The universe contains about 100 billion galaxies.
宇宙中有着大约1000亿个星系。
Each of those galaxies contains about 100 billion stars.
每个星系包含着大约1000亿个恒星。
Many of those stars have planets orbiting them.
许多恒星拥有绕着它们旋转的行星。
So how do we look for life in all that immensity?
那么我们怎样在这浩瀚的空间中找到生命呢？
It's like searching for a needle in trillions of haystacks.
这简直就是海底捞针。
We might want to focus our search on planets that we know can support life as we know it -- what we call habitable worlds.
我们集中注意力在可以支持生命的星球——所谓的“宜居星球”。
What do such planets look like?
这些星球是什么样子的呢？
To answer that question, we don't look out there.
来回答这个问题，我们勿需把眼光放在外太空。
Instead, we look at ourselves. At Earth.
而是反观自身——我们的地球。
Because this is the one planet in the universe that we know for certain is habitable.
因为这是宇宙中我们所确定能支持生命的一个星球。
When we look at Earth from space, we see a blue, watery world.
当我们从太空观察地球，我们会看见一个蓝色的水之世界。
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盖并不是出于偶然。
Because of its unique chemical and physical properties, water is absolutely essential for all life as we know it.
由于水独一无二的化学和物理特性，它是生命不可缺少的物质。
And so we get especially excited about other worlds on which water is abundant.
正是如此，我们才会对同样有着丰富水源的星球而激动异常。
Fortunately, water is very common in the universe.
幸运的是，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态水，不是冰块，也不是蒸汽，这样的话，情况就没那么乐观了。
For a planet to have liquid water at its surface, three things are important.
行星若要有液态水存在于其表面， 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，这个行星需要有足够的体积来产生，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.
比如说火星，它比地球体积小，因此重力更小，这就是为什么火星仅有一层薄薄的大气，然而表面却没有海洋。
Second, the planet needs to have an atmosphere. Why?
第二，这个行星需要有大气层。为什么呢？
Because without an atmosphere, the planet is in a vacuum, and liquid water isn't stable in a vacuum.
因为没有大气层，行星就处于真空状态，而液态水在真空中是不稳定的。
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，我们的月球没有大气层，所以如果你将水洒在月球上，它要么会沸腾蒸发掉，要么会冻结成冰块。
Without the pressure of an atmosphere, liquid water can't survive.
没有大气层的气压，液态水也难以“生存"。
Third, the planet needs to be at the right distance from its star.
第三，行星需要与恒星保持一个合适的距离。
Too close, and the surface temperature will exceed the boiling point of water, and oceans will turn to vapor.
太接近恒星，其表面温度会超过水的沸点，海洋将会蒸发。
Too far, and the surface temperature will fall below the freezing point of water, causing the oceans to turn to ice.
太遥远的话，其表面温度又会低于水的冰点，导致海洋结冰。
Fire or ice. For life as we know it, neither will suffice.
无论是冰还是火，都无法满足我们所知的生命所需的生存条件。
You can imagine that the perfect zone where water stays liquid looks kind of like a belt around a star.
现在你能想象到，液态水的“完美区域”是个围绕着恒星的环形地带。
We call that belt the habitable zone.
我们称这个环形地带为“宜居带”。
So when we search for habitable worlds, we definitely want to look for planets in the habitable zones around their stars.
所以寻找宜居星球时，我们当然想要把目光放在宜居带内绕着恒星旋转的行星上面。
Those regions are the best bets to find planets like Earth.
那些地区最有可能找到像地球这样的行星。
But while habitable zones are a pretty good place to begin the search for planets with life, there are a couple of complications.
不过，尽管宜居带是寻找地外生命的良好开端，但是我们仍然会碰到一些麻烦。
First, a planet isn't necessarily habitable just because it's in the habitable zone.
首先，一颗行星在宜居带内并不一定代表它就是宜居的。
Consider the planet Venus in our solar system.
