Mars is one of the four planets in the solar system that is a rocky planet.

火星是太阳系中四颗岩态行星之一。

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Earth is a rocky planet where there is liquid water, which is necessary for life to exist.

地球是一个岩石行星，这里有液态水，这是生命存在的必要条件。

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The surface formed mostly in the last billion years, which makes it fresher and more recently active than any rocky planet other than earth.

它的表面大部分是在过去十亿年里形成的，使得它比地球以外的任何岩石行星都更新鲜活跃。

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"Everything that we've discovered in the last few years tells us that where we find one small, rocky planet there are likely to be others, " he told BBC News.

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Kepler-22b is quite a bit larger than the biggest rocky planet in this Solar System (which happens to be Earth), but much smaller than the smallest of our local gaseous worlds (Neptune).

BBC: Viewpoint: The obsession with a 'twin Earth'

Mr. JACK LISSAUER (NASA): This is likely to be or plausibly the first rocky planet around a normal star other than the sun, and in that sense it's a class of planets like Earth.

NPR: Astronomers Find Planet with Similarities to Earth