BROADBAND TRANSMISSION SPECTROSCOPY OF THE SUPER-EARTH GJ 1214b SUGGESTS A LOW MEAN MOLECULAR WEIGHT ATMOSPHERE
- Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4 (Canada)
- Departement de Physique, Universite de Montreal, C.P. 6128 Succ. Centre-Ville, Montreal, QC H3C 3J7 (Canada)
- Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064 (United States)
- Canadian Institute for Theoretical Astrophysics, 60 St. George Street, University of Toronto, Toronto, ON M5S 3H8 (Canada)
- Argelander-Institut fuer Astronomie, Auf dem Huegel 71, D-53121 Bonn (Germany)
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 ({approx}1.25 {mu}m) observations we find a transit depth, analogous to the planet-to-star radius ratio squared, in this band of (R{sub PJ} /R{sub *}){sup 2} = (1.338 {+-} 0.013)%-a value consistent with the optical transit depth reported by Charbonneau and collaborators. However, our best-fit combined K{sub s}-band ({approx}2.15 {mu}m) transit depth is deeper: (R{sub PKs} /R{sub *}){sup 2} = (1.438 {+-} 0.019)%. Formally, our K{sub s}-band transits are deeper than the J-band transits observed simultaneously by a factor of (R{sub PKs} /R{sub PJ}){sup 2} = 1.072 {+-} 0.018-a 4{sigma} discrepancy. The most straightforward explanation for our deeper K{sub 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 {mu}m, 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{sub s}-band transit depth or detect features at other wavelengths.
- OSTI ID:
- 21578288
- Journal Information:
- Astrophysical Journal, Vol. 736, Issue 2; Other Information: DOI: 10.1088/0004-637X/736/2/78; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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