拿我们太阳系内的金星作为例子。
If you were an alien astronomer, you'd think Venus is a pretty good bet for life.
如果你是个外星天文学家，你会认为金星是个很适宜生命繁衍的地方。
It's the right size, it has an atmosphere, and it's in the habitable zone of our sun.
它大小适中，拥有大气层而且它在太阳系的宜居带内。
An alien astronomer might see it as Earth's twin.
一个外星天文学家会将它视作地球的双胞胎兄弟姐妹。
But Venus is not habitable, at least not at its surface.
但是金星并不适宜生命存在，至少在它的表面是这样。
Not by life as we know it. It's too hot.
或者说不适宜我们已知的生命形式。 它太热了。
That's because Venus' atmosphere is full of carbon dioxide, an important greenhouse gas.
这是由于金星的大气层充斥着二氧化碳，一种重要的温室气体。
In fact, its atmosphere is almost entirely carbon dioxide, and is almost 100 times thicker than our own.
事实上，它的大气层几乎全是二氧化碳，而且其厚度比地球大气层高100倍。
As a result, the temperature on Venus is hot enough to melt lead, and the planet is dry as a bone.
造成的结果就是金星上的气温高到足以融化铅块，而且整个星球干燥透顶。
So finding planets of the right size and distance from their stars is only a beginning.
所以发现那些大小适中并且与恒星距离合适的行星仅仅是一个开端。
We also want to know about the makeup of their atmospheres.
我们同样需要了解它们的大气成分。
The second complication emerges when we look a little more deeply at planet Earth.
当更进一步探索，我们碰到寻找地外生命的第二个麻烦。
In the last 30 years, we've discovered microbes living in all sorts of extreme environments.
在过去30年，我们在所有极端环境中都发现了微生物的存在。
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.
在我们脚下的岩缝里，海底岩床的沸水中，地热温泉的酸性泉水中，我们头顶几英里上方滴落的雨水中。
These so-called extremophiles aren't rare.
这些所谓的极端环境其实并不罕见。
Some scientists estimate that the mass of microbes living deep underground equals the mass of all the life at Earth's surface.
一些科学家估计生活在地下的微生物的数量，跟所有生存在地球表面的生物的数量差不多。
These subterranean microbes don't need oceans or sunshine.
这些地下微生物不需要海洋或者阳光。
These discoveries suggest that Earth-like planets may be only the tip of the astrobiological iceberg.
这些发现表明类地行星可能仅仅是天体生物学冰山的小小一角。
It's possible that life might persist in aquifers beneath the surface of Mars.
有可能生命就顽强地生活在火星地表以下的含水层内。
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卫星——欧罗巴上繁荣生长，在欧罗巴的冰层下可能存在液态水海洋。
Another ocean beneath the surface of Saturn's moon Enceladus is the source of geysers erupting into space.
另外土星的卫星——土卫二地表下的海洋是其间歇泉的水汽来源。
Could these geysers be raining microbes？
间歇泉能滋养微生物的生命吗？
Could we fly through them to find out?
我们能够穿越时空去发现问题的答案吗？
And what about life as we don't know it, using a liquid other than water?
在未发现的生命形式中，有依靠除了水以外的其他液体存活？
Maybe we are the crazy creatures living in an unusual and extreme environment.
可能人类才是在不同寻常的极端环境中生存的“疯狂生物”。
Maybe the real habitable zone is so large that there are billions of needles in those trillions of haystacks.
可能真正的宜居带的范围相当巨大如同在海底有无数的针可捞。
Maybe in the big scheme of things, Earth is only one of many different kinds of habitable worlds.
可能在整个宇宙的大设计中，地球仅仅是众多不同种类的宜居世界中的一种。
The only way to find out is to go out and explore.
找到这些问题答案的唯一办法就是走向那无垠的太空，去勇敢地探索。
@TED
