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Because from trillions of miles away, these small planets get lost in the blinding glare of their stars'brightness.
因為從幾萬億光年外看，這些小行星會被周圍恒星的耀眼光芒所埋沒。
The sad truth of the matter is that earth-like planets are near cosmic specks of dust.
據說類地球的形體好像宇宙中微塵。
And therefore detecting them is really quite hard.
所以說偵測到它們時間十分困難的事情。
Hard, but not impossible. Every now and again, the planet passes directly between us and the star.
困難，但并不是沒有可能，行星有時會從我們地球和恒星之間穿過。
And if it does that, it's like a little eclipse. And the star-light drops.
當它穿過時就形成了類似蝕的情況，那么恒星的光亮就會減弱變得灰暗。
It gets darker because the planet is blocking the light from that star. And it turns out we can see that.
因為行星擋住了恒星所發出的光，而結果是我們可以看到那顆行星。
With this in mind, scientists built one of the world's most sensitive telescopes and launched it into space.
具備了這種理論，科學家們制造了世界上最敏銳的望遠鏡之一并發射入太空。
The mission called Kepler may not help us find E.T, but it could help us find its home.
這項任務叫做開普勒計劃，雖然不太可能幫助我們找到外星人，但是可能幫助我們找到他們的家。
Kepler will simply do one thing unbelievably well--measure the brightness of 100,000 stars over and over and over,looking for a few of those 100,000 stars that dim.
開普勒能將它唯一的一件工作做到極致就是一次又一次地測量幾十萬星星的光亮度，試圖找到其中少數發暗的星體。
Seeing the tiniest dim from a small planet travelling in front of a star thousands of light-years away isn't easy to imagine.
從幾千光年外觀測穿過其恒星的小行星發出的暗光是難以想象的。
But the Kepler telescope is one of the most sensitive cameras ever built.
但開普勒望遠鏡是世界上最敏銳的照相機。
To give you an idea of just how sensitive Kepler is, I'm gonna demonstrate using this massive World War II search light.
為了讓你們知道開普勒到底敏銳到什么程度，我要用這個二戰時期的巨大探照燈為你們做演示。
The brightness of that search light represents the extreme luminosity of stars.
探照燈的光亮代表著恒星極高的亮度。
And I'm gonna use this tiny marble to represent the planet.
而我將用這個小彈珠來代表行星。
When this planet passes in front of the star, it blocks out a tiny fraction of the light, dimming the star by a very small amount, and that's exactly what the Kepler space craft can detect.
當這個小行星從恒星面前通過時，它會擋住微小的一部分光，使恒星變暗一小小點，而開普勒就能夠偵測到這種十分微小的變化。
Kepler can detect a 0.01% change in the star's luminosity, but even if it spots a dimming star, there's still another problem.
開普勒能夠識別恒星萬分之一的亮度變化，但即便它檢測到一個變暗的恒星，我們還有一個問題需要解決。
When Kepler sees a star that dims, it could be due to a speck of dust that crossed in front of the telescope.
當開普勒檢測到恒星變暗有可能是因為望遠鏡前的一些塵土。
There's a possibility of binary stars, two stars that orbit each other thereby dimming the stars as one blocks the other.
還有可能是雙星兩個形體相互環繞，就好像是一個擋住了另一個從而使得恒星變暗。
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