How do scientists know so much about planets so far away?
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Because of light. It sounds oversimplified, but really it’s incredibly fascinating to me.
Light is generated by energy output by stars. Different stars output different kinds and amounts of light, including light we can’t see with our eyes. This is the electromagnetic spectrum and includes radio waves, gamma rays, and all the fancy rays. We can call all of that just **light**, and the lucky thing is that we know *exactly* how fast any kind of light should travel. Because we know this speed very precisely and under pretty much any condition, it is actually quite easy to know how far away a star is.*
But if you stare at a star long enough, you’ll notice that every so often the star gets slightly dimmer in certain parts. This means something is partially blocking its light. The thing blocking its light periodically must be *orbitting* the star, so it must probably be a planet. We can tell how big it is by how much light it blocks, and how fast it moves by how quickly it moves across the star, and many other fascinating things can be deduced just by looking at how a star’s light is blocked by objects.
Then there’s the fact that planets reflect light coming from other stars around them. At the right angle, we can see light from a star in one direction bounce off the planet and shine towards us. Again, if we estimate the time that takes, we can know how far away the planet is. Furthermore, because different elements reflect different *parts* of light, which we interpret as colours, we can compare the parts we see to our own catalogue of chemical elements and compounds and deduce what the planets are made of.
For eg, we have iron here on earth, and we know that iron *absorbs* a lot of colours but mostly *reflects* red light, which is why iron is red. If we see a planet out there, and it absorbs and reflects the same parts of the light spectrum as iron does on earth, we can be pretty confident that there’s iron on the planet’s surface. Then we ask people who study rocks to tell us more details about *how* iron could have formed on the planet, because there’s only a few ways we know of that explain it.
The last really cool thing, and this really blew my mind, is that we learned recently that light interacts with gravity, meaning objects that are really big and heavy that have a lot of gravity, actually *bend* light around them. So even if light isn’t reflecting off of a planet, and the planet isn’t obscuring a star’s light, It could still be possible to “see” things out there because they are *affecting* the light around them. If we see a lot of heavy things bending light, this just gives us more interesting ways to deduce what’s happening out in the cosmos. All because of light.
*Note, astrophysics is not “easy”, but once we figured out the principle we’re trying to identify, a lot of stuff becomes easy to learn. That’s why you hear about scientists all of a sudden “discovering” a dozen interesting things a week
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