Its an aspect of the game that is still WIP.
In relation to the other planets, the game is layered, so yes, they are technically a ceiling way up in the sky. That's why portals are required for players to initially get there.
Moon and Mars are pretty stable planets. I'm working on either adding more, some asteroids, or adding my own planets. Like a water planet or something.
Moons is at +5300 or so, Mars is at +22000