  In the past decade, observations from space and ground have found H$_2$O to
be the most abundant molecular species, after hydrogen, in the atmospheres of
hot, gaseous, extrasolar planets. Being the main molecular carrier of oxygen,
H$_2$O is a tracer of the origin and the evolution mechanisms of planets. For
temperate, terrestrial planets, the presence of H$_2$O is of great significance
as an indicator of habitable conditions. Being small and relatively cold, these
planets and their atmospheres are the most challenging to observe, and
therefore no atmospheric spectral signatures have so far been detected.
Super-Earths -- planets lighter than ten M$_\oplus$ -- around later-type stars
may provide our first opportunity to study spectroscopically the
characteristics of such planets, as they are best suited for transit
observations. Here we report the detection of an H$_2$O spectroscopic signature
in the atmosphere of \planet\ -- an eight M$_\oplus$ planet in the
habitable-zone of an M-dwarf -- with high statistical confidence (ADI = 5.0,
$\sim$3.6$\sigma$). In addition, the derived mean molecular weight suggests an
atmosphere still containing some hydrogen. The observations were recorded with
the Hubble Space Telescope/WFC3 camera, and analysed with our dedicated,
publicly available, algorithms. While the suitability of M-dwarfs to host
habitable worlds is still under discussion, \planet\ offers an unprecedented
opportunity to get insight into the composition and climate of habitable-zone
planets.
