Each of those galaxies contains about 100 billion stars.

這些星系中的每一個都包含了大約1000億顆恆星。

Many of those stars have planets orbiting them.

其中許多恆星都有行星圍繞著它們運行。

So how do we look for life in all that immensity?

那麼，我們該如何在這無量無邊中尋找生命呢？

It's like searching for a needle in trillions of haystacks.

這就像在數萬億個大海撈針一樣。

We might want to focus our search on planets that we know can support life as we know it --

我們可能想把搜索重點放在我們知道能支持生命的星球上...

what we call habitable worlds.

我們稱之為可居住的世界。

What do such planets look like?

這樣的行星是什麼樣子的？

To answer that question, we don't look out there.

回答這個問題，我們不看外面。

Instead, we look at ourselves. At Earth.

相反，我們看著自己。在地球上。

Because this is the one planet in the universe that we know for certain is habitable.

因為這是宇宙中唯一一個我們可以確定是適合居住的星球。

When we look at Earth from space, we see a blue, watery world.

當我們從太空看地球時，我們看到的是一個藍色的、水汪汪的世界。

It's no coincidence that three quarters of the surface is covered by oceans.

這不是巧合，四分之三的地表被海洋覆蓋。

Because of its unique chemical and physical properties,

由於其獨特的化學和物理特性。

water is absolutely essential for all life as we know it.

水對於我們所知的所有生命來說絕對是必不可少的。

And so we get especially excited about other worlds on which water is abundant.

所以我們對其他水豐富的世界特別興奮。

Fortunately, water is very common in the universe.

幸運的是，水在宇宙中很常見。

But life needs water in the form of liquid, not ice, and not vapor,

但生命需要的是液體形式的水，而不是冰，也不是蒸汽。

and that's a little bit less common.

而這'就有點不太常見了。

For a planet to have liquid water at its surface, three things are important.

一個星球要想在其表面有液態水，有三件事很重要。

First, the planet needs to be large enough that the force of gravity

首先，行星需要足夠大，引力的力量

keeps the water molecules from flying off into space.

讓水分子不至於飛向太空。

For example, Mars is smaller than Earth, and so has less gravity,

例如，火星比地球小，所以重力較小。

and that's one important reason that Mars has a very thin atmosphere,

而這也是火星大氣層非常稀薄的一個重要原因。

and no oceans at its surface.

而在其表面沒有海洋。

Second, the planet needs to have an atmosphere. Why?

第二，地球需要有大氣層。為什麼？

Because without an atmosphere, the planet is in a vacuum,

因為沒有大氣層，地球就處於真空狀態。

and liquid water isn't stable in a vacuum.

而液態水在真空中並不穩定。

For example, our moon has no atmosphere, and so if you spill some water on the moon,

比如說，我們的月球沒有大氣層，所以如果你在月球上灑一些水。

it will either boil away as vapor, or freeze solid to make ice.

它要麼以蒸汽的形式沸騰，要麼凝固成冰。

Without the pressure of an atmosphere, liquid water can't survive.

沒有大氣的壓力，液態水就無法生存。

Third, the planet needs to be at the right distance from its star.

第三，行星與恆星的距離要合適。

Too close, and the surface temperature will exceed the boiling point of water,

過近，表面溫度會超過水的沸點。

and oceans will turn to vapor.

和海洋將變成水汽。

Too far, and the surface temperature will fall below the freezing point of water,

太遠，地表溫度就會降到水的冰點以下。

causing the oceans to turn to ice.

導致海洋變成冰。

Fire or ice. For life as we know it, neither will suffice.

火或冰。對於我們所知的生命來說，兩者都不夠。

You can imagine that the perfect zone where water stays liquid looks kind of like a belt around a star.

你可以想象一下，水保持液態的完美區域看起來有點像圍繞著恆星的一條帶子。

We call that belt the habitable zone.

我們稱那條帶子為可居住區。

So when we search for habitable worlds, we definitely want to look for planets in the habitable zones around their stars.

所以我們在尋找宜居世界的時候，一定要在恆星周圍的宜居區尋找行星。

Those regions are the best bets to find planets like Earth.

這些地區是尋找像地球這樣的行星的最佳選擇。

But while habitable zones are a pretty good place to begin the search for planets with life,

但是，雖然宜居區是一個相當不錯的地方，可以開始尋找有生命的行星。

there are a couple of complications.

