The universe contains about 100 billion galaxies.
宇宙中有著大約1000億個(gè)星系。
Each of those galaxies contains about 100 billion stars.
每個(gè)星系包含著大約1000億個(gè)恒星。
Many of those stars have planets orbiting them.
許多恒星擁有繞著它們旋轉(zhuǎn)的行星。
So how do we look for life in all that immensity?
那么我們?cè)鯓釉谶@浩瀚的空間中找到生命呢？
It's like searching for a needle in trillions of haystacks.
這簡(jiǎn)直就是海底撈針。
We might want to focus our search on planets that we know can support life as we know it -- what we call habitable worlds.
我們集中注意力在可以支持生命的星球——所謂的“宜居星球”。
What do such planets look like?
這些星球是什么樣子的呢？
To answer that question, we don't look out there.
來(lái)回答這個(gè)問(wèn)題，我們勿需把眼光放在外太空。
Instead, we look at ourselves. At Earth.
而是反觀自身——我們的地球。
Because this is the one planet in the universe that we know for certain is habitable.
因?yàn)檫@是宇宙中我們所確定能支持生命的一個(gè)星球。
When we look at Earth from space, we see a blue, watery world.
當(dāng)我們從太空觀察地球，我們會(huì)看見(jiàn)一個(gè)藍(lán)色的水之世界。
It's no coincidence that three quarters of the surface is covered by oceans.
它表面的四分之三被海洋覆蓋并不是出于偶然。
Because of its unique chemical and physical properties, water is absolutely essential for all life as we know it.
由于水獨(dú)一無(wú)二的化學(xué)和物理特性，它是生命不可缺少的物質(zhì)。
And so we get especially excited about other worlds on which water is abundant.
正是如此，我們才會(huì)對(duì)同樣有著豐富水源的星球而激動(dòng)異常。
Fortunately, water is very common in the universe.
幸運(yùn)的是，水在宇宙中是非常普遍的。
But life needs water in the form of liquid, not ice, and not vapor, and that's a little bit less common.
但是生命需要的是液態(tài)水，不是冰塊，也不是蒸汽，這樣的話，情況就沒(méi)那么樂(lè)觀了。
For a planet to have liquid water at its surface, three things are important.
行星若要有液態(tài)水存在于其表面， 有三件事是很重要的。
First, the planet needs to be large enough that the force of gravity keeps the water molecules from flying off into space.
第一，這個(gè)行星需要有足夠的體積來(lái)產(chǎn)生，足以防止水分子逃逸到外太空的重力。
For example, Mars is smaller than Earth, and so has less gravity, and that's one important reason that Mars has a very thin atmosphere, and no oceans at its surface.
比如說(shuō)火星，它比地球體積小，因此重力更小，這就是為什么火星僅有一層薄薄的大氣，然而表面卻沒(méi)有海洋。
Second, the planet needs to have an atmosphere. Why?
第二，這個(gè)行星需要有大氣層。為什么呢？
Because without an atmosphere, the planet is in a vacuum, and liquid water isn't stable in a vacuum.
因?yàn)闆](méi)有大氣層，行星就處于真空狀態(tài)，而液態(tài)水在真空中是不穩(wěn)定的。
For example, our moon has no atmosphere, and so if you spill some water on the moon, it will either boil away as vapor, or freeze solid to make ice.
例如，我們的月球沒(méi)有大氣層，所以如果你將水灑在月球上，它要么會(huì)沸騰蒸發(fā)掉，要么會(huì)凍結(jié)成冰塊。
Without the pressure of an atmosphere, liquid water can't survive.
沒(méi)有大氣層的氣壓，液態(tài)水也難以“生存"。
Third, the planet needs to be at the right distance from its star.
第三，行星需要與恒星保持一個(gè)合適的距離。
Too close, and the surface temperature will exceed the boiling point of water, and oceans will turn to vapor.
太接近恒星，其表面溫度會(huì)超過(guò)水的沸點(diǎn)，海洋將會(huì)蒸發(fā)。
Too far, and the surface temperature will fall below the freezing point of water, causing the oceans to turn to ice.
太遙遠(yuǎn)的話，其表面溫度又會(huì)低于水的冰點(diǎn)，導(dǎo)致海洋結(jié)冰。
Fire or ice. For life as we know it, neither will suffice.
無(wú)論是冰還是火，都無(wú)法滿足我們所知的生命所需的生存條件。
You can imagine that the perfect zone where water stays liquid looks kind of like a belt around a star.
現(xiàn)在你能想象到，液態(tài)水的“完美區(qū)域”是個(gè)圍繞著恒星的環(huán)形地帶。
We call that belt the habitable zone.
我們稱這個(gè)環(huán)形地帶為“宜居帶”。
So when we search for habitable worlds, we definitely want to look for planets in the habitable zones around their stars.
所以尋找宜居星球時(shí)，我們當(dāng)然想要把目光放在宜居帶內(nèi)繞著恒星旋轉(zhuǎn)的行星上面。
Those regions are the best bets to find planets like Earth.
那些地區(qū)最有可能找到像地球這樣的行星。
But while habitable zones are a pretty good place to begin the search for planets with life, there are a couple of complications.
