So when you look out at the stars at night,

當你擡頭看夜空中的繁星，

it's amazing what you can see.

你所見到的，是多麼奇妙。

It's beautiful.

多麽美好。

But what's more amazing is what you can't see,

但更奇妙的是那些你看不見的，

because what we know now

因為根據人類已知，

is that around every star or almost every star,

幾乎在每一顆恒星周圍

there's a planet,

都有一颗行星，

or probably a few.

甚至若干個。

So what this picture isn't showing you

這幅圖片沒能告訴你的是

are all the planets that we know about

在這個宇宙中存在著的

out there in space.

所有的行星。

But when we think about planets, we tend to think of faraway things

當我們提起行星，我們總會覺得， 它們是離我們很遙遠的東西，

that are very different from our own.

跟我們自己的星球很不一樣。

But here we are on a planet,

但我們現在正身處在一個行星，

and there are so many things that are amazing about Earth

而且有太多美妙的事物在這個地球，

that we're searching far and wide to find things that are like that.

我們正在深入發掘和探索。

And when we're searching, we're finding amazing things.

當我們探索，就發現奇妙的事物。

But I want to tell you about an amazing thing here on Earth.

但今天我想告訴你們一件 關於地球的奇妙的事，

And that is that every minute,

就是每分鐘內，

400 pounds of hydrogen

四百磅的氫氣（181.2 公斤）

and almost seven pounds of helium

和將近七磅的氦氣（3.18 公斤）

escape from Earth into space.

正在從地球洩漏到宇宙中。

And this is gas that is going off and never coming back.

這些氣體一去不復返。

So hydrogen, helium and many other things

氫氣、氮氣、和很多其他氣體

make up what's known as the Earth's atmosphere.

共同組成了我們所知的地球大氣。

The atmosphere is just these gases that form a thin blue line

大氣層從國際空間站的視角來看，

that's seen here from the International Space Station,

就只是一層薄薄的藍色。

a photograph that some astronauts took.

這是太空人拍下的照片。

And this tenuous veneer around our planet

正是這層稀薄的大氣，

is what allows life to flourish.

讓地球上的生命欣欣向榮。

It protects our planet from too many impacts,

大氣保護地球，免受太多侵害，

from meteorites and the like.

例如隕石墜落。

And it's such an amazing phenomenon

但是事實上，

that the fact that it's disappearing

這個奇妙的現象正在消失。

should frighten you, at least a little bit.

這應該讓你感到害怕， 哪怕只是一點點。

So this process is something that I study

這消失的過程就是我研究的領域——

and it's called atmospheric escape.

大氣逃離。

So atmospheric escape is not specific to planet Earth.

大氣逃離的現象 不僅僅發生在地球上；

It's part of what it means to be a planet, if you ask me,

這個現象是被稱為行星的條件之一，

because planets, not just here on Earth but throughout the universe,

因為不僅在地球上， 在宇宙中的所有行星上，

can undergo atmospheric escape.

都會發生大氣逃離現象。

And the way it happens actually tells us about planets themselves.

實際上，這個現象 可以告訴我們關於行星本身。

Because when you think about the solar system,

因為當你想到太陽系，

you might think about this picture here.

你可能會想到這張圖片。

And you would say, well, there are eight planets, maybe nine.

你可能會說： 這有八顆行星，可能是九顆。

So for those of you who are stressed by this picture,

那些為這張圖片感到苦惱的朋友，

I will add somebody for you.

我要給你們加上一顆。

(Laughter)

（笑）

Courtesy of New Horizons, we're including Pluto.

我們尊重「新視野號」， 所以連冥王星也算上。

And the thing here is,

我們現在討論的是：

for the purposes of this talk and atmospheric escape,

為了關於大氣逃離的研究課題，

Pluto is a planet in my mind,

我認為冥王星屬於行星。

in the same way that planets around other stars that we can't see

同樣的，在我們看不見的恒星周圍

are also planets.

公轉的行星都應該被稱作行星。

So fundamental characteristics of planets

因此，行星的基本特徵

include the fact that they are bodies

包括他們受萬有引力吸引 而凝聚成一體

that are bound together by gravity.

這個事實。

So it's a lot of material just stuck together

這股吸引力令很多物質

with this attractive force.

凝聚在一起。

And these bodies are so big and have so much gravity.

因為這些行星體積巨大 而且引力很強，

That's why they're round.

