Astronomers studying images from the Hubble Space Telescope have finally settled a debate about the atmosphere of the exoplanet Gliese 1214b, first discovered in 2009.

The planet, which sits around 42 light years from the Sun in the constellation Ophichus, has long puzzled researchers because its density is too low to be purely rock and too high to be a gas giant.

We knew it had some sort of atmosphere from measurements of the infrared colour of the planet’s sunsets, but couldn’t tell if it was a heavy molecule like water vapour, making it a “steamy waterworld”, or whether it was enveloped in thick, high-altitude clouds.

A team of researchers led by Laura Kreidberg tackled the question by searching 96 hours of telescope data captured over 11 months for the chemical fingerprints of water, methane, carbon monoxide, nitrogen and carbon dioxide. They failed to find them, ruling out the steamy hypothesis.

“The planet’s atmosphere must contain clouds to be consistent with the data,” they wrote in their paper, published in Nature. “We rule out cloud-free atmospheric models with compositions dominated by water, methane, carbon monoxide, nitrogen or carbon dioxide.”

What the clouds are made of, however, is still unknown. “You would expect very different kinds of clouds to form than you would expect, say, on Earth,” Kriedberg told The Verge. It may be potassium chloride or zinc sulphide, but working that out is going to require another round of data analysis, perhaps with the aid of Hubble’s successor, the James Webb Space Telescope, which will be launched in 2018.