[ELI5] How can scientists know about the makeup of distant celestial bodies?

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I can understand the mass stuff since you can see how they attract/orbit etc, but how can they say stuff like “This planet has a thin atmosphere of 80% sulphur monoxide and 18% helium and it rains nano-diamonds periodically”.

How can you get all this stuff from looking through a telescope?

In: Physics

Like different colored filters for a camera, different materials will reflect/pass different bands of electromagnetic radiation.

By looking at these different reflected (or filtered if they’re passing through the body) waves and comparing them to the source star we can make an educated guess as to what it *might* be made of.

They find planets by observing distant stars. When a planet passes between its star and the telescope the light which we see from that star changes. First of all, it becomes dimmer, and that’s how we determine that there’s a planet there. But also other properties of the light (like spectrum) change after it passes through the planets atmosphere, so we can figure out what that atmosphere is made of, and then infer the conditions on the planets surface. The light still looks like a single point no matter how big the telescope, there is no way to actually “see” an extrasolar planet.

So we have a fairly good understanding of how stars work; we can discern quite a bit about them from their size and color and luminosity.

So now pass a planet in front of that light, and the light will change. Not only will it dim, because it’s partially obscured, but the spectral lines will change. From how they change, we can deduce the chemistry of the object that passed in front of it. We can, for example, tell that the object is gaseous, or has an atmosphere, or is rocky, and some of its basic chemical makeup. We can deduce more from other clues, like the period of the orbit, so it’s orbital distance, its gravity, other things.

A lot of the conclusions, like it rains diamonds, come from deduction. We understand pretty well how chemistry and physics work, so on some given planet, we know IT MUST rain nano diamonds, our understanding of physics tells us so, it’s predictive, and it tends to be pretty accurate.

The scientific name for it is [Astronomical Spectroscopy](https://en.wikipedia.org/wiki/Astronomical_spectroscopy). As the others have explained, atoms can absorb or emit light, and depending on their electron configuration, the frequencies of light (colors) that are absorbed or emitted will have certain bands that a sensitive spectrometer instrument can detect. It’s possible to identify each element, hydrogen, helium, oxygen, etc., because they each have different electron configurations, and thus different [spectra](https://en.wikipedia.org/wiki/Emission_spectrum).

Each element (hydrogen, nitrogen, helium, etc) have their own “fingerprint” of specific colours, specifically their frequency. By splitting the light that passes through a planet’s atmosphere from the host star, you can identify what elements are present.