How do planets form?
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Out in space, it’s mostly empty, but there exists “stuff.” Gasses, solids, rocks. Stuff that was spewed out from exploding stars. All of the stuff eventually gets attracted to each other by gravity (anything with mass has gravity).
If it’s mostly hydrogen, you get something like Jupiter. If Jupiter were to attract enough mass to crush all the hydrogen to the point it achieves hydrogen fusion, you get a star.
If you have heavier elements that don’t fuse (iron, for example), and you get enough mass, the gravity of that mass will turn the object into a (rough) sphere. That’s the difference between a giant asteroid and a planet/dwarf planet. Having enough mass to form a sphere due to gravity. We also have some other criteria for what makes a planet, like being able to clear other stuff out of your orbit. When that criterion was added to the definition of a planet, that’s what demoted Pluto to a dwarf planet. We now know there are thousands of objects out there in its area – some about the same size as Pluto.
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