NIRISS vs WFC3 – Exoplanet Showdown : A Comparative Analysis of Transmission Spectra Retrievals

With the dawn of the JWST era comes an explosion in the resolution and wavelength coverage of transmission spectra of exoplanets from space-based instruments. This brings an expected increase in the information content of these spectra, when compared with previous data from the Hubble Space Telescope. However, the degree and nature of this increase is not obvious. In this work, we attempt to quantify the information gain when comparing spectra from HST’s WFC3 instrument with its most similar JWST counterpart – NIRISS. We perform atmospheric retrievals on WFC3 and NIRISS transmission spectra of the hot Jupiter WASP-39b, and find inconsistencies in the water abundances. This suggests water abundance values in the literature from WFC3 retrievals could be unreliable. By comparing the retrieval results across all parameters, we quantify the increase in information from NIRISS vs WFC3. We also determine whether the causes of the improvement are due to the increase in wavelength coverage or resolution/precision, which varies for different parameters. Furthermore, we combine the NIRISS and WFC3 spectra with that of STIS and discuss inconsistencies in the data and retrieved quantities. We attempt to mitigate this by adding a shift to the STIS data, but this has no effect. This highlights issues that can arise from joint retrievals, and motivates the need for new reductions of archival data.