In contrast to planets with masses similar to that of Jupiter and higher, thebulk compositions of planets in the so-called super-Earth regime cannot beuniquely determined from a mass and radius measurement alone. For theseplanets, there is a degeneracy between the mass and composition of the interiorand a possible atmosphere in theoretical models. The recently discoveredtransiting super-Earth GJ1214b is one example of this problem. Three distinctmodels for the planet that are consistent with its mass and radius have beensuggested, and breaking the degeneracy between these models requires obtainingconstraints on the planet's atmospheric composition. Here we report aground-based measurement of the transmission spectrum of GJ1214b between 780and 1000 nm. The lack of features in this spectrum rules out cloud-freeatmospheres composed primarily of hydrogen at 4.9 sigma confidence. If theplanet's atmosphere is hydrogen-dominated, then it must contain clouds or hazesthat are optically thick at the observed wavelengths at pressures less than 200mbar. Alternatively, the featureless transmission spectrum is also consistentwith the presence of a dense water vapor atmosphere.
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