I know how they stay in orbit but how do random planets and stuff have the exact speed needed to get to orbit? Is it pure chance or some phenomenon that speds them up or slows them down?
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The “exact speed” in a circular orbit assuming the orbiter is significantly lighter than the object it is orbiting around, is simply solved by a trivial equation:
a = v^2/r = GM/r^2
=> v = sqrt(GM/r)
Where v is orbital velocity, G the gravitational constant, r the orbit radius and M the mass of the object being orbited. So for every orbital speed, there is an orbit radius r where it is stable. Essentially what happens when an object is set into orbit is that as it passed near the other object, if it’s going slow enough, the other object’s gravity will “capture” it and set it into a proper orbit fitting its speed. If it’s too fast, it will escape instead and not be set in an orbit.
In most cases of course, the orbits are not neatly circular, but elliptical, because not every object conveniently comes in at such an angle and speed that they fit into the neat circular orbit. However, there is also a set of elliptic orbits which any captured object can get caught in. Unlike the circular orbit, an elliptical orbit does not have a constant speed, but the speed of the orbiter will vary depending on where on the path it is. But I’ll leave the math for that as an exercise for the reader, as it doesn’t fit into ELI5 and they essentially work the same as circular orbits, since circular is just an edge case of elliptic.
Also, many of the classic nicely orbiting objects (planets around the star, moons around the planet), the object does not “enter” the orbit, but it is formed in orbit from dust that is already orbiting at on average the right speed for that orbit. That’s why these orbits are much cleaner than, say, comets’ orbits.
TL;DR: The objects come in at whatever speed they like, and end up in an orbit that matches their speed, not the other way around.
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