How do scientists know so much about planets so far away?
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We actually don’t know very much at all about exoplanets. A huge amount of what you read in articles about them is exaggerated or sometimes outright made up. At uni I did a project on exoplanets and my supervisor was keen to point out how unreliable science journalism is when it comes to exoplanets.
What we can work out about them is roughly how big they are and what their mass is, and maybe how far away from the star it is.
You can work out its size by observing the star over long periods and looking for a drop in the amount of light from the star. The light is dimmed by a fraction of a percent, so you need a good telescope to observe this, and you need repeated observations to make sure it’s not caused by some other factor.
You can work out its mass using the ‘Doppler wobble’ method. If a large enough planet is orbiting a star, it causes it to wobble back and forth in its orbit slight, and you can observe this as a doppler shift in the light from the star. Again, the change is a fraction of a percentage point, so you need a good telescope and careful observation.
This gives you its mass and size, from which you can work out its density, and with all of that information you have a pretty good idea of what sort of planet it is (and by that I mean ‘is it a terrestrial planet or a gas giant’) and how far from the host star it is, and that’s about it.
If you want to learn anything about its atmosphere, you need spectroscopy. This means you look at the light that has passed through the planet’s atmosphere, and you compare it to white light to see which part of the spectrum have been absorbed. From this, you can tell roughly what elements make up the atmosphere. This is a fairly recent method so we haven’t done it for a ton of exoplanets yet.
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