The planet, spotted by a team led by Giovanna Tinetti, a European Space Agency (ESA) fellow, is 15 per cent bigger than Jupiter and orbits a star in the constellation Vulpecula (The Fox), 60 light years from Earth.

Clear signs

"Although HD 189733b is far from being habitable, and actually provides a rather hostile environment, our discovery shows that water might be more common out there than previously thought"

Giavanna Tinetti, research team leader

A light year is the distance a beam of light travels in one year at 300,000km per second or close to 10 trillion km.

The Earth's moon is 1.3 light seconds from our planet. The Sun only eight light minutes.

"We're thrilled to have identified clear signs of water on a planet that is trillions of miles away", said Tinetti.

"Although HD 189733b is far from being habitable, and actually provides a rather hostile environment, our discovery shows that water might be more common out there than previously thought", he said.

HD 189733 b is known as a "hot Jupiter" planet - like the solar system's gas planet Jupiter but far hotter.

Orbiting cheek by jowl to its star at a distance 30 times closer than that between the Earth and the Sun, the planet's part that faces its star reaches a temperature of 2,000C, while the other side is a much less hostile 555C.

Signal hidden

The finding contradicted earlier studies showing no evidence of water, said Heather Knutson, an astronomer at Harvard University who reviewed the findings in the same issue of Nature.

However, the earlier studies looked at light emitted from the day side of the planet while the latest research used a different method that measured light transmitted through the outer edges of the planet's atmosphere, Knutson said.

This suggested there might be something hiding a water signal in the previous measurements, she said.

"In the long term we could evaluate other planets that could support life and have water in their atmosphere," Knutson said.