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; BARRAGAN Oscar; LIVINGSTON John H.; PERSSON Carina M.; LAM Kristine W. F.; VIDOTTO Aline; LUND Michael B.; D'ANGELO Carolina Villarr; COLLINS Karen A.; FOSSATI Luca; HOWARD Andrew W.; KUBYSHKINA Daria; BRAHM Rafael; OKLOPCIC Antonija; MOLLIERE Paul; REDFIELD Seth; SERRANO Luisa Maria; DAI Fei; FRIDLUND Malcolm; BORSA Francesco; KORTH Judith; ESPOSITO Massimiliano; DIAZ Matias R.; NIELSEN Louise Dyregaard; HELLIER Coel; MATHUR Savita; DEEG Hans J.; HATZES Artie P.; BENATTI Serena; RODLER Florian; ALARCON Javier; SPINA Lorenzo; SANTOS Angela R. G.; GEORGIEVA Iskra; GARCIA Rafael A.; GONZALEZ-CUESTA Lucia; 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 Francois; 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; EIGMULLER 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; JORDAN Andres; KABATH Petr; KANE Stephen R.; KNUDSTRUP Emil; LUBIN Jack; LUQUE Rafael; MIRELES Ismael; NARITA Norio; NESPRAL David; NIRAULA Prajwal; NOWAK Grzegorz; PALLE Enric; PATZOLD Martin; PETIGURA Erik A.; PRIETO-ARRANZ Jorge; RAUER Heike; ROBERTSON Paul; ROSE Mark E.; ROY Arpita; SARKIS Paula; SCHLIEDER Joshua E.; SEGRANSAN 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 Aurelien
• Year of publication: 2020
• Type: Article in Periodical
• Magazine / Source: Astronomical Journal
• MU Faculty or unit: Faculty of Science
• Web: https://doi.org/10.3847/1538-3881/aba124
• Doi: http://dx.doi.org/10.3847/1538-3881/aba124
• Keywords: Exoplanet astronomy; Exoplanet systems; Exoplanets; Mini Neptunes; Hot Neptunes; Fundamental parameters of stars; High resolution spectroscopy; Transit photometry; Radial velocity
• 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 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 Echelle Spectrograph, CORALIE, High Accuracy Radial velocity Planet Searcher, High Resolution echelle 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-circle plus, and a radius of R-b = R-circle plus, whereas the outer warm Neptune, TOI-421 c, has a period of P-c = 16.06819 +/- 0.00035 days, a mass of M-c = 16.42(-1.04)(+1.06)M(circle plus), a radius of R-c = 5.09(-0.15)(+0.16)R(circle plus), and a density of rho(c) = 0.685(-0.072)(+0.080) cm(-3). With its characteristics, the outer planet (rho(c) = 0.685(-0.0072)(+0.080) 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 Ly alpha 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.