Clouds may hide what goes on in planetary atmospheres

THE WATER ‘MISSING’ from a handful of hot Jupiter exoplanets appears to have been found. Scientists had struggled to explain why the planets appeared to have less water in their atmospheres than expected, but a new study suggests that it was never missing at all, merely hidden – concealed behind layers of cloud and haze.

The worlds were all hot Jupiters, which occupy orbits close to their host stars, making them prime targets for atmospheric analysis by both the Hubble and Spitzer Space Telescopes. Astronomers used these instruments to study 10 such worlds, the largest number collectively examined. By looking at the starlight filtering through the each planet’s atmosphere, they could look for ‘fingerprints’ in the light and work out what elements it was made of.

On some planets, the vital signal that betrayed the presence of water was less than expected. Yet on closer inspection, the planetary atmospheres of these waterless worlds seemed remarkably flat – devoid of any fingerprints at all. This means it is likely that clouds cover the planet, blocking out our view of the atmosphere and its water. And the cloudier a planet appeared, the less water there was to be found.

“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 David Sing of the University of Exeter. “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.”

The discovery that water is not missing, merely hidden, comes as a great relief to planetary scientists as water is an important part of the way planets form.

“The alternative 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,” says Jonathan Fortney of the University of California, Santa Cruz. “Our results have ruled out the dry scenario, and strongly suggest it’s simply clouds hiding the water from prying eyes.” http://hubblesite.org