And how do we know their sizes and distances?
In: Physics
Stars are literally defined as suns. A star is a stellar object, a ball of plasma that emits light as a result of nuclear fusion.
Not all stars have planets. The ones where we can determine that they do, we can by seeing that planet pass in front of them (i.e. between us and the star) we can see that by the tiny change in luminosity as the planet passes in front of the star. This is why we could only detect gas giants like Jupiter for the longest time, because these things are big and caused a relatively large change in luminosity. With better and better detectors, we’re now capable of seeing even the rather small change an earth-like planet passing in front of a star causes.
By the speed such planets pass in front of the star, we can determine the distance the planet has from its star. The closer a planet is to a star, the faster it orbits around the star and the faster it passes in front of it.
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