  Recent surveys have revealed that planets intermediate in size between Earth
and Neptune ("super-Earths") are among the most common planets in the Galaxy.
Atmospheric studies are the next step toward developing a comprehensive
understanding of this new class of object. Much effort has been focused on
using transmission spectroscopy to characterize the atmosphere of the
super-Earth archetype GJ 1214b, but previous observations did not have
sufficient precision to distinguish between two interpretations for the
atmosphere. The planet's atmosphere could be dominated by relatively heavy
molecules, such as water (e.g., a 100% water vapor composition), or it could
contain high-altitude clouds that obscure its lower layers. Here we report a
measurement of the transmission spectrum of GJ 1214b at near-infrared
wavelengths that definitively resolves this ambiguity. These data, obtained
with the Hubble Space Telescope, are sufficiently precise to detect absorption
features from a high mean molecular mass atmosphere. The observed spectrum,
however, is featureless. We rule out cloud-free atmospheric models with water-,
methane-, carbon monoxide-, nitrogen-, or carbon dioxide-dominated compositions
at greater than 5$\sigma$ confidence. The planet's atmosphere must contain
clouds to be consistent with the data.
