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This is our planet just 50 million years after it formed.
這是我們剛剛形成五千萬年后的地球
It's thought that back then Earth had a twin, Theia.
據分析那時候地球有個孿生星球，叫西亞
The two planets were born in similar orbits.
這兩個星球誕生在類似的軌道位置
The result, 3 catastrophic collision.
結果是一次災難性的撞擊
Theia was obliterated,leaving a ring of debris orbiting the Earth.
西亞被毀滅了 留下了一圏殘骸繞著地球旋轉
This may have been a vital first stepin creating the conditions for complex life.
這是創造在復雜生命存活條件的重要第一步
That's because Earth consumed a part of its twin,
因為地球吸收了其孿生星球的一部分
and became a significantly larger planet.
成為了更大的星球
The result was to increase the pull of Earth's gravity.
結果增加了地球的引力
Without this,Earth's atmosphere might have gradually leaked into space.
沒有這個 地球的大氣層可能會漸漸泄漏到太空中
Even an atmosphere needs gravity to hold it down.
連大氣層都需要重力將它牢牢抓住
It keeps our planet warm.It drives the climate.
它讓我們的星球溫暖 改變氣候
And provides us with the oxygen we breathe.
給我們提供需要呼吸的氧氣
But the atmosphere also gives us something else that's vital for life on Earth,Protection.
但是大氣層也給我們其它 對于地球上的生命很重要的東西 那就是保護
