  The atmospheric composition of low-mass exoplanets is the object of intense
observational and theoretical investigations. GJ3470b is a warm uranus recently
detected in transit across a bright late-type star. The transit of this planet
has already been observed in several band passes from the ground and space,
allowing observers to draw an intriguing yet incomplete transmission spectrum
of the planet atmospheric limb. In particular, published data in the visible
suggest the existence of a Rayleigh scattering slope, making GJ3470b a unique
case among the known neptunes, while data obtained beyond 2 um are consistent
with a flat infrared spectrum. The unexplored near-infrared spectral region
between 1 and 2 um, is thus key to undertanding the atmospheric nature of
GJ3470b. Here, we report on the first space-borne spectrum of GJ3470, obtained
during one transit of the planet with WFC3 on board HST, operated in stare
mode. The spectrum covers the 1.1--1.7-um region with a resolution of about
300. We retrieve the transmission spectrum of GJ3470b with a chromatic
planet-to-star radius ratio precision of 0.15% (about one scale height) per
40-nm bins. At this precision, the spectrum appears featureless, in good
agreement with ground-based and Spitzer infrared data at longer wavelengths,
pointing to a flat transmission spectrum from 1 to 5 um. We present new
simulations of transmission spectra for GJ3470b, which allow us to show that
the HST/WFC3 observations rule out cloudless hydrogen-rich atmospheres (>10
sigma) as well as hydrogen-rich atmospheres with tholin haze (>5 sigma). Adding
our near-infrared measurements to the full set of previously published data
from 0.3 to 5 um, we find that a cloudy, hydrogen-rich atmosphere can explain
the full transmission spectrum if, at the terminator, the clouds are located at
low pressures (<1 mbar) or the water mixing ratio is extremely low (<1 ppm).
