A survey of so-called hot-Jupiters – giant exoplanets that orbit very close to their host stars – conducted with NASA’s Spitzer and the NASA/ESA Hubble space telescopes has solved a long-standing mystery: why some of these huge alien worlds seem to have less water than expected.

Previously, astronomers had been puzzled that they had not observed water on some of hot-Jupiters, but the new study, led by Prof. David Sing of the University of Exeter, has revealed that their view of the water was only obscured by haze and cloud.

“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,” Prof. Sing said.

“We found the planetary atmospheres to be much more diverse than we expected, and this significantly progresses our understanding of what makes up these planets and how they were created.”

“Our results suggest it’s simply clouds hiding the water from prying eyes, and therefore rule out dry hot Jupiters,” added co-author Dr Jonathan Fortney of the University of California, Santa Cruz.

Prof. Sing, Dr Fortney and their colleagues used Hubble and Spitzer observations of two well-studied hot-Jupiter exoplanets, HD 209458b and HD 189733b, and used Hubble to observe eight other planets – WASP-6b, WASP-12b, WASP-17b, WASP-19b, WASP-31b, WASP-39b, HAT-P-1b, HAT-P-12b.

An exoplanet’s atmosphere leaves its unique fingerprint on the starlight, which astronomers can study when the light reaches Earth. These fingerprints allowed the sceintists to extract the signatures from various elements and molecules – including water – and distinguish between cloudy and cloud-free exoplanets.

“We report results from a comparative study of 10 hot Jupiters covering the wavelength range 0.3–5 micrometers, which allows us to resolve both the optical scattering and infrared molecular absorption spectroscopically,” the astronomers said. “Our results reveal a diverse group of hot Jupiters that exhibit a continuum from clear to cloudy atmospheres.”

The team’s models revealed that, while apparently cloud-free hot-Jupiters showed strong signs of water, the atmospheres of those gaseous giants with faint water signals also contained clouds and haze – both of which are known to hide water from view.

“The alternative theory to this is that planets form in an environment deprived of water, but this would require us to completely rethink our current theories of how planets are born,” Dr Fortney said.

The results of this study were published online this week in the journal Nature.

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David K. Sing et al. A continuum from clear to cloudy hot-Jupiter exoplanets without primordial water depletion. Nature, published online December 14, 2015; doi: 10.1038/nature16068