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Hubble finds water in atmospheres of alien worlds

Paul Sutherland, Feature writer
Dec 6, 2013, 8:00 UTC

Sen—Scientists have used the Hubble Space Telescope to detect signs of water in the atmospheres of five giant planets orbiting nearby stars.

It is the first time these faint signatures have been found on multiple worlds outside our own Solar System and had their profiles and intensities measured and compared.

The presence of water had been reported before on a few individual alien worlds.

The five planets have the uninspiring labels WASP-17b, HD209458b, WASP-12b, WASP-19b and XO-1b. The strengths of the signals indicating water varied. WASP-17b, a planet with a particularly puffed-up atmosphere, and HD209458b had the strongest signals.

The signatures for the other three planets, WASP-12b, WASP-19b and XO-1b, also showed the presence of water. All the signals were less strong than they were expected to be. The astronomers suspect this is because a layer of haze or dust blankets each of the five planets.

They say that this haze can reduce the intensity of all signals from the atmosphere in the same way that fog can dull colours in a photograph we might take on Earth. At the same time, the haze alters the profiles of water signals and other important molecules in a distinctive way.

Dr Avi Mandell, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said: “We’re very confident that we see a water signature for multiple planets. This work really opens the door for comparing how much water is present in atmospheres on different kinds of exoplanets, for example hotter versus cooler ones.”

Dr Mandell is lead author of an Astrophysical Journal paper, published today, describing the findings for WASP-12b, WASP-17b and WASP-19b.

A NASA video explains how astronomers investigate the atmospheres of exoplanets. Credit: NASA Goddard/ESA/Hubble

The studies were part of a census of exoplanet atmospheres led by Dr L. Drake Deming of the University of Maryland. Both teams used Hubble’s Wide Field Camera 3 to explore the details of absorption of light through the planets’ atmospheres. The observations were made in a range of infrared wavelengths where the water signature, if present, would appear.

The teams compared the shapes and intensities of the absorption profiles, and the consistency of the signatures gave them confidence they saw water.

“To actually detect the atmosphere of an exoplanet is extraordinarily difficult. But we were able to pull out a very clear signal, and it is water,” said Deming, whose team reported results for HD209458b and XO-1b in a Sept. 10 paper in the same journal. Deming’s team employed a new technique with longer exposure times, which increased the sensitivity of their measurements.

Sen asked Dr Deming whether the discovery increased the chances that any rocky worlds in the habitable zones of these alien star systems might have water. He told us: “I think that seeing water vapour in the atmospheres of giant planets is something like a necessary but not sufficient condition to have water in the atmospheres of any habitable-zone planets.

“But actually seeing water vapour in one planet tells us that the overall composition of the planets is probably similar to our Solar System. And water is very common in virtually all planets of our solar system - even Mercury has hydrated minerals, but the water is locked up geochemically.”

We also asked how the water got to the planets. Might it have been delivered by comets, as are thought to have filled the oceans of Earth?

Dr Deming replied: “Water could form in the giant planet from the primordial material in the system, just as it forms in comets. So I would expect that giant planets would be born with a minimum level of water from the cosmic gases that precede them.

“But also, a “hot Jupiter” orbiting in the inner parts of a planetary system has probably eaten many comets by collisions, further enhancing its water content.”