  We present a population study of 20 exoplanets, ranging from Neptune-like to
inflated hot-Jupiter planets, observed during transit with the STIS and WFC3
instruments aboard the Hubble Space Telescope. To obtain spectral information
from the near-UV to the near-infrared, we reanalysed sixteen WFC3 and over
fifty STIS archival data sets with our dedicated HST pipeline. We also include
twenty-four WFC3 data sets previously reduced with the same software. Across
our target sample we observe significant divergence among multiple observations
conducted with the same STIS grating at various epochs, whilst we do not detect
variations in the WFC3 data sets. These results are suggestive of stellar
contamination, which we have investigated further using known Bayesian tools
and other tailored metrics, facilitating a more objective assessment of stellar
activity intensity within each system. Our findings reveal that stellar
activity contaminates up to half of the studied exoplanet atmospheres, albeit
at varying extents. Accounting for stellar activity can significantly alter
planetary atmospheric parameters like molecular abundances (up to 6 orders of
magnitude) and temperature (up to 145 %), contrasting with the results of
analyses that neglect activity. Our results emphasise the importance of
considering the effects of stellar contamination in exoplanet transit studies;
this issue is particularly true for data sets obtained with facilities that do
not cover the optical and/or UV spectral range where the activity is expected
to be more impactful but also more easily detectable. Our results also provide
a catalogue of potentially active stars for further investigation and
monitoring.
