Where does the solar system end? It doesn’t end even where there seems to be not much out there. It ends when another solar system begins, where the stellar wind of other stars is active. Did you know the sun made wind?
There are many worlds in the solar system. The other worlds in our solar system are not just the other planets, but the moons as well. Some moons are bigger than some of the planets.
What makes a moon? A moon orbits a planet. The planets orbit the sun. Otherwise, they are all just rocks.
Moons are different, just as planets are different. Jupiter has the most unique moons. It has more than one. One of those has an active volcano. Another is bigger than Mercury. Some think that large moon may have an ocean on it under its surface ice.
The next largest moon orbits Saturn. We don’t know much about it because it has a thick atmosphere so our satellites can’t take pictures. The other moons of Saturn have unique surfaces that have been shaped by collisions with comets and asteroids.