a moon of Jupiter, has a very thin oxygen atmosphere, and HD 209458 b, a Jupiter-sized

木星的衛星之一，有一層非常稀薄的氧氣大氣層， 而 HD 209458 b，一顆環繞154光年外 HD 209458

exoplanet orbiting the star HD 209458 which is 154 lightyears away, has an atmosphere

的木星大小的系外行星，擁有一層大氣

that contains hydrogen, carbon, oxygen, sodium, carbon dioxide, methane, and even water vapor.

包含氫、碳、氧、鈉、二氧化碳、甲烷，甚至水蒸氣。

All this even though we haven’t visited any of these places to directly sample the

雖然我們未曾到過任何一顆星直接取得空氣樣本，

air... but we don’t need to.

但我們也不用。

We can study the air on other planets, moons, and exoplanets just by looking at them. In

我們可以研究其他行星、衛星與系外行星的空氣， 只需觀看它們。

particular, by looking at light that bounces off or passes through their atmospheres, because

尤其，靠觀測光線反射或穿越大氣層，因為

when you shine light on a gas, the molecules absorb and scatter different frequencies of

當你朝氣體發射光，分子會吸收光線並散射不同頻率

that light in different amounts. If we then split the transmitted or scattered light apart

及強弱的光。如果我們使用三稜鏡或衍射光柵分離

into its constituent colors using a prism or diffraction grating, we can see a molecule’s

氣體吸收或放射的光，我們可以看到分子的

light-absorption fingerprint, or its light-emission fingerprint. This is hydrogen. This is nitrogen.

光線吸收指紋，或它的光線發射指紋。 這是氫。這是氮。

Oxygen. Methane. Carbon dioxide. Water. Sulfuric acid.

氧。甲烷。二氧化碳。水。

So when we look at the sunlight bouncing off of the atmospheres of planets and notice spikes

所以當我們看到陽光從行星的大氣反射，又注意到

of certain heights in certain frequencies, we can carefully match those to the known

某些光線波段有某些高峰，我們可以小心將它們比對

fingerprints of gas molecules, and learn not just what gases make up the air, but even

已知氣體分子的指紋，不只可以知道大氣組成，甚至是

their relative abundances!

氣體的相對豐度！

In fact, we don’t even need to be able to see a planet at all to learn about its atmosphere

事實上，我們根本不用看見行星本身就能研究大氣：

– many exoplanets have been discovered because they pass in front of their parent

很多系外行星因為經過母恆星前方才被發現，

star, which we see as a dip in the overall intensity of the star’s light. But if an

我們會看到恆星的光規律的下降。但若

exoplanet has an atmosphere, the gas molecules in its atmosphere will block some frequencies

系外行星有大氣，其中的氣體分子會擋下某些額外的

an extra amount, according to their molecular fingerprints, and we can do the same gas-matching

光譜頻率，根據其分子指紋，我們可以進行同樣的

process as before. And that’s how we know what’s in the atmosphere of HD 209458 b!.

氣體比對。 這就是我們知道HD 209458 b大氣成分的原因！

Of course, in practice it’s pretty darn challenging to use molecular fingerprints

當然，實際上要用分子指紋做系外行星的大氣層研究

to study exoplanet atmospheres, because air is thin so the fingerprints are super faint

是項艱鉅的挑戰，因為空氣稀薄，而指紋也極不明顯。

and we need big sensitive telescopes and spectrometers; and because atmospheres are complicated and

因此我們需要高解析望遠鏡和分析儀器， 而因為大氣成分複雜

their fingerprints can be ambiguous or hard to match; and because different parts of a

其指紋可能不明確或難以比對， 又因為一顆恆星的各部分

single star emit different amounts of different colors of light, so a planet’s effect on

都會發出不同強弱和顏色的光線，因此一顆行星影響

the star’s spectrum will depend on which part the planet passes in front of.

其恆星的光譜會取決於其掠過恆星的哪個部分。

But all of these difficulties can be dealt with by clever astronomers, and thus we have

然而聰明的天文學家早已看穿這一切，因此我們

been able to figure out what the air is like on planets hundreds of light years away that

能夠知道幾百光年外我們看都看不到的

we can’t even see.

行星大氣組成。