  We present an investigation of the transmission spectrum of the 6.5 M_earth
planet GJ1214b based on new ground-based observations of transits of the planet
in the optical and near-infrared, and on previously published data.
Observations with the VLT+FORS and Magellan+MMIRS using the technique of
multi-object spectroscopy with wide slits yielded new measurements of the
planet's transmission spectrum from 0.61 to 0.85 micron, and in the J, H, and K
atmospheric windows. We also present a new measurement based on narrow-band
photometry centered at 2.09 micron with the VLT+HAWKI. We combined these data
with results from a re-analysis of previously published FORS data from 0.78 to
1.00 micron using an improved data reduction algorithm, and previously reported
values based on Spitzer data at 3.6 and 4.5 micron. All of the data are
consistent with a featureless transmission spectrum for the planet. Our K-band
data are inconsistent with the detection of spectral features at these
wavelengths reported by Croll and collaborators at the level of 4.1 sigma. The
planet's atmosphere must either have at least 70% water by mass or optically
thick high-altitude clouds or haze to be consistent with the data.
