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This is our planet just 50 million years after it formed.
这是我们刚刚形成五千万年后的地球
It's thought that back then Earth had a twin, Theia.
据分析那时候地球有个孪生星球，叫西亚
The two planets were born in similar orbits.
这两个星球诞生在类似的轨道位置
The result, 3 catastrophic collision.
结果是一次灾难性的撞击
Theia was obliterated,leaving a ring of debris orbiting the Earth.
西亚被毁灭了 留下了一圏残骸绕着地球旋转
This may have been a vital first stepin creating the conditions for complex life.
这是创造在复杂生命存活条件的重要第一步
That's because Earth consumed a part of its twin,
因为地球吸收了其孪生星球的一部分
and became a significantly larger planet.
成为了更大的星球
The result was to increase the pull of Earth's gravity.
结果增加了地球的引力
Without this,Earth's atmosphere might have gradually leaked into space.
没有这个 地球的大气层可能会渐渐泄漏到太空中
Even an atmosphere needs gravity to hold it down.
连大气层都需要重力将它牢牢抓住
It keeps our planet warm.It drives the climate.
它让我们的星球温暖 改变气候
And provides us with the oxygen we breathe.
给我们提供需要呼吸的氧气
But the atmosphere also gives us something else that's vital for life on Earth,Protection.
但是大气层也给我们其它 对于地球上的生命很重要的东西 那就是保护
