  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
$T_{eq}\sim$2200K 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 to 4.5 $\mu m$ with data from TESS, HST WFC3/G141,
and Spitzer 4.5 $\mu m$ channel. KELT-20b has a 1.4 $\mu m$ water feature
strength metric of S$_{H_2O}$ = -0.097$\pm$0.02 and a blackbody brightness
temperature difference of 528K between WFC3/G141 (T$_b$=2402$\pm$14K) and
Spitzer 4.5 $\mu m$ channel (T$_b$=2930$\pm59$K). These very large H$_2$O 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 (T$_{eff}=8720\pm250$K) is expected to be the key
that drives its strong thermal inversion and prominent emission features based
on previous PHOENIX models calculations.
