'Hot Jupiters' - gaseous planets that share characteristics with Jupiter.
Image: European Space Agency.

Mystery of missing exoplanet water solved

Astronomers from the University of Exeter have helped shed light on the atmospheres of a group of planets which orbit stars outside our solar system, known as exoplanets.

Exeter academics led an international team of experts in analysing observations from the NASA/ESA Hubble Space Telescope and the NASA Spitzer Space Telescope. Their combined power gave a detailed study of the atmospheres of 10 hot-Jupiter exoplanets – the largest number ever collectively studied – in a bid to understand their atmospheres. Previously, scientists had been puzzled that they had not observed water on some of these planets – but the latest study, published in Nature on Monday December 14, has revealed that their view of the water was only obscured by haze and cloud. Although no life could ever exist on such gaseous hot planets, the presence of water has significant implications for theories over how they were formed.

Known as “hot Jupiters”, these gaseous planets share characteristics with Jupiter. However, they orbit very close to their stars, making their surface hot, and the planets difficult to study in detail without being overwhelmed by bright starlight. Because of this difficulty, Hubble has only explored a handful of hot Jupiters in the past.

Professor David Sing, of the University of Exeter, who led the study, said: “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. 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.”

All of the planets have an orbit that brings them between their parent star and Earth. As the exoplanet passes in front of its host star, as seen from Earth, some of this starlight travels through the planet’s outer atmosphere. “The atmosphere leaves its unique fingerprint on the starlight, which we can study when the light reaches us,” explains co-author Dr Hannah Wakeford, a postgraduate student at the University of Exeter during the study who is now at NASA Goddard Space Flight Center, USA.

These fingerprints allowed the team to extract the signatures from various elements and molecules — including water — and distinguish between cloudy and cloud-free exoplanets.

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

Co-author Professor Jonathan Fortney of the University of California, Santa Cruz, USA, said: “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. Our results have ruled out the dry scenario, and strongly suggest it’s simply clouds hiding the water from prying eyes.”

To date, astronomers have discovered nearly 2000 planets orbiting other stars. The study of exoplanetary atmospheres is currently in its infancy, with only a handful of observations taken so far. Hubble’s successor, the James Webb Space Telescope (JWST), will open a new infrared window on the study of exoplanets and their atmospheres.

Date: 14 December 2015

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