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.
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