科學家發現地球“雙胞胎”

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.'據英國《每日郵報》報道，科學家發現在距離地球200光年的地方有一顆與地球質量相同的神祕行星。

儘管科學家認爲在銀河系中五分之一的恆星都有一個與地球相似的行星圍繞其運行，但是天文學家們卻從未找到另外一個與地球真正相似的世界。

不過，研究者們最近離這一目標又近了一步，在距離地球200光年的外星系中發現了地球的“雙胞胎”。

這顆被稱爲“KOI-314c”的行星，質量與地球相當，但是體積比地球大60%。科學家懷疑這是因爲它的大氣層更厚。

這顆行星圍繞一顆暗紅色的小行星近距離運行，因此其表面溫度高達104攝氏度，這個溫度對於地球上的大部分生物來說都太高。

行星”KOI-314C“的密度只比水高30%。這意味着它被厚達幾百英里的氫氣和氦氣像毯子一樣包裹着。但是科學家認爲，在其大氣受到來自恆星的強輻射而爆炸之前，這顆行星可能已經像一顆迷你海王星那樣運行。

哈佛大學史密森尼天體物理學中心的David Kipping博士則認爲：“這顆行星可能只是與地球質量相同，但肯定不是與地球相似的行星。”

此前，科學家試圖從開普勒太空望遠鏡的數據中尋找與地球相似的衛星，而這顆行星是機緣巧合下的新發現。