HD 21520 b: a warm sub-Neptune transiting a bright G dwarf

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 ₄₅₅ = 5871 62 K, R = 1. 04 0. 02\, R). 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 0. 0001 d and radius of 2. 70 0. 09\, R. 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\, M, with a 3 upper limit of 17. 7 M. 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.