The Multiplanet System TOI-421: A Warm Neptune and a Super Puffy Mini-Neptune Transiting a G9 V Star in a Visual Binary

Creators: Carleo, Ilaria; Gandolfi, Davide; Barragán, Oscar; Livingston, John H.; Persson, Carina M.; Lam, Kristine W. F.; Vidotto, Aline; Lund, Michael B.; Villarreal D'Angelo, Carolina; 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.; Dyregaard Nielsen, Louise; Hellier, Coel; Mathur, Savita; Deeg, Hans J.; Hatzes, Artie P.; Benatti, Serena; Rodler, Florian; Alarcon, Javier; Spina, Lorenzo; Santos, Ângela R. G.; Georgieva, Iskra; Garcia, 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, Néstor; 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.; Van Eylen, Vincent; Wang, Sharon; Weiss, Lauren M.; Wyttenbach, Aurélien

Abstract

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 = 2.68^(+0.19)_(-0.18) R⊕, whereas the outer warm Neptune, TOI-421 c, has a period of Pc = 16.06819 ± 0.00035 days, a mass of M_c = 16.42^(+1.06)_(-1.04) M⊕, a radius of R_c = 5.09^(+0.16)_(-0.15) R⊕ and a density of ρ_c = 0.685^(+0.080)_(-0.072) g cm⁻³. With its characteristics, the outer planet (ρ_c = 0.685^(+0.080)_(-0.072) 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.

Additional Information

© 2020 The American Astronomical Society. Received 2020 April 16; revised 2020 June 19; accepted 2020 June 22; published 2020 August 14. Based on observations made with ESO Telescopes at the La Silla Observatory under programs ID 1102.C-0923, 0103.C-0874, 0103.C-0759, 0103.C-0442, and 60.A-970. Based on observations obtained with the Nordic Optical Telescope (NOT), operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos (ORM) of the Instituto de Astrofísica de Canarias (IAC). This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile. This work makes use of observations from the LCOGT network. This work was supported by the KESPRINT collaboration, an international consortium devoted to the characterization and research of exoplanets discovered with space-based missions. We acknowledge the use of public TESS Alert data from pipelines at the TESS Science Office and at the TESS Science Processing Operations Center. Funding for the TESS mission is provided by NASA's Science Mission directorate. We acknowledge the use of public TESS Alert data from pipelines at the TESS Science Office and at the TESS Science Processing Operations Center. Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center for the production of the SPOC data products. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (NASA). We are very grateful to the NOT and ESO staff members for their unique and superb support during the observations. A.A.V. and D.K. acknowledge funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement No 817540, ASTROFLOW). P.M. thanks Thomas Mikal-Evans and Laura Kreidberg for help computing predicted HST uncertainties for TOI-421. P.M. acknowledges support from the European Research Council under the European Union's Horizon 2020 research and innovation program under grant agreement No. 832428. L.M.S. and D.G. gratefully acknowledge financial support from the CRT foundation under grant No. 2018.2323 "Gaseous or rocky? Unveiling the nature of small worlds." R.B. acknowledges support from FONDECYT Post-doctoral Fellowship Project 3180246. A.J. and R.B. acknowledge support from project IC120009 "Millennium Institute of Astrophysics (MAS)" of the Millennium Science Initiative, Chilean Ministry of Economy. A.J. acknowledges additional support from FONDECYT project 1171208. C.V.D. acknowledges the funding from the Irish Research Council through the postdoctoral fellowship (Project ID: GOIPD/2018/659). C.M.P., M.F., and I.G. gratefully acknowledge the support of the Swedish National Space Agency (DNR 65/19 and 136/13). L.S. acknowledges financial support from the Australian Research Council (Discovery Project 170100521). D.D. acknowledges support from NASA through Caltech/JPL grant RSA-1006130 and through the TESS Guest Investigator Program grant 80NSSC19K1727. D.H. acknowledges support from the Alfred P. Sloan Foundation, the National Aeronautics and Space Administration (80NSSC18K1585, 80NSSC19K0379), and the National Science Foundation (AST-1717000). S.M. acknowledges support from the Spanish Ministry with the Ramon y Cajal fellowship number RYC-2015-17697. L.G.C. thanks the support from grant FPI-SO from the Spanish Ministry of Economy and Competitiveness (MINECO), under research project SEV-2015-0548-17-2 and predoctoral contract BES-2017-082610. A.R.G.S. acknowledges support from NASA under grant No. NNX17AF27G. J.N.W. thanks the Heising-Simons foundation for support. R.A.G. acknowledges support from PLATO and GOLF CNES grants. M.R.D. acknowledges support from CONICYT-PFCHA/Doctorado Nacional-21140646, Chile. K.W.F.L., J.K., S.z.C.s., M.E., S.G., A.P.H., M.P., and H.R. acknowledge support by DFG grants PA525/18-1, PA525/19-1, PA525/20-1, HA3279/12-1, and RA714/14-1 within the DFG Schwerpunkt SPP 1992, "Exploring the Diversity of Extrasolar Planets" This research has made use of the Exoplanet Follow-up Observation Program website, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program.

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