科学家发现地球“双胞胎”

Scientists believe one in five stars in our galaxy have Earth-like planets orbiting them.

But the ultimate goal of finding a world that truly resembles our own has continued to elude astronomers.

Now researchers have come a step closer by finding Earth's gassy twin in another solar system 200 light years away.

It orbits a dim red dwarf star at such a close distance that temperatures on its surface could be as high as 104°C - too hot for most forms of life on Earth.

KOI-314C is only 30 per cent more dense than water. This suggests that the world is enveloped by a blanket of hydrogen and helium hundreds of miles thick.

Scientists believe it may have started life as a mini-Neptune before some of its atmospheric gases were blasted away by intense radiation from the parent star.

'This planet might have the same mass as Earth, but it is certainly not Earth-like,' said lead astronomer Dr David Kipping from the Harvard-Smithsonian Centre for Astrophysics.

'It proves that there is no clear dividing line between rocky worlds like Earth and fluffier planets like water worlds or gas giants.'

The findings were presented at the annual meeting of the American Astronomical Society in Washington DC.

To weigh KOI-314c, the scientists used a new technique called transit timing variations (TTV), which only works when more than one planet orbits a star.

The two planets tug on each other, slightly altering the time they take to cross or 'transit' the star's face.

Analysing the way the planetary wobbles affect light coming from the star makes it possible to calculate their mass.

KOI-314c's companion world is similar to it in size but weighs four times more than the Earth.

The new discovery was made by chance as scientists scoured data from the Kepler space telescope looking for evidence of moons rather than planets.

'When we noticed this planet showed transit timing variations, the signature was clearly due to the other planet in the system and not a moon,' said Dr Kipping.

'At first we were disappointed it wasn't a moon, but then we soon realised it was an extraordinary measurement.'