Strong H 2 O and CO Emission Features in the Spectrum of KELT-20b Driven by Stellar UV Irradiation

Know thy star, know thy planetary atmosphere. Every exoplanet with atmospheric measurements orbits around a star, and the stellar environment directly affects the planetary atmosphere. Here we present the emission spectrum of ultra-hot Jupiter KELT-20b which provides an observational link between host-star properties and planet atmospheric thermal structure. It is currently the only planet with thermal emission measurements in the Teq ∼ 2200 K range that orbits around an early A-type star. By comparing it with other similar ultra-hot Jupiters around FGK stars, we can better understand how different host-star types influence planetary atmospheres. The emission spectrum covers 0.6–4.5 μm with data from TESS, HST WFC3/G141, and Spitzer 4.5 μm channel. KELT-20b has a 1.4 μm water feature strength metric of ${{\rm{S}}}_{{{\rm{H}}}_{2}{\rm{O}}}$ = −0.097 ± 0.02 and a blackbody brightness temperature difference of 528 K between WFC3/G141 (Tb = 2402 ± 14 K) and Spitzer 4.5 μm channel (Tb = 2930 ± 59 K). These very large H2O and CO emission features combined with the A-type host star make KELT-20b a unique planet among other similar hot Jupiters. The abundant FUV, NUV, and optical radiation from its host star (Teff = 8720 ± 250 K) is expected to be the key that drives its strong thermal inversion and prominent emission features based on previous PHOENIX model calculations.