Astronomers Detect Signs of Water on Five Exoplanets

Two teams of scientists using the Wide Field Camera 3 aboard NASA’s Hubble Space Telescope have detected faint signatures of water in the atmospheres of five big exoplanets.

This is an artist's impression of a star's light illuminating the atmosphere of an exoplanet. Image credit: NASA's Goddard Space Flight Center.

This is an artist’s impression of a star’s light illuminating the atmosphere of an exoplanet. Image credit: NASA’s Goddard Space Flight Center.

The five exoplanets, WASP-17b, HD 209458b, WASP-12b, WASP-19b and XO-1b, are hot Jupiters, massive worlds that orbit close to their host stars.

Discovered in August 2009, WASP-17b orbits a main sequence star about 1,000 light-years away in the constellation Scorpius. The planet has an especially puffed-up atmosphere.

HD 209458b orbits a Sun-like star in the constellation Pegasus, about 150 light-years away. This exoplanet has the strongest signals of water.

The signatures for the other three planets, WASP-12b, WASP-19b and XO-1b, also are consistent with water.

Dr Avi Mandell of NASA’s Goddard Space Flight Center in Greenbelt, who is the lead author of the paper describing the WASP-12b, WASP-17b and WASP-19b findings in the Astrophysical Journal (, 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 with colleagues explored the details of absorption of light through atmospheres the five exoplanets. They 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 Dr Drake Deming of the University of Maryland in College Park, who is the lead author of the study on HD209458b and XO-1b published in the Astrophysical Journal (

The water signals were all less pronounced than expected, and the astronomers suspect this is because a layer of haze or dust blankets each of the five planets. This haze can reduce the intensity of all signals from the atmosphere in the same way fog can make colors in a photograph appear muted. At the same time, haze alters the profiles of water signals and other important molecules in a distinctive way.


Avi M. Mandell et al. 2013. Exoplanet Transit Spectroscopy Using WFC3: WASP-12 b, WASP-17 b, and WASP-19 b. ApJ 779, 128; doi: 10.1088/0004-637X/779/2/128

Drake Deming et al. 2013. Infrared Transmission Spectroscopy of the Exoplanets HD 209458b and XO-1b Using the Wide Field Camera-3 on the Hubble Space Telescope. ApJ 774, 95; doi: 10.1088/0004-637X/774/2/95