所以它們是球體狀。

So when you look at all of these,

所以當你看到圖中所示的所有星球，

including Pluto,

包括冥王星，

they're round.

它們都是球體。

So you can see that gravity is really at play here.

萬有引力是一個很重要的因素。

But another fundamental characteristic about planets

行星的另一個基本特徵

is what you don't see here,

是你在這個圖裡沒有看到的，

and that's the star, the Sun,

那就是我們的恒星——太陽。

that all of the planets in the solar system are orbiting around.

太陽系中所有 行星都圍繞著太陽公轉，

And that's fundamentally driving atmospheric escape.

這也是大氣逃離現象的基本原因。

The reason that fundamentally stars drive atmospheric escape from planets

恒星造成大氣從行星中逃離是因為

is because stars offer planets particles and light and heat

恒星為行星提供的微粒、光、和熱

that can cause the atmospheres to go away.

會促使大氣逃離。

So if you think of a hot-air balloon,

你可以想象一個熱氣球，

or you look at this picture of lanterns in Thailand at a festival,

或者看看這幅泰國水燈節的照片，

you can see that hot air can propel gasses upward.

你會見到熱空氣可以驅使氣體上升。

And if you have enough energy and heating,

如果能量和熱量充足，

which our Sun does,

像太陽那樣，

that gas, which is so light and only bound by gravity,

那麽質量輕而且受到引力吸引的氣體

it can escape into space.

就會逃離進入宇宙，

And so this is what's actually causing atmospheric escape

造成大氣逃離

here on Earth and also on other planets --

不僅在地球，而且在其他行星上，

that interplay between heating from the star

這互相作用都因恒星給予的熱量

and overcoming the force of gravity on the planet.

和掙脫行星自身的引力而產生。

So I've told you that it happens

我曾經提到過：

at the rate of 400 pounds a minute for hydrogen

氣體逃離的速度是 每分鐘四百磅的氫氣

and almost seven pounds for helium.

和將近七磅的氦氣，

But what does that look like?

但這現象看起來是什麽樣子的呢？

Well, even in the '80s,

從八十年代開始，

we took pictures of the Earth

美國國家航空暨太空總署

in the ultraviolet

動態探測器飛船

using NASA's Dynamic Explorer spacecraft.

就在為地球拍攝紫外線照片。

So these two images of the Earth

這兩張照片告訴你

show you what that glow of escaping hydrogen looks like,

氫氣逃離時的光是什麽樣的——

shown in red.

是紅色的那部分。

And you can also see other features like oxygen and nitrogen

你也可以看到其他部分：

in that white glimmer

氧氣和氮氣是白色的、

in the circle showing you the auroras

發亮的圓圈的那部分。

and also some wisps around the tropics.

還有回歸線附近的小束光線。

So these are pictures that conclusively show us

所以總結來看，

that our atmosphere isn't just tightly bound to us here on Earth

我們的大氣層並不是只與 地球緊密相連的，

but it's actually reaching out far into space,

它們用，我會說是驚人的速度，

and at an alarming rate, I might add.

逃離到宇宙深處。

But the Earth is not alone in undergoing atmospheric escape.

但地球並非唯一 存在大氣逃離現象的行星，

Mars, our nearest neighbor, is much smaller than Earth,

我們最近的鄰居火星比地球小得多，

so it has much less gravity with which to hold on to its atmosphere.

所以它對於它的大氣層 具有較小的引力。

And so even though Mars has an atmosphere,

所以盡管火星有大氣層，

we can see it's much thinner than the Earth's.

火星大氣層比地球大氣層薄得多。

Just look at the surface.

火星表面的大坑就可以告訴我們：

You see craters indicating that it didn't have an atmosphere

火星並沒有一層

that could stop those impacts.

足以阻止大氣逃離的大氣。

Also, we see that it's the "red planet,"

火星被稱為「紅色行星」，

and atmospheric escape plays a role

而大氣逃離

in Mars being red.

是火星呈紅色的原因之一。

That's because we think Mars used to have a wetter past,

火星原本是濕潤的，

and when water had enough energy, it broke up into hydrogen and oxygen,

當水擁有了足夠能量 就會分解成氫氣和氧氣，

and hydrogen being so light, it escaped into space,

質量很輕的氫氣逃離了火星，

and the oxygen that was left

剩下來的氧氣

oxidized or rusted the ground,

氧化了土地，

making that familiar rusty red color that we see.