有幾個併發症。

First, a planet isn't necessarily habitable just because it's in the habitable zone.

首先，一個星球不一定適合居住，只是因為它'在可居住區。

Consider the planet Venus in our solar system.

考慮到太陽系中的金星。

If you were an alien astronomer, you'd think Venus is a pretty good bet for life.

如果你是一個外星天文學家，你'會認為金星是一個相當不錯的生命賭注。

It's the right size, it has an atmosphere, and it's in the habitable zone of our sun.

它大小合適 有大氣層 而且在太陽的可居住區裡

An alien astronomer might see it as Earth's twin.

外星天文學家可能會把它看作是地球的孿生兄弟。

But Venus is not habitable, at least not at its surface.

但金星不適合居住，至少在其表面不適合居住。

Not by life as we know it. It's too hot.

不是由我們所知道的生活。它太熱。

That's because Venus' atmosphere is full of carbon dioxide, an important greenhouse gas.

那是因為金星'大氣中充滿了二氧化碳，這是一種重要的溫室氣體。

In fact, its atmosphere is almost entirely carbon dioxide,

事實上，它的大氣中幾乎全是二氧化碳。

and is almost 100 times thicker than our own.

並且比我們自己的厚度幾乎厚100倍。

As a result, the temperature on Venus is hot enough to melt lead,

是以，金星上的溫度足以融化鉛。

and the planet is dry as a bone.

而這個星球卻乾枯如骨。

So finding planets of the right size and distance from their stars is only a beginning.

所以，尋找大小合適、距離恆星距離合適的行星只是一個開始。

We also want to know about the makeup of their atmospheres.

我們也想知道他們大氣層的構成。

The second complication emerges when we look a little more deeply at planet Earth.

當我們更深入地觀察地球時，就會發現第二個複雜問題。

In the last 30 years, we've discovered microbes living in all sorts of extreme environments.

在過去的30年裡，我們'發現了生活在各種極端環境中的微生物。

We find them in fissures of rock miles beneath our feet,

我們在腳下數英里處的岩石裂縫中找到它們。

in boiling waters of the ocean floor,

在沸騰的海底水裡。

in acidic waters of thermal springs,

在酸性溫泉水中。

and in cloud droplets miles above our heads.

並在我們頭頂數英里的雲滴中。

These so-called extremophiles aren't rare.

這些所謂的極端分子並不罕見。

Some scientists estimate that the mass of microbes living deep underground

據一些科學家估計，生活在地下深處的微生物的品質。

equals the mass of all the life at Earth's surface.

等於地球表面所有生命的品質'。

These subterranean microbes don't need oceans or sunshine.

這些地下微生物不需要海洋和陽光。

These discoveries suggest that Earth-like planets may be only the tip of the astrobiological iceberg.

這些發現表明，類地行星可能只是天體生物學冰山的一角。

It's possible that life might persist in aquifers beneath the surface of Mars.

它'生命有可能存在於火星表面下的含水層中。

Microbes may thrive on Jupiter's moon Europa,

微生物可能在木星'的衛星歐羅巴上茁壯成長。

where liquid water ocean probably lies beneath the icy crust.

其中液態水海洋可能位於冰殼之下。

Another ocean beneath the surface of Saturn's moon Enceladus is the source of geysers erupting into space.

土星'的衛星恩克拉多斯表面下的另一片海洋是向太空噴發的噴泉的來源。

Could these geysers be raining microbes?

難道這些間歇泉是在下微生物雨？

Could we fly through them to find out?

我們能不能飛過他們去看看？

And what about life as we don't know it, using a liquid other than water?

那我們不知道的生命呢'，使用水以外的液體？

Maybe we are the crazy creatures living in an unusual and extreme environment.

也許我們是生活在不尋常的極端環境中的瘋狂生物。

Maybe the real habitable zone is so large

也許真正的可居住區就是這麼大

that there are billions of needles in those trillions of haystacks.

在那數萬億個乾草堆裡有數十億根針。

Maybe in the big scheme of things, Earth is only one of many different kinds of habitable worlds.

也許從大的方面來說，地球只是許多不同種類的可居住世界中的一個。

The only way to find out is to go out and explore.

唯有走出去探索，才能找到答案。