HAT-P-18b JWST NIRISS transmission spectrum

The early release observation (ERO) program provides us with the first look at the scientific data and the spectral capabilities. One of the targets from the ERO is HAT-P-18b, an inflated Saturn-mass planet with an equilibrium temperature of ~850K.

We present the NIRISS/SOSS transmission spectrum of HAT-P-18b from 0.6 to 2.8{mu}m and reveal the planet in the infrared beyond 1.6{mu}m for the first time. From the spectrum, we see clear water and escaping helium tail features in an otherwise very hazy atmosphere. Our free chemistry retrievals with ATMO show moderate Bayesian evidence (3.79) supporting the presence of methane, but the spectrum does not display any clearly identifiable methane absorption features.