BROADBAND TRANSMISSION SPECTROSCOPY OF THE SUPER-EARTH GJ 1214b SUGGESTS A LOW MEAN MOLECULAR WEIGHT ATMOSPHERE

We use the Wide-field Infrared Camera (WIRCam) on the Canada–France–Hawaii Telescope to observe four transits of the super-Earth planet GJ 1214b in the near-infrared. For each transit, we observe GJ 1214 in two bands nearly simultaneously by rapidly switching the WIRCam filter wheel back and forth for the duration of the observations. By combining all our J -band (∼1.25 μm) observations we find a transit depth, analogous to the planet-to-star radius ratio squared, in this band of ( R PJ / R * ) 2 = (1.338 ± 0.013)%—a value consistent with the optical transit depth reported by Charbonneau and collaborators. However, our best-fit combined K s -band (∼2.15 μm) transit depth is deeper: ( R PKs / R * ) 2 = (1.438 ± 0.019)%. Formally, our K s -band transits are deeper than the J -band transits observed simultaneously by a factor of ( R PKs / R PJ ) 2 = 1.072 ± 0.018—a 4σ discrepancy. The most straightforward explanation for our deeper K s -band transit depth is a spectral absorption feature from the limb of the atmosphere of the planet; for the spectral absorption feature to be this prominent, the atmosphere of GJ 1214b must have a large-scale height and a low mean molecular weight. That is, its atmosphere would have to be hydrogen/helium dominated and this planet would be better described as a mini-Neptune. However, recently published observations from 0.78 to 1.0 μm, by Bean and collaborators, show a lack of spectral features and transit depths consistent with those obtained by Charbonneau and collaborators. The most likely atmospheric composition for GJ 1214b that arises from combining all these observations is less clear; if the atmosphere of GJ 1214b is hydrogen/helium dominated, then it must have either a haze layer that is obscuring transit-depth differences at shorter wavelengths or significantly different spectral features from what current models predict. Our observations disfavor a water-world composition, but such a composition will remain a possibility for GJ 1214b until observations reconfirm our deeper K s -band transit depth or detect features at other wavelengths.