使土地變成我們 所看到的生銹的紅色。

So it's fine to look at pictures of Mars

看到火星的照片，

and say that atmospheric escape probably happened,

我們就可以推斷 大氣逃離現象的存在。

but NASA has a probe that's currently at Mars called the MAVEN satellite,

美國太空總署在火星上 放置的 MAVEN 衛星，

and its actual job is to study atmospheric escape.

它的實際任務 就是研究大氣逃離現象。

It's the Mars Atmosphere and Volatile Evolution spacecraft.

MAVEN 是「火星大氣 與揮發物演化任務」的縮寫。

And results from it have already shown pictures very similar

它發現火星的大氣逃離現象

to what you've seen here on Earth.

與地球上發生的很相似。

We've long known that Mars was losing its atmosphere,

我們一直知道火星正在 失去它的大氣，

but we have some stunning pictures.

但讓我們來看看令人震驚的照片。

Here, for example, you can see in the red circle

比方說這裏，你可以看到紅色的圓圈

is the size of Mars,

代表火星的大小，

and in blue you can see the hydrogen escaping away from the planet.

藍色的部分代表逃離火星的氫氣，

So it's reaching out more than 10 times the size of the planet,

而這塊藍色的區域是 火星大小的十倍，

far enough away that it's no longer bound to that planet.

足以證明它們已經 不再受到火星引力的吸引，

It's escaping off into space.

而是逃離進入了宇宙。

And this helps us confirm ideas,

這幫助我們證實了

like why Mars is red, from that lost hydrogen.

火星呈紅色的原因是消失的氫氣。

But hydrogen isn't the only gas that's lost.

但氫氣並非唯一逃離的氣體，

I mentioned helium on Earth and some oxygen and nitrogen,

還有我之前提到地球上的 氦氣、氧氣、和氮氣。

and from MAVEN we can also look at the oxygen being lost from Mars.

我們還可以通過 MAVEN 了解火星上消失的氧氣。

And you can see that because oxygen is heavier,

你可以看到，因為氧氣質量重，

it can't get as far as the hydrogen,

它們不能像氫氣一樣逃離得那麽遠，

but it's still escaping away from the planet.

但它們仍在逃離火星。

You don't see it all confined into that red circle.

圖中綠色的部分 不局限在紅色的圓圈以內。

So the fact that we not only see atmospheric escape on our own planet

因此，我們不僅研究 地球上的大氣逃離現象，

but we can study it elsewhere and send spacecraft

還研究其他行星，並且通過發射飛船

allows us to learn about the past of planets

我們可以得知行星的歷史、

but also about planets in general

行星的一般特徵、

and Earth's future.

以及地球的未來。

So one way we actually can learn about the future

我們能預測未來的一個途徑，

is by planets so far away that we can't see.

是通過我們看不見的遙遠行星。

And I should just note though, before I go on to that,

在我繼續講下去之前， 我要告訴你們的是，

I'm not going to show you photos like this of Pluto,

我沒有冥王星的類似圖片，

which might be disappointing,

這可能會令你們失望，

but that's because we don't have them yet.

這是因為我們還沒能 拍攝到這樣的照片。

But the New Horizons mission is currently studying atmospheric escape

但是新視野號目前正在研究

being lost from the planet.

大氣逃離，

So stay tuned and look out for that.

所以你們不久之後就能看到。

But the planets that I did want to talk about

我想介紹的行星

are known as transiting exoplanets.

叫作「淩日系外行星」。

So any planet orbiting a star that's not our Sun

圍繞除太陽外的 其他恒星公轉的行星，

is called an exoplanet, or extrasolar planet.

都被稱為「外行星」 或者「太陽系外行星」。

And these planets that we call transiting

我們之所以稱它們為 「淩」日系外行星，

have the special feature

是因為它們的特殊特徵：

that if you look at that star in the middle,

你看圖中間的那顆恒星，

you'll see that actually it's blinking.

它實際上正在閃爍，

And the reason that it's blinking

而它閃爍的原因是

is because there are planets that are going past it all the time,

一直有行星在圍繞它公轉，

and it's that special orientation

而且眾行星公轉的特殊方向

where the planets are blocking the light from the star

導致它們擋住了恒星的光芒，

that allows us to see that light blinking.

所以我們才能看到閃爍的光芒。

And by surveying the stars in the night sky

通過研究夜空中的星星

for this blinking motion,

閃爍的現象，

we are able to find planets.

