HD 56414 b: A Warm Neptune Transiting an A-type Star

Abstract

We report the discovery in TESS data and validation of HD 56414 b (a.k.a. TOI-1228 b), a Neptune-size (Rp = 3.71 ± 0.20 R⊕) planet with a 29 day orbital period transiting a young (age = 420 ± 140 Myr) A-type star in the TESS southern continuous-viewing zone. HD 56414 is one of the hottest stars (Teff = 8500 ± 150 K) to host a known sub-Jovian planet. HD 56414 b lies on the boundary of the hot Neptune desert in the planet radius– bolometric insolation flux space, suggesting that the planet may be experiencing mass loss. To explore this, we apply a photoevaporation model that incorporates the high near-ultraviolet continuum emission of A-type stars. We find that the planet can retain most of its atmosphere over the typical 1 Gyr main-sequence lifetime of an A-type star if its mass is 8 M⊕. Our model also predicts that close-in Neptune-size planets with masses <14 M⊕ are susceptible to total atmospheric stripping over 1 Gyr, hinting that the hot Neptune desert, which has been previously observed around FGKM-type stars, likely extends to A-type stars.