  The super-Earth LHS 1140 b is an interesting target for atmospheric
observations since it is close to the habitable zone of its star and falls in
the gap of the radius distribution of small exoplanets, in the region thought
to correspond to the transition between planets with and without atmospheres.
Observations of the primary transit with WFC3 on board of the Hubble Space
Telescope (HST) revealed a modulation in the planet transmission spectrum
compatible with the presence of water; however this modulation may be also due
to stellar activity-related phenomena. Here we present a detailed analysis of
the WFC3/HST observations to probe the nature of this modulation and to
understand if it can be attributable to the presence of unocculted spots on the
stellar surface. Our analysis strongly suggests that LHS1140 is a rather quiet
star with subsolar [Fe/H] and enriched in alpha elements. Therefore, we rule
out the possibility that the planetary spectrum is affected by the presence of
spots and faculae. This analysis shows the importance of a proper modelling of
the stellar spectrum when analyzing transit observations. Finally, we modelled
the planetary atmosphere of LHS1140 b to retrieve its atmospheric composition.
However, the low resolution and the narrow spectral range of HST observations
prevented us from definitively determining whether the spectral features are
attributable to the presence of water or of other molecules in the planetary
atmosphere.
