Characterizing a World Within the Hot-Neptune Desert: Transit Observations of LTT 9779 b with the Hubble Space Telescope/WFC3

Abstract We present an atmospheric analysis of LTT 9779 b, a rare planet situated in the hot-Neptune desert, that has been observed with Hubble Space Telescope (HST)/WFC3 G102 and G141. The combined transmission spectrum, which covers 0.8–1.6 μ m, shows gradual increase transit depth wavelength. Our preferred model evidence for H 2 O, CO , FeH significance 3.1 σ 2.4 2.1 respectively. In attempt to constrain rate escape this planet, we search 1.083 m helium line data but find no excess absorption would indicate escaping atmosphere using tracer. refine orbital ephemerides b our HST observations from TESS, searching decay or apsidal precession, are not found. phase-curve observation JWST NIRISS should provide deeper insights into expected might be detected further concentrated on other tracers such as Ly α .