Between the fall of 2012 and the fall of 2013, astronomers took a deep, long look into the atmosphere of GJ 1214b, a distant Earth-sized planet in a tight orbit around a cool, red star. Fifteen times it crossed the face of its star as the Hubble Space Telescope watched. Each time, starlight would pass through the thin ring of atmosphere around the planet. Some of it would catch on airborne molecules—methane, carbon dioxide, or maybe even water vapor—leaving a spectral fingerprint.
At least that’s what the astronomers hoped. It had worked for bigger, hotter planets, and now we’d be able to understand the atmosphere of one of the most earth-like exoplanets known. Going forward, we would use the same method to look at ever more familiar worlds, sniffing for chemicals that might have been exhaled by living organisms. With this trial run on GJ 1214 b, however, there was just one problem.
We saw nothing.
Or, to say it in a kinder way, we saw something else amazing: a world enshrouded in alien clouds. The spectrum, painstakingly measured from light filtered through the planet’s atmosphere, didn’t show the telltale spikes and wiggles of molecules. Instead, it was featureless and flat.
In a paper published in Nature, astronomers inferred that an opaque layer of clouds had blocked Hubble’s view. Suddenly, small planets around other stars seemed lot harder to probe, even for the world’s most powerful observatory. “They got a tremendous amount of Hubble Space Telescope time just to study that one planet in exquisite detail,” says Jonathan Fortney, an astronomer at the University of California, Santa Cruz. “And that spectrum came back flat as a line.”