A Close-in Puffy Neptune with Hidden Friends: The Enigma of TOI 620
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 
Published: May 2022

Abstract
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 ≳0.″2. 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 M

P
< 7.1 M
⊕ and ρ

P
< 0.74 g cm−3, and we identify a nontransiting 17.7 day candidate. We also find evidence for a substellar (1–20 M

J
) 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.