Astronomers have had their first glimpse of the atmosphere of exo-planet GJ 1214b. GJ 1214b has a mass 6.5 times that of Earth and is 2.6 times its radius. By measuring the absorption spectra of the planet’s atmosphere, astronomers were able to shed light onto its composition.
Knowing the mass and radius of GJ 1214b is not enough information for us to know how it is made. For example, it could be a small gas giant like Neptune, or it could be a rocky planet with a hydrogen atmosphere produced by geological processes. Models show that both options could produce the same mass and radius. To distinguish between different models, astronomers set out to measure the composition of GJ 1214b’s atmosphere directly.
As the planet passes in front of its host star, its atmosphere will absorb some of the star’s light. The absorption by the atmosphere at different wavelengths of light will be dependent on the materials present. By recording the intensity at different wavelengths, from 780-1100nm, as the planet transits its star, researchers were able to determine the absorption spectrum of the planet’s atmosphere, and hence determine its composition.
So, do astronomers know what GJ 1214b is made of? Not quite. What they found was a rather featureless absorption spectra. While this does rule out an extended hydrogen-only atmosphere that would have strong distinct absorption lines, there are still two other possibilities. The absorption spectrum is consistent with both a dense “steam” atmosphere made up of 70 percent water, as well as a cloudy hydrogen atmosphere where the clouds would hide the absorption spectra. This is a great first step towards characterizing the atmosphere of an exo-planet, but we’re not quite where we’d like to be yet.