  We have conducted a re-analysis of publicly available Hubble Space Telescope
Wide Field Camera 3 (HST WFC3) transmission data for the hot-Jupiter exoplanet
WASP-43b, using the Bayesian retrieval package Tau-REx. We report evidence of
AlO in transmission to a high level of statistical significance (> 5-sigma in
comparison to a flat model, and 3.4-sigma in comparison to a model with H2O
only). We find no evidence of the presence of CO, CO2, or CH4 based on the
available HST WFC3 data or on Spitzer IRAC data. We demonstrate that AlO is the
molecule that fits the data to the highest level of confidence out of all
molecules for which high-temperature opacity data currently exists in the
infrared region covered by the HST WFC3 instrument, and that the subsequent
inclusion of Spitzer IRAC data points in our retrieval further supports the
presence of AlO. H2O is the only other molecule we find to be statistically
significant in this region. AlO is not expected from the equilibrium chemistry
at the temperatures and pressures of the atmospheric layer that is being probed
by the observed data. Its presence therefore implies direct evidence of some
disequilibrium processes with links to atmospheric dynamics. Implications for
future study using instruments such as the James Webb Space Telescope (JWST)
are discussed, along with future opacity needs. Comparisons are made with
previous studies into WASP-43b.
