  In contrast to planets with masses similar to that of Jupiter and higher, the
bulk compositions of planets in the so-called super-Earth regime cannot be
uniquely determined from a mass and radius measurement alone. For these
planets, there is a degeneracy between the mass and composition of the interior
and a possible atmosphere in theoretical models. The recently discovered
transiting super-Earth GJ1214b is one example of this problem. Three distinct
models for the planet that are consistent with its mass and radius have been
suggested, and breaking the degeneracy between these models requires obtaining
constraints on the planet's atmospheric composition. Here we report a
ground-based measurement of the transmission spectrum of GJ1214b between 780
and 1000 nm. The lack of features in this spectrum rules out cloud-free
atmospheres composed primarily of hydrogen at 4.9 sigma confidence. If the
planet's atmosphere is hydrogen-dominated, then it must contain clouds or hazes
that are optically thick at the observed wavelengths at pressures less than 200
mbar. Alternatively, the featureless transmission spectrum is also consistent
with the presence of a dense water vapor atmosphere.