不過(guò)，盡管宜居帶是尋找地外生命的良好開(kāi)端，但是我們?nèi)匀粫?huì)碰到一些麻煩。
First, a planet isn't necessarily habitable just because it's in the habitable zone.
首先，一顆行星在宜居帶內(nèi)并不一定代表它就是宜居的。
Consider the planet Venus in our solar system.
拿我們太陽(yáng)系內(nèi)的金星作為例子。
If you were an alien astronomer, you'd think Venus is a pretty good bet for life.
如果你是個(gè)外星天文學(xué)家，你會(huì)認(rèn)為金星是個(gè)很適宜生命繁衍的地方。
It's the right size, it has an atmosphere, and it's in the habitable zone of our sun.
它大小適中，擁有大氣層而且它在太陽(yáng)系的宜居帶內(nèi)。
An alien astronomer might see it as Earth's twin.
一個(gè)外星天文學(xué)家會(huì)將它視作地球的雙胞胎兄弟姐妹。
But Venus is not habitable, at least not at its surface.
但是金星并不適宜生命存在，至少在它的表面是這樣。
Not by life as we know it. It's too hot.
或者說(shuō)不適宜我們已知的生命形式。 它太熱了。
That's because Venus' atmosphere is full of carbon dioxide, an important greenhouse gas.
這是由于金星的大氣層充斥著二氧化碳，一種重要的溫室氣體。
In fact, its atmosphere is almost entirely carbon dioxide, and is almost 100 times thicker than our own.
事實(shí)上，它的大氣層幾乎全是二氧化碳，而且其厚度比地球大氣層高100倍。
As a result, the temperature on Venus is hot enough to melt lead, and the planet is dry as a bone.
造成的結(jié)果就是金星上的氣溫高到足以融化鉛塊，而且整個(gè)星球干燥透頂。
So finding planets of the right size and distance from their stars is only a beginning.
所以發(fā)現(xiàn)那些大小適中并且與恒星距離合適的行星僅僅是一個(gè)開(kāi)端。
We also want to know about the makeup of their atmospheres.
我們同樣需要了解它們的大氣成分。
The second complication emerges when we look a little more deeply at planet Earth.
當(dāng)更進(jìn)一步探索，我們碰到尋找地外生命的第二個(gè)麻煩。
In the last 30 years, we've discovered microbes living in all sorts of extreme environments.
在過(guò)去30年，我們?cè)谒袠O端環(huán)境中都發(fā)現(xiàn)了微生物的存在。
We find them in fissures of rock miles beneath our feet, in boiling waters of the ocean floor, in acidic waters of thermal springs, and in cloud droplets miles above our heads.
在我們腳下的巖縫里，海底巖床的沸水中，地?zé)釡厝乃嵝匀校覀冾^頂幾英里上方滴落的雨水中。
These so-called extremophiles aren't rare.
這些所謂的極端環(huán)境其實(shí)并不罕見(jiàn)。
Some scientists estimate that the mass of microbes living deep underground equals the mass of all the life at Earth's surface.
一些科學(xué)家估計(jì)生活在地下的微生物的數(shù)量，跟所有生存在地球表面的生物的數(shù)量差不多。
These subterranean microbes don't need oceans or sunshine.
這些地下微生物不需要海洋或者陽(yáng)光。
These discoveries suggest that Earth-like planets may be only the tip of the astrobiological iceberg.
這些發(fā)現(xiàn)表明類地行星可能僅僅是天體生物學(xué)冰山的小小一角。
It's possible that life might persist in aquifers beneath the surface of Mars.
有可能生命就頑強(qiáng)地生活在火星地表以下的含水層內(nèi)。
Microbes may thrive on Jupiter's moon Europa, where liquid water ocean probably lies beneath the icy crust.
微生物可能在木星的衛(wèi)星——?dú)W羅巴上繁榮生長(zhǎng)，在歐羅巴的冰層下可能存在液態(tài)水海洋。
Another ocean beneath the surface of Saturn's moon Enceladus is the source of geysers erupting into space.
另外土星的衛(wèi)星——土衛(wèi)二地表下的海洋是其間歇泉的水汽來(lái)源。
Could these geysers be raining microbes？
間歇泉能滋養(yǎng)微生物的生命嗎？
Could we fly through them to find out?
我們能夠穿越時(shí)空去發(fā)現(xiàn)問(wèn)題的答案嗎？
And what about life as we don't know it, using a liquid other than water?
在未發(fā)現(xiàn)的生命形式中，有依靠除了水以外的其他液體存活？
Maybe we are the crazy creatures living in an unusual and extreme environment.
可能人類才是在不同尋常的極端環(huán)境中生存的“瘋狂生物”。
Maybe the real habitable zone is so large that there are billions of needles in those trillions of haystacks.
可能真正的宜居帶的范圍相當(dāng)巨大如同在海底有無(wú)數(shù)的針可撈。
Maybe in the big scheme of things, Earth is only one of many different kinds of habitable worlds.
可能在整個(gè)宇宙的大設(shè)計(jì)中，地球僅僅是眾多不同種類的宜居世界中的一種。
The only way to find out is to go out and explore.
找到這些問(wèn)題答案的唯一辦法就是走向那無(wú)垠的太空，去勇敢地探索。
@TED
