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arxiv: 2004.10095 · v3 · pith:UVE6B5TAnew · submitted 2020-04-21 · 🌌 astro-ph.EP · astro-ph.SR

The multi-planet system TOI-421 -- A warm Neptune and a super puffy mini-Neptune transiting a G9 V star in a visual binary

Ilaria Carleo , Davide Gandolfi , Oscar Barrag\'an , John H. Livingston , Carina M. Persson , Kristine W. F. Lam , Aline Vidotto , Michael B. Lund
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Carolina Villarreal D'Angelo Karen A. Collins Luca Fossati Andrew W. Howard Daria Kubyshkina Rafael Brahm Antonija Oklop\v{c}i\'c Paul Molli\`ere Seth Redfield Luisa Maria Serrano Fei Dai Malcolm Fridlund Francesco Borsa Judith Korth Massimiliano Esposito Mat\'ias R. D\'iaz Louise Dyregaard Nielsen Coel Hellier Savita Mathur Hans J. Deeg Artie P. Hatzes Serena Benatti Florian Rodler Javier Alarcon Lorenzo Spina \^Angela R. G. Santos Iskra Georgieva Rafael A. Garc\'ia Luc\'ia Gonz\'alez-Cuesta George R. Ricker Roland Vanderspek David W. Latham Sara Seager Joshua N. Winn Jon M. Jenkins Simon Albrecht Natalie M. Batalha Corey Beard Patricia T. Boyd Fran\c{c}ois Bouchy Jennifer A. Burt R. Paul Butler Juan Cabrera Ashley Chontos David Ciardi William D. Cochran Kevin I. Collins Jeffrey D. Crane Ian Crossfield Szilard Csizmadia Diana Dragomir Courtney Dressing Philipp Eigm\"uller Michael Endl Anders Erikson Nestor Espinoza Michael Fausnaugh Fabo Feng Erin Flowers Benjamin Fulton Erica J. Gonzales Nolan Grieves Sascha Grziwa Eike W. Guenther Natalia M. Guerrero Thomas Henning Diego Hidalgo Teruyuki Hirano Maria Hjorth Daniel Huber Howard Isaacson Matias Jones Andr\'es Jord\'an Petr Kab\'ath Stephen R. Kane Emil Knudstrup Jack Lubin Rafael Luque Ismael Mireles Norio Narita David Nespral Prajwal Niraula Grzegorz Nowak Enric Palle Martin P\"atzold Erik A Petigura Jorge Prieto-Arranz Heike Rauer Paul Robertson Mark E. Rose Arpita Roy Paula Sarkis Joshua E. Schlieder Damien S\'egransan Stephen Shectman Marek Skarka Alexis M. S. Smith Jeffrey C. Smith Keivan Stassun Johanna Teske Joseph D. Twicken Vincent Van Eylen Sharon Wang Lauren M. Weiss Aur\'elien Wyttenbach
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classification 🌌 astro-ph.EP astro-ph.SR
keywords daysfoundneptuneperiodstarsystemtoi-421transit
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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.

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