A needle in countless haystacks: Finding habitable worlds - Ariel Anbar

大海撈針：找尋可居住的星球 - Ariel Anbar

Translator: tom carter Reviewer: Bedirhan Cinar

譯者: Jephian Lin 審譯者: Coco Shen

The universe contains about 100 billion galaxies.

宇宙中大約有一千億個星系。

Each of those galaxies contains about 100 billion stars.

每個星系大約有一千億個恆星。

Many of those stars have planets orbiting them.

而許多恆星有行星繞著它們旋轉。

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

所以我們要怎麼在這麼片浩瀚中 找尋其他生物？

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

這就像大海撈針一樣。

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

也許我們會先從一些行星著手， 這些行星要能夠維持我們已經認識的生命形態

what we call habitable worlds.

──也就是我們所說的「適居地區」。

What do such planets look like?

這樣的星球會有什麼特徵？

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

要回答這個問題，我們不用向外找答案。

Instead, we look at ourselves. At Earth.

而是看看我們自己。 我們的地球。

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

因為這顆宇宙中獨一無二的星球， 我們確定它上面是可住人的。

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

當我們從宇宙中看著地球， 我們會看到一個藍色的、充滿水的世界。

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

地球表面有確實有四分之三 被海洋所覆蓋。

Because of its unique chemical and physical properties,

因為水獨特的物理性質與化學性質，

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

它對於我們已知的生命來說 是絕對重要的。

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

所以我們對於富含水的行球 感到特別期待。

Fortunately, water is very common in the universe.

幸運的是，水在宇宙中蠻常見的。

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

然而生命須要的是液態水， 不是冰、也不是水氣，

and that's a little bit less common.

這就變得有一點點不常見了。

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

要找到表面上有水的行星， 有三件事情是重要的。

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

第一，這行星必須夠大， 這樣才能有足夠的重力

keeps the water molecules from flying off into space.

來把水分子抓住、 不讓它們飛回宇宙。

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

比如說，火星比地球還小，所以重力也比較小，

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

而這是火星上大氣稀薄

and no oceans at its surface.

並且沒有海洋的原因。

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

第二，這行星必須要有大氣層。 為什麼？

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

因為如果沒有大氣層， 那這星球就是真空的，

and liquid water isn't stable in a vacuum.

而液態水在真空中 並不是穩定的。

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

舉例來說，月球上沒有大氣， 所以如果你在月球上潑一些水，

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

它要不就是沸騰變成氣體， 要不就是凝結成冰。

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

如果沒有大氣壓力， 液態水就沒辦法存在。

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

第三，這行星必須要和它的恆星 保持適當的距離。

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

太近，表面溫度會超過水的沸點，

and oceans will turn to vapor.

那海洋就會變為水蒸氣。

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

太遠，那表面溫度就會降到水的凝固點以下

causing the oceans to turn to ice.

這樣海洋就會凝結成冰。

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

炎熱如火或是寒冷如冰。 對於已知的生命來說，兩種都是不行的。

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

你可以想像那段水能以液態存在的完美區域 就像是恆星周圍的一個環。

We call that belt the habitable zone.

我們把這個環叫作 「適居帶」。

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

所以當我們在找尋適居地區的時候， 我們一定會去找尋落在恆星適居帶的行星。

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

這些區域最有可能找到 和地球相似的行星。

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

儘管適居帶是 找尋有生命行星的一個好地方，

there are a couple of complications.

但還是有一些些困難。

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

首先，適居帶裡的行星 並不一定就是適居的星球。

Consider the planet Venus in our solar system.

太陽系裡的金星就是個例子。

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

如果你是外星人的天文學家， 也許金星看上去很有可能。

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

它大小適中、它有大氣層、 又在太陽的適居帶裡。

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

外星天文學家 應該會覺得它是地球的孿生兄弟。

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

不過金星並不適合生存， 至少它的表面沒辦法。

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

已知生命無法生存。 它太熱了。

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

這是因為金星的大氣充滿著二氧化碳， 這是種重要的溫室氣體。

In fact, its atmosphere is almost entirely carbon dioxide,

事實上，它的大氣 幾乎全部都是二氧化碳，

and is almost 100 times thicker than our own.

而且是我們大氣的一百倍厚。

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

因此， 金星的氣溫熱到可以將鉛熔化，

and the planet is dry as a bone.

而且乾到不行。

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

所以找尋大小適中、 離恆星距離適中只是個開始。

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

我們也會想要知道 它們大氣的組成。

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

而當我們稍微瞄一下地球深處 第二個困難就會浮現出來。

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

在過去的三十年之中，我們發現許多 活在各種極端環境的微生物。

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

我們在腳下數英里的岩石裂縫裡找到它們，

in boiling waters of the ocean floor,

在海洋底層的熱水中找到它們，

in acidic waters of thermal springs,

在溫泉的酸性水質中找到它們，

and in cloud droplets miles above our heads.

還有在頭上數英里的雲滴裡找到它們。

These so-called extremophiles aren't rare.

這些所謂的嗜極生物並不少見。

Some scientists estimate that the mass of microbes living deep underground

有些科學家估計 地底深處所有微生物的質量

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

相當於 所有地表生物的質量。

These subterranean microbes don't need oceans or sunshine.

這些地底微生物 並不依賴海洋和陽光。

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

這些發現告訴我們 地球這類的行星可能只是宇宙生命的冰山一角。

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

也許有生命存活在火星地底的含水層。

Microbes may thrive on Jupiter's moon Europa,

也許有大量生命存活在 木星的衛星歐羅巴（Europa）上，

where liquid water ocean probably lies beneath the icy crust.

在那裡液態水可能是躲在 結冰的地殼之下。

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

其他地底海洋則是在土星衛星恩克拉多斯（Enceladus）上，那也是噴到空中的間歇泉的源頭。

Could these geysers be raining microbes?

這些間歇泉有可能噴著 含有微生物的泉水嗎？

Could we fly through them to find out?

我們能飛到那裡然後找出答案嗎？

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

或者說是其他我們不知道的生物， 它們仰賴其他液體存活而不是水？

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

也許反而是我們才是一個瘋狂的物種， 生活在奇怪又異端的環境裡。

Maybe the real habitable zone is so large

也許實際上的適居帶大到

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

像是大片海洋裡的 數十億根針。

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

也許整體看來， 地球才是許多不同形式的適居地區裡的其中一種。

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

而要找出答案唯一的方法 就是向外探索。