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The latest planet is only a stone's throw away in astronomical terms, meaning scientists will be able to turn the Hubble Space Telescope towards it and analyse its atmosphere, potentially revealing signs of life. Charbonneau's team has already requested time on the space telescope.

"Using the Hubble, we can look at the atmosphere and say not only whether it's habitable, but whether it's inhabited," Charbonneau told the Guardian. "If we find oxygen in the atmosphere things will get really interesting, because on Earth all the oxygen in the atmosphere comes from life."

After spotting GJ1214b in orbit, the astronomers measured tiny movements in the parent star as the planet circled around it. From these wobbles they calculated the mass of the planet to be 6.6 times as great as the Earth's. The most likely composition of the planet is 75% water, with 22% silicon and 3% iron forming a solid core, the scientists report.

In an accompanying article, Geoffrey Marcy, an astronomer at the University of California in Berkeley, said the extrasolar planet or "exoplanet" provides "the most watertight evidence so far for a planet that is something like our own Earth outside our solar system".

Zachory Berta, a co-author on the latest paper, said: "Despite its hot temperature, this appears to be a waterworld. It is much smaller, cooler and more Earth-like than any other known exoplanet." Some of the planet's water is expected to be in an exotic form called ice VII, a crystalline form of water that exists under immense pressures.

Astronomers have discovered more than 400 planets beyond our solar system in the past twenty years. Two dedicated space missions, the French space agency's Corot telescope and Nasa's Kepler telescope have been launched to look for Earth-sized rocky planets in stars' Goldilocks zone that could be hospitable to life.

http://www.guardian.co.uk/science/2009/dec/16/waterworld-planet-earth-life