NASA says the Hubble Space Telescope and its counterpart the Spitzer Space Telescope have surveyed 10 Jupiter-sized distant exoplanets to delve into the mysteries of their atmospheres.

The survey has provided the solution to at least one long-standing puzzle, namely why some of these exoplanets — known as hot Jupiters orbiting closely around their host stars — appear to have a lot less water in their atmosphere than is expected, astronomers say.

Using both the Hubble and Spitzer telescopes allowed the international team of astronomers to amass the largest-ever catalog of spectroscopic data on exoplanet atmospheres. They studied 10 giant gas planets of various masses, size and temperatures.

"I'm really excited to finally see this wide group of planets together, as this is the first time we've had sufficient wavelength coverage to compare multiple features from one planet to another," says lead study author David Sing of the University of Exeter in Britain. "We found the planetary atmospheres to be much more diverse than we expected."

The survey enabled the team to distinguish between cloudy and cloud-free exoplanets in the study group, an atmospheric quality that could explain the mystery of the apparently missing water, they said.

Strong signs of water were seen on cloud-free exoplanets, while weaker water signals were detected on those planets with clouds and haze.

Such a cloudy atmosphere can hide water from view, the researchers explain, even on a class of expoplanets known for their extreme temperatures.

"Our results suggest it's simply clouds hiding the water from prying eyes, and therefore rule out dry, hot Jupiters," explained study co-author Jonathan Fortney, an astronomer and astrophysicist at the University of California, Santa Cruz.

Dry, hot Jupiters would require that planets form in an environment without water, which would need a complete rethinking of current theories of how planets are born, he says.

"It was good to see with a large sample size that the water was there the whole time and the ones where we couldn't see the spectral signature for water were just the ones that were the cloudiest," he adds.

The confirmation of water vapor on all of the 10 planets shows water vapor is a common, abundant molecule in the universe, he says.

"We found that water is prominent in all these planets, so we think that water makes up a lot of the heavier elements that goes into planet formation," he notes.

The study was published online in the journal Nature.

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