High Resolution, Differential, Near-infrared Transmission Spectroscopy of GJ 1214b

TL;DR

This study used high-resolution near-infrared spectroscopy during GJ 1214b's transit to search for atmospheric signatures, ruling out certain models and supporting a hydrogen-helium atmosphere with methane depletion.

Contribution

First high-resolution near-infrared transmission spectroscopy of GJ 1214b that constrains atmospheric composition and rules out specific models with wavelength-dependent transit depth variations.

Findings

Ruled out models with >5e-4 transit depth variations between 2.1-2.4 microns

No positive atmospheric signatures detected

Supports a hydrogen-helium atmosphere with methane depletion

Abstract

The nearby star GJ 1214 hosts a planet intermediate in radius and mass between Earth and Neptune, resulting in some uncertainty as to its nature. We have observed this planet, GJ 1214b, during transit with the high-resolution, near-infrared NIRSPEC spectrograph on the Keck II telescope, in order to characterize the planet's atmosphere. By cross-correlating the spectral changes through transit with a suite of theoretical atmosphere models, we search for variations associated with absorption in the planet atmosphere. Our observations are sufficient to rule out tested model atmospheres with wavelength-dependent transit depth variations >5e-4 over the wavelength range 2.1 - 2.4 micron. Our sensitivity is limited by variable slit loss and telluric transmission effects. We find no positive signatures but successfully rule out a number of plausible atmospheric models, including the default assumption of a gaseous, H-dominated atmosphere in chemical equilibrium. Such an atmosphere can be made consistent if the absorption due to methane is reduced. Clouds can also render such an atmosphere consistent with our observations, but only if they lie higher in the atmosphere than indicated by recent optical and infrared measurements. When taken in concert with constraints from other groups, our results support a consensus model in which the atmosphere of GJ 1214b contains significant H and He, but where methane is depleted. If this depletion is the result of photochemical processes, it may also produce a haze that suppresses spectral features in the optical.