  We are using archived data from HST of transiting exoplanet L~98-59~b to
place constraints on its potentially hot atmosphere. We analyze the data from
five transit visits and extract the final combined transmission spectrum using
Iraclis. Then we use the inverse atmospheric retrieval code TauREx to analyze
the combined transmission spectrum. There is a weak absorption feature near
1.40~$\mu m$ and 1.55~$\mu m$ in the transmission spectrum, which can be
modeled by a cloudy atmosphere with abundant HCN. However, the unrealistically
high abundance of HCN derived cannot be explained by any equilibrium chemical
model with reasonable assumptions. Thus, the likeliest scenario is that
L~98-59~b has a flat, featureless transmission spectrum in the WFC3/G141
bandpass due to a thin atmosphere with high mean molecular weight, an
atmosphere with an opaque aerosol layer, or no atmosphere, and it is very
unlikely for L~98-59~b to have a clear hydrogen-dominated primary atmosphere.
Due to the narrow wavelength coverage and low spectral resolution of HST/WFC3
G141 grism observation, we cannot tell these different scenarios apart. Our
simulation shows future higher precision measurements over wider wavelengths
from the James Webb Space Telescope (JWST) can be used to better characterize
the planetary atmosphere of L~98-59~b.
