But the ultimate goal of finding a world that truly resembles our own has continued to elude²（逃避） astronomers⁴.

 

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

 

It orbits a dim red dwarf⁵ 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 dense⁶ than water. This suggests that the world is enveloped⁷ 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 atmospheric⁸ 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 astronomer³ 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 fluffier⁹ planets like water worlds or gas giants.'

 

The findings were presented at the annual meeting of the American Astronomical¹⁰ Society in Washington DC.

 

To weigh KOI-314c, the scientists used a new technique called transit¹¹ timing¹² variations (TTV), which only works when more than one planet orbits a star.

 

The two planets tug¹³ on each other, slightly altering the time they take to cross or 'transit' the star's face.

 

Analysing the way the planetary wobbles（摇晃） 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 scoured¹⁴ 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.'

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