我們就能找到行星。

This is how we've now been able to detect over 5,000 planets

這是我們能夠在銀河系中

in our own Milky Way,

找到超過五千顆行星的方法，

and we know there are many more out there, like I mentioned.

我們也知道還有更多 等待被發現的行星存在。

So when we look at the light from these stars,

當我們看到恒星的光芒時，

what we see, like I said, is not the planet itself,

我們看到的，如我所言， 並不是恒星本身，

but you actually see a dimming of the light

而是瞬間減弱的光，

that we can record in time.

我們可以記錄這段時間。

So the light drops as the planet decreases in front of the star,

當行星公轉到恒星前方時，

and that's that blinking that you saw before.

光線變暗，

So not only do we detect the planets

這就是我們看到的閃爍的光芒。

but we can look at this light in different wavelengths.

我們不僅能夠發現行星，

So I mentioned looking at the Earth and Mars in ultraviolet light.

還可以看到光的不同波長。

If we look at transiting exoplanets with the Hubble Space Telescope,

我曾經提到通過紫外線 觀測地球和火星，

we find that in the ultraviolet,

如果我們用哈伯望遠鏡 觀測淩日系外行星，

you see much bigger blinking, much less light from the star,

我們能發現：在紫外線下

when the planet is passing in front.

可以看到更強的閃爍， 以及行星經過時

And we think this is because you have an extended atmosphere of hydrogen

恒星散發出更少的光芒。

all around the planet

這個現象的原因是：在行星周圍

that's making it look puffier

有一層被延伸了的氫氣層，

and thus blocking more of the light that you see.

這使得它看上去更厚，

So using this technique, we've actually been able to discover

所以擋住了更多的光。

a few transiting exoplanets that are undergoing atmospheric escape.

利用這個方法，我們已經成功發現了

And these planets can be called hot Jupiters,

一些有著大氣逃離現象的 淩日系外行星，

for some of the ones we've found.

其中有一些

And that's because they're gas planets like Jupiter,

被稱為「熱木星」，

but they're so close to their star,

這是因為它們是 類似木星的氣體行星，

about a hundred times closer than Jupiter.

但它們離恒星更近，

And because there's all this lightweight gas that's ready to escape,

大約比木星離太陽的距離短一百倍。

and all this heating from the star,

由於這些輕的氣體已經 準備好要逃離，

you have completely catastrophic rates of atmospheric escape.

加上來自恒星的熱量，

So unlike our 400 pounds per minute of hydrogen being lost on Earth,

大氣逃離的速度之快是災難性的。

for these planets,

不同於地球上每分鐘 四百磅氫氣的逃離速度，

you're losing 1.3 billion pounds of hydrogen every minute.

在這些行星上，

So you might think, well, does this make the planet cease to exist?

每分鐘有十三億磅氫氣消失。

And this is a question that people wondered

你可能會問： 大氣逃離會讓行星滅亡嗎？

when they looked at our solar system,

這是很多人想問的問題。

because planets closer to the Sun are rocky,

因為在我們的太陽系中，

and planets further away are bigger and more gaseous.

離太陽近的行星都是佈滿岩石的，

Could you have started with something like Jupiter

而離太陽遠的行星 更大而且有更多氣體。

that was actually close to the Sun,

毀滅不會先從離太陽近的木星

and get rid of all the gas in it?

先開始嗎？

We now think that if you start with something like a hot Jupiter,

因為會把木星上的所有氣體都排盡。

you actually can't end up with Mercury or the Earth.

我們認為如果 從熱木星這類行星開始，

But if you started with something smaller,

最後並不能以水星或者地球結束。

it's possible that enough gas would have gotten away

但如果從更小的行星開始，

that it would have significantly impacted it

有可能消失掉足夠的氣體，

and left you with something very different than what you started with.

對這顆行星造成巨大的影響，

So all of this sounds sort of general,

而且最後的結果 可能跟開始時完全不同。

and we might think about the solar system,

這些聽起來都很籠統，

but what does this have to do with us here on Earth?

我們可能會聯想到太陽系，

Well, in the far future,

但這和我們的地球有什麽關係？

the Sun is going to get brighter.

在很遠的未來，

And as that happens,

太陽將會變得更亮，

the heating that we find from the Sun is going to become very intense.

當這樣的情況發生時，