  Transmission spectroscopy to date has detected atomic and molecular
absorption in Jupiter-sized exoplanets, but intense efforts to measure
molecular absorption in the atmospheres of smaller (Neptune-sized) planets
during transits have revealed only featureless spectra. From this it was
concluded that the majority of small, warm planets evolve to sustain high mean
molecular weights, opaque clouds, or scattering hazes in their atmospheres,
obscuring our ability to observe the composition of these atmospheres. Here we
report observations of the transmission spectrum of HAT-P-11b (~4 Earth radii)
from the optical to the infrared. We detected water vapour absorption at 1.4
micrometre wavelength. The amplitude of the water absorption (approximately 250
parts-per- million) indicates that the planetary atmosphere is predominantly
clear down to ~1 mbar, and sufficiently hydrogen-rich to exhibit a large scale
height. The spectrum is indicative of a planetary atmosphere with an upper
limit of ~700 times the abundance of heavy elements relative to solar. This is
in good agreement with the core accretion theory of planet formation, in which
gas giant planets acquire their atmospheres by directly accreting hydrogen-rich
gas from the protoplanetary nebulae onto a large rocky or icy core.
