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We present the analysis of the atmospheres of 70 gaseous extrasolar planetsvia transit spectroscopy with Hubble's Wide Field Camera 3 (WFC3). For overhalf of these, we statistically detect spectral modulation which our retrievalsattribute to molecular species. Among these, we use Bayesian HierarchicalModelling to search for chemical trends with bulk parameters. We use theextracted water abundance to infer the atmospheric metallicity and compare itto the planet's mass. We also run chemical equilibrium retrievals, fitting forthe atmospheric metallicity directly. However, although previous studies havefound evidence of a mass-metallicity trend, we find no such relation within ourdata. For the hotter planets within our sample, we find evidence for thermaldissociation of dihydrogen and water via the H$^-$ opacity. We suggest that thegeneral lack of trends seen across this population study could be due to i) theinsufficient spectral coverage offered by HST WFC3 G141, ii) the lack of asimple trend across the whole population, iii) the essentially random nature ofthe target selection for this study or iv) a combination of all the above. Weset out how we can learn from this vast dataset going forward in an attempt toensure comparative planetology can be undertaken in the future with facilitiessuch as JWST, Twinkle and Ariel. We conclude that a wider simultaneous spectralcoverage is required as well as a more structured approach to target selection.

Exploring the Ability of HST WFC3 G141 to Uncover Trends in Populations  of Exoplanet Atmospheres Through a Homogeneous Transmission Survey of 70  Gaseous Planets