Asteroids are large clumps of rocks organized in a belt, or can be found lone. In Spore, asteroids are found naturally.
In the cell stage, a large asteroid brings the player's cell to the planet (possibly a reