A European probe has discovered the first planet orbiting another star that resembles one in our own solar system. The distant world is a similar size to Jupiter and is cool enough to contain liquid water – an essential for life as we know it.

Most of the 400 alien worlds – or exoplanets – detected out in the galaxy have been searingly hot gas-balls zipping round their parent suns in just a few days.

But the new discovery, called Corot-9b, takes 95.274 days to orbit its star, 1,500 light-years away in the constellation of Serpens Cauda, a little longer than Mercury’s own year.

It was detected by planet-hunting satellite Corot as it passed in front of the star – dubbed a transit – causing the star’s light to dip for a short time. Changes during the transit allow astronomers to measure the planet’s mass, size and temperature.

Because it orbits a star cooler than our Sun, calculations estimate that Corot-9b’s temperature could lie somewhere between –23°C and 157°C. The team of 60 astronomers credited with the discovery made follow-up observations to confirm its nature with giant telescopes in Tenerife and Chile.

One of them Dr Suzanne Aigrain of Oxford University, said: “Over the past decade, densities could be measured only for hot planets orbiting very close to their host stars. Their evolution is completely dominated by the huge amount of radiation they receive from their host stars.

“Corot-9b is much cooler, and provides us with a clean, isolated test of our theories of how giant planets evolve.”

French colleague Tristan Guillot said: “The planet is mostly made of hydrogen and helium, but it may contain up to 20 Earth-masses of high-pressure ices and rocks. It is thus very similar to the solar system’s giant planets, Jupiter and Saturn. Its density agrees well with theoretical expectations.”

Hans Deeg, chief author of a scientific paper reporting the discovery, said: “Corot-9b is the first exoplanet that is definitely similar to a planet in our Solar System.”

Picture: An artist’s impression of a planet in transit in front of a star. (Credit: NASA).

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