HD 21520 b: a warm sub-Neptune transiting a bright G dwarf
Hellier, Coel; Latham, David W.; Littlefield, Colin; Kane, Stephen R.; Stassun, Keivan G.; Wittenmyer, Robert A.; Howell, Steve B.; Horner, Jonathan; Ziegler, Carl; Collins, Karen A.; Watanabe, David; Ricker, George R.; Winn, Joshua N.; Seager, S.; Shporer, Avi; Mireles, Ismael; Palle, Enric; Dragomir, Diana; Lendl, Monika; Mann, Andrew W.; Clark, Catherine A.; Bowler, Brendan P.; Ting, Eric B.; Bouchy, François; Plavchan, Peter; Schwarz, Richard P.; Christiansen, Jessie L.; Kielkopf, John; Okumura, Jack; Wright, Duncan J.; Zhang, Hui; Law, Nicholas; Conti, Dennis M.; Briceño, César; Dumusque, Xavier; Santos, Nuno C.; Gnilka, Crystal L.; Hobson, Melissa; Liu, Huigen; Mengel, Matthew W.; Eisner, Nora L.; Psaridi, Angelica; Edwards, Billy; Ballard, Sarah; Lavie, Baptiste; Addison, Brett; Villaseñor, Joel; Sousa, Sergio G.; Oddo, Dominic; Medina, Jennifer V.; Nicholson, Belinda A.; Nies, Molly
United States, Switzerland, Australia, United Kingdom, Portugal, Chile, Netherlands, China, Spain
Abstract
We report the discovery and validation of HD 21520 b, a transiting planet found with Transiting Exoplanet Survey Satellite and orbiting a bright G dwarf (V = 9.2, $T_{\rm eff} = 5871 \pm 62$ K, $R_{\star } = 1.04\pm 0.02\, {\rm R}_{\odot }$). HD 21520 b was originally alerted as a system (TOI-4320) consisting of two planet candidates with periods of 703.6 and 46.4 d. However, our analysis supports instead a single-planet system with an orbital period of $25.1292\pm 0.0001$ d and radius of $2.70 \pm 0.09\, {\rm R}_{{\oplus }}$. Three full transits in sectors 4, 30, and 31 match this period and have transit depths and durations in agreement with each other, as does a partial transit in sector 3. We also observe transits using CHEOPS and LCOGT. SOAR and Gemini high-resolution imaging do not indicate the presence of any nearby companions, and MINERVA-Australis and CORALIE radial velocities rule out an on-target spectroscopic binary. Additionally, we use ESPRESSO radial velocities to obtain a tentative mass measurement of $7.9^{+3.2}_{-3.0}\, {\rm M}_{{\oplus }}$, with a 3$\sigma$ upper limit of 17.7 ${\rm M}_{{\oplus }}$. Due to the bright nature of its host and likely significant gas envelope of the planet, HD 21520b is a promising candidate for further mass measurements and for atmospheric characterization.