在远离太阳的外太阳系，在木星轨道之外，温度低到足以使冰从水以及甲烷和二氧化碳等气体中形成。
Far from the sun, in the outer solar system, beyond Jupiter's orbit, temperatures are low enough to permit ices to form out of water and out of gases like methane and carbon dioxide.
一些研究人员认为，拥有厚厚大气层和液态甲烷湖的“泰坦”是太阳系内最有可能发现地外生命的天体。
Some researchers think that Titan, with its thick atmosphere and lakes of liquid methane, is the likeliest spot to find extraterrestrial life in our solar system.
另外三个生命标志是水、甲烷和氧气，然而现在只有氧气还未在太阳系之外的行星中被发现。
The other three are water, methane and oxygen - and now oxygen is the only one left to be observed in the atmosphere of a planet outside the Solar System.
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