We present observations of WASP-63b by the Hubble Space Telescope (HST) as
part of "A Preparatory Program to Identify the Single Best Transiting Exoplanet
for JWST Early Release Science". WASP-63b is one of the community targets under
consideration for the James Webb Space Telescope (JWST) Early Release Science
(ERS) program. We present a spectrum derived from a single observation by HST
Wide Field Camera 3 in the near infrared. We engaged groups across the
transiting exoplanet community to participate in the analysis of the data and
present results from each. There is general agreement amongst all results that
we find an H2O absorption feature with 3.5-4.0 sigma significance. However, the
feature is muted in comparison to a clear atmosphere at solar composition.
Although the detection of the water feature is robust, the reasons for the
muting of this feature are ambiguous due to a degeneracy between clouds and
composition. The data does not yield robust detections of any molecular species
other than H2O. The group was motivated to perform an additional set of
retrieval exercises to investigate an apparent bump in the spectrum at ~ 1.55
um. We explore possible disequilibrium chemistry and find this feature is
consistent with super-solar HCN abundance but it is questionable if the
required mixing ratio of HCN is chemically and physically plausible. The
ultimate goal of this study is to vet WASP-63b as a potential community target
to best demonstrate the capabilities and systematics of JWST instruments for
transiting exoplanet science. In the case of WASP-63b, the presence of a
detectable water feature indicates that WASP-63b remains a plausible target for
ERS observations