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Arxiv: JWST/NIRISS and HST: Exploring the improved ability to characterise exoplanet atmospheres in the JWST era Published: 9/25/2024 6:51:26 PM Updated: 9/25/2024 6:51:26 PM

Paper abstract: The Hubble Space Telescope has been a pioneering instrument for studying theatmospheres of exoplanets, specifically its WFC3 and STIS instruments. With thelaunch of JWST, we are able to observe larger spectral ranges at higherprecision. NIRISS/SOSS covers the range 0.6--2.8 microns, and thus can serve asa direct comparison to WFC3 (0.8--1.7 microns). We perform atmosphericretrievals of WFC3 and NIRISS transmission spectra of WASP-39 b in order tocompare their constraining power. We find that NIRISS is able to retrieveprecise H2O abundances that do not suffer a degeneracy with the continuumlevel, due to the coverage of multiple spectral features. We also combine thesedatasets with spectra from STIS, and find that challenges associated withfitting the steep optical slope can bias the retrieval results. In an effort todiagnose the differences between the WFC3 and NIRISS retrievals, we perform theanalysis again on the NIRISS data cut to the same wavelength range as WFC3. Wefind that the water abundance is in strong disagreement with both the WFC3 andfull NIRISS retrievals, highlighting the importance of wide wavelengthcoverage. Finally, we carry out mock retrievals on the different instruments,which shows further evidence of the challenges in constraining water abundancefrom the WFC3 data alone. Our study demonstrates the vast information gain ofJWST's NIRISS instrument over WFC3, highlighting the insights to be obtainedfrom our new era of space-based instruments.