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  • We know that Jupiter has an atmosphere made up mainly of hydrogen and helium. Europa,

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

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

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

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

  • air... but we don’t need to.

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

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

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

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

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

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

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

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

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

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

  • their relative abundances!

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

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

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

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

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

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

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

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

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

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

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

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

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

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

  • we can’t even see.

We know that Jupiter has an atmosphere made up mainly of hydrogen and helium. Europa,

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