The Multiplanet System TOI-421: A Warm Neptune and a Super Puffy Mini-Neptune Transiting a G9 V Star in a Visual Binary*
Authors: Carleo Ilaria , Gandolfi Davide , Barragán Oscar , Livingston John H. , Persson Carina M. , Lam Kristine W. F. , Vidotto Aline , Lund Michael B. , D’Angelo Carolina Villarreal , Collins Karen A. , Fossati Luca , Howard Andrew W. , Kubyshkina Daria , Brahm Rafael , Oklopčić Antonija , Mollière Paul , Redfield Seth , Serrano Luisa Maria , Dai Fei , Fridlund Malcolm , Borsa Francesco , Korth Judith , Esposito Massimiliano , Díaz Matías R. , Nielsen Louise Dyregaard , Hellier Coel , Mathur Savita , Deeg Hans J. , Hatzes Artie P. , Benatti Serena , Rodler Florian , Alarcon Javier , Spina Lorenzo , Santos Ângela R. G. , Georgieva Iskra , García Rafael A. , González-Cuesta Lucía , Ricker George R. , Vanderspek Roland , Latham David W. , Seager Sara , Winn Joshua N. , Jenkins Jon M. , Albrecht Simon , Batalha Natalie M. , Beard Corey , Boyd Patricia T. , Bouchy François , Burt Jennifer A. , Butler R. Paul , Cabrera Juan , Chontos Ashley , Ciardi David R. , Cochran William D. , Collins Kevin I. , Crane Jeffrey D. , Crossfield Ian , Csizmadia Szilard , Dragomir Diana , Dressing Courtney , Eigmüller Philipp , Endl Michael , Erikson Anders , Espinoza Nestor , Fausnaugh Michael , Feng Fabo , Flowers Erin , Fulton Benjamin , Gonzales Erica J. , Grieves Nolan , Grziwa Sascha , Guenther Eike W. , Guerrero Natalia M. , Henning Thomas , Hidalgo Diego , Hirano Teruyuki , Hjorth Maria , Huber Daniel , Isaacson Howard , Jones Matias , Jordán Andrés , Kabáth Petr , Kane Stephen R. , Knudstrup Emil , Lubin Jack , Luque Rafael , Mireles Ismael , Narita Norio , Nespral David , Niraula Prajwal , Nowak Grzegorz , Palle Enric , Pätzold Martin , Petigura Erik A , Prieto-Arranz Jorge , Rauer Heike , Robertson Paul , Rose Mark E. , Roy Arpita , Sarkis Paula , Schlieder Joshua E. , Ségransan Damien , Shectman Stephen , Skarka Marek , Smith Alexis M. S. , Smith Jeffrey C. , Stassun Keivan , Teske Johanna , Twicken Joseph D. , Eylen Vincent Van , Wang Sharon , Weiss Lauren M. , Wyttenbach Aurélien 
Published: August 2020

We report the discovery of a warm Neptune and a hot sub-Neptune transiting TOI-421 (BD-14 1137, TIC 94986319), a bright (V = 9.9) G9 dwarf star in a visual binary system observed by the Transiting Exoplanet Survey Satellite (TESS) space mission in Sectors 5 and 6. We performed ground-based follow-up observations—comprised of Las Cumbres Observatory Global Telescope transit photometry, NIRC2 adaptive optics imaging, and FIbre-fed Echellé Spectrograph, CORALIE, High Accuracy Radial velocity Planet Searcher, High Resolution Échelle Spectrometer, and Planet Finder Spectrograph high-precision Doppler measurements—and confirmed the planetary nature of the 16 day transiting candidate announced by the TESS team. We discovered an additional radial velocity signal with a period of five days induced by the presence of a second planet in the system, which we also found to transit its host star. We found that the inner mini-Neptune, TOI-421 b, has an orbital period of P
b = 5.19672 ± 0.00049 days, a mass of M
b = 7.17 ± 0.66 M
⊕, and a radius of R
b = 

 R
⊕, whereas the outer warm Neptune, TOI-421 c, has a period of P
c = 16.06819 ± 0.00035 days, a mass of M
c = 

 M
⊕, a radius of R
c = 

 R
⊕, and a density of ρ
c =

g cm−3. With its characteristics, the outer planet (ρ
c = 

g cm−3) is placed in the intriguing class of the super-puffy mini-Neptunes. TOI-421 b and TOI-421 c are found to be well-suited for atmospheric characterization. Our atmospheric simulations predict significant Lyα transit absorption, due to strong hydrogen escape in both planets, as well as the presence of detectable CH4 in the atmosphere of TOI-421 c if equilibrium chemistry is assumed.