An international team has discovered water on an extra-solar planet for only the second time. The researchers, led by Giovanna Tinetti of the European Space Agency and University College London's department of physics and astronomy, found that as the planet, known as HD189733b, passes in front of its Sun, it absorbs light in a way that can only be explained by the presence of water vapour in its atmosphere. The discovery was made using Nasa's Spitzer earth-orbiting telescope and is reported in the journal Nature .