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 TESS and orbiting a bright G dwarf (V=9.2, $T_{eff}=5871\pm 62$ K, $R_{\star }=1.04\pm 0.02R_{\odot }$). HD 21520 b was originally alerted as a system (TOI-4320) consisting of two planet candidates with periods of 703.6 and 46.4 days. However, our analysis supports instead a single-planet system with an orbital period of $25.1292\pm 0.0001$ days and radius of $2.70\pm 0.09R_{\oplus }$. 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 $7.9_{-3.0}^{+3.2}{M}_{\oplus }$, with a 3-$\ddot{I}ƒ$ upper limit of 17.7 $M_{\oplus }$. Due to the bright nature of its host and likely significant gas envelope of the planet, HD 21520 b is a promising candidate for further mass measurements and for atmospheric characterization.

Submitted to MNRAS