Clouds in the Atmosphere of a Super-Earth Exoplanet
An artist's illustration of the super-Earth exoplanet GJ 1214b. Spectroscopic observations with the Hubble Space Telescope provide evidence of high clouds blanketing the planet, and hiding information about the world's lower atmosphere and surface. The composition of the clouds is unknown. GJ1214b is located 40 light-years from Earth, in the constellation Ophiuchus.
As of the start of the new year, 2014, there were about 1056 confirmed planets around stars other than our Sun ("exoplanets"), including 175 multiple planet systems. Surveys have found that planets intermediate in size between Earth and Neptune, the so-called "super-Earths," are among the most common planets in the galaxy (about 140 are known). Modeling the atmospheres of these objects is the next step towards developing a comprehensive understanding of the class, and astronomers are working to obtain spectra of the atmospheres to characterize them.
There are three basic spectroscopic techniques used to study an exoplanet's atmosphere: transmission (the star's light is transmitted or absorbed as it passes through the planet's atmosphere), emission (light is emitted directly by the atmospheric gases), or reflection (the atmospheric clouds or gases selectively reflect the star's light). Transmission spectroscopy is particularly apt for transiting exoplanets because they pass in front of the star, and there have been numerous attempts to use it in the case of the transiting super-Earth archetype GJ 1214b. Astronomers suspected that its atmosphere was either dominated by relatively heavy molecules like water, or that it contained high-altitude clouds that obscured its lower layers, but the observations so far have not had enough precision to distinguish between these options.
CfA astronomer Zachory Berta-Thompson and his colleagues used the Hubble Space Telescope to measure successfully, for the first time, the transmission spectrum of GJ 1214b at near-infrared wavelengths in an observation that definitively resolves this ambiguity. They followed the exoplanet over fifteen transits, and combined the data to obtain a spectrum in the near infrared. They found a clear lack of any features: neither water, nor nitrogen, carbon dioxide, nor any other potential molecules. This null result signals that the atmosphere consists of grey and opaque clouds. The result rules out the cloud-free option, and marks the start of a new era in exoplanet technology in which their atmospheres will gradually reveal their secrets.
"Clouds in the Atmosphere of the Super-Earth Exoplanet GJ 1214b," Laura Kreidberg, Jacob L. Bean, Jean-Michel De´sert, Bjorn Benneke, Drake Deming, Kevin B. Stevenson, Sara Seager, Zachory Berta-Thompson, Andreas Seifahrt & Derek Homeier, Nature 505, 69, 2014.