  TOI-270d is a temperate sub-Neptune discovered by the Transiting Exoplanet
Survey Satellite (TESS) around a bright (J=9.1mag) M3V host star. With an
approximate radius of 2RE and equilibrium temperature of 350K, TOI-270d is one
of the most promising small exoplanets for atmospheric characterisation using
transit spectroscopy. Here we present a primary transit observation of TOI-270d
made with the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3)
spectrograph across the 1.126-1.644 micron wavelength range, and a 95% credible
upper limit of $8.2 \times 10^{-14}$ erg s$^{-1}$ cm$^{-2}$ A$^{-1}$
arcsec$^{-2}$ for the stellar Ly-alpha emission obtained using the Space
Telescope Imaging Spectrograph (STIS). The transmission spectrum derived from
the TESS and WFC3 data provides evidence for molecular absorption by a
hydrogen-rich atmosphere at 4-sigma significance relative to a featureless
spectrum. The strongest evidence for any individual absorber is obtained for
H2O, which is favoured at 3-sigma significance. When retrieving on the WFC3
data alone and allowing for the possibility of a heterogeneous stellar
brightness profile, the detection significance of H2O is reduced to 2.8-sigma.
Further observations are therefore required to robustly determine the
atmospheric composition of TOI-270d and assess the impact of stellar
heterogeneity. If confirmed, our findings would make TOI-270d one of the
smallest and coolest exoplanets to date with detected atmospheric spectral
features.
