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

TL;DR

This paper reports the discovery of HD 56414 b, a warm Neptune-sized planet transiting a young, hot A-type star, and explores its atmospheric retention and implications for the hot Neptune desert around such stars.

Contribution

It presents the first validation of a Neptune-sized planet orbiting a hot A-type star and models its atmospheric evolution considering the star's ultraviolet emission.

Findings

HD 56414 b has a radius of 3.71 R_⊕ and a 29-day orbit.

The planet can retain its atmosphere if its mass is ≥ 8 M_⊕ over 1 Gyr.

The hot Neptune desert likely extends to A-type stars.

Abstract

We report the discovery in TESS data and validation of HD 56414 b (a.k.a. TOI-1228 b), a Neptune-size ($R_{\rm p} = 3.71 \pm 0.20\, R_\oplus$) planet with a 29-day orbital period transiting a young (Age = $420 \pm 140$ Myr) A-type star in the TESS southern continuous viewing zone. HD 56414 is one of the hottest stars ($T_{\rm eff} = 8500 \pm 150 \, {\rm K}$) to host a known sub-Jovian planet. HD 56414 b lies on the boundary of the hot Neptune desert in 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 $\ge 8 \, M_\oplus$. Our model also predicts that close-in Neptune-size planets with masses $< 14 \, M_\oplus$ 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.