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Comment by Loughla | Hacker News Reader

It's absolutely amazing that we can even figure out molecules at that distance. I'm sure it's accurate but it's so sciency to me that it might as well be made up entirely. I can't begin to fathom that process.

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Spectroscopy is cool! Basically whenever light shines through a thing, the thing can absorb or emit light, and that happens at specific wavelengths depending on what the thing is made of. You can get small spectroscope pretty cheap and play around with a bunch of hone experiments - a common lab project in class is putting salts in a flame and based on the spectra figuring out what atoms are in the salt. It's how we discovered helium must exist in the sun before actually finding any on earth. So basically we just do that, except on a planet that's far away. When it's between it's star and us, we look at the spectra of the light that goes through the planets atmosphere

> I can't begin to fathom that process.

It's basically "look at far away object with a fancy pair of sunglasses."

Because of quantum mechanical reasons, molecules can only absorb and emit light with very particular energy levels (which correspond to frequency/wavelength). So point your camera at an exoplanet and carefully record the amount of the light you see at different wavelengths. My guess is that it's based on IR spectroscopy, since there's a nice region of that spectra where you can "fingerprint" a molecule based on the peaks in absorption in that region (it's literally called the "fingerprint region").

It isn't that simple. It took a lot of careful calibration to be able to determine the amount of ratio of materials from spectroscopy. This is what led to determining the Sun is mostly hydrogen.

If you haven't already, I recommend learning about Astronomical spectroscopy, which helped me get a toehold on how this stuff works: https://en.wikipedia.org/wiki/Astronomical_spectroscopy

It's only possible if the planet passes directly between its star and Earth, so we can see sunlight passing through its atmosphere, and detect which specific frequencies of light got absorbed.

I haven't read the paper but it's likely just some fancy spectroscopy: looking at the wavelengths that are absorbed. Each molecule has a characteristic pattern.

> I can't begin to fathom that process.

I write about it in on page 5 of my coffee table photobook:

https://impacts.to/downloads/lowres/impacts.pdf#page=5

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