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

description: 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 TESS space mission in Sectors 5 and 6. We performed ground-based follow-up observations -- comprised of LCOGT transit photometry, NIRC2 adaptive optics imaging, and FIES, CORALIE, HARPS, HIRES, and PFS 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 5 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-421b, has an orbital period of Pb =5.19672 +- 0.00049 days, a mass of Mb = 7.17 +- 0.66 Mearth and a radius of Rb = 2.68+0.19-0.18 Rearth, whereas the outer warm Neptune, TOI-421 c, has a period of Pc =16.06819 +- 0.00035 days, a mass of Mc = 16.42+1.06-1.04 Mearth, a radius of Rc = 5.09+0.16-0.15 Rearth and a density of rho_c =0.685+0.080-0.072 g cm-3 . With its characteristics the inner planet (rho_b=2.05+0.52-0.41 g cm-3) is placed in the intriguing class of the super-puffy mini-Neptunes. TOI-421b and TOI-421c are found to be well suitable for atmospheric characterization. Our atmospheric simulations predict significant Ly-alpha transit absorption, due to strong hydrogen escape in both planets, and the presence of detectable CH_4 in the atmosphere of TOI-421c if equilibrium chemistry is assumed.