under scrutiny, that we can exclude both of these scenarios from the multiwavelength transit photometry, thus validating TOI 620 b as a low-density exoplanet transiting the central star in this system. The low density of TOI 620 b makes it one of the most amenable exoplanets for atmospheric characterization, such as with the James Webb Space Telescope and Ariel, validated or confirmed by the TESS mission to date. © 2022. The Author(s). Published by the American Astronomical Society.
M.A.R. and P.P.P. acknowledge support from NASA (Exoplanet Research Program Award #80NSSC20K0251, TESS Cycle 3 Guest Investigator Program Award #80NSSC21K0349, JPL Research and Technology Development, and Keck Observatory Data Analysis) and the NSF (Astronomy and Astrophysics grant Nos. 1716202 and 2006517), and the Mt Cuba Astronomical Foundation. R.L. acknowledges financial support from the Spanish Ministerio de Ciencia e Innovación, through project PID2019-109522GB-C52, and the Centre of Excellence "Severo Ochoa" award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709). This work is partly supported by JSPS KAKENHI grant No. P17H04574, JP18H05439, JP20K14518, JP21K13975, JST CREST grant No. JPMJCR1761, and the Astrobiology Center of National Institutes of Natural Sciences (NINS) (grant Nos. AB022006, AB031010, AB031014). This work is partly financed by the Spanish Ministry of Economics and Competitiveness through grant No. PGC2018-098153-B-C31. V.K. gratefully acknowledges support from NASA via grant No. NNX17AF81G. M.L. acknowledges support from the Swiss National Science Foundation under grant No. PCEFP2_194576. The contribution of M.L. has been carried out within the framework of the NCCR PlanetS supported by the Swiss National Science Foundation. C.I.C. acknowledges support by NASA Headquarters under the NASA Earth and Space Science Fellowship Program through grant No. 80NSSC18K1114. NEID is funded by NASA/JPL under contract 1547612. We acknowledge support from NSF grant Nos. AST-190950 and 1910954.