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.
Gravity pulls objects together. The greater the mass the stronger the pull. In the beginning there was only Hydrogen gas, and gravity gradually pulled it together to form stars.
At the centre of stars, atomic fusion takes place and elements like Helium (basically two hydrogen ions smushed together) all the way through iron can form. For certain reasons, iron is as heavy as it gets during a star’s normal life. However, when a star of sufficient size dies and goes supernova, the energy is enough to fuse heavier elements (cobalt right the way on up to uranium and such) right as the star explodes and scatters all its material in a big cloud.
After a few generations of stars doing this, there’s a lot of these heavier gases and metals floating around and gravity pulls that stuff together as well, and when the clumps of matter are too small to be stars (not enough pressure for nuclear fusion to take place), they form planets.
Stars form from big clouds of dust and gas. Most of the material ends up in the center as part of the star, as gravity pulls it in. There’s some left over that doesn’t make it into the center, and it forms a disk around the new star. Stuff collides in the disk, and some of it sticks together. As the clump of stuff gets bigger, its gravity attracts more stuff to collide with it. Eventually it gets big enough that gravity pulls it into a roughly spherical shape. If it gets big enough, all the stuff near its orbit either becomes part of it or gets thrown out of the disk.
Follow up question: why are planets usually round? I know Earth is a little squashed at the poles.
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