Astronomers have so far discovered almost 250 planets beyond our solar system. Most of these "exoplanets" are gaseous giants similar to Jupiter, which itself contains large quantities of water. Researchers had therefore suspected that water also exists on planets of a similar size outside our solar system.

Now Giovanna Tinetti of the European Space Agency and University College London, along with colleagues in France, Taiwan, Spain and the US, has studied exoplanet HD 189733b, a gas giant that crosses between the Earth and its companion star every 2.2 days. As it does so, some of the light from the star is absorbed by the exoplanet, causing the star to dim.

To search for water, the astronomers paid close attention to three wavelengths of infrared light – 3.6, 5.8 and 8.0µm – as the edge of the exoplanet’s atmosphere crossed the star. They discovered that the atmosphere absorbed less light at 3.6 µm than it did at the longer wavelengths. According to Tinetti, this could only occur if the atmosphere contains a significant amount of water vapour.

While it is unlikely that life exists on HD 189733b or any similar exoplanet, the research supports the idea that significant quantities of water could exist in other planetary systems. According to Tinetti the technique could someday be used to search for Earth-like exoplanets with water – which she describes as “the ‘holy grail’ for today’s planet hunters”. However, such studies would be very difficult to do using existing telescopes, which are not capable of performing such measurements on Earth-sized exoplanets. As a result the quest will probably have to wait for NASA’s James Webb Space Telescope, which should launch in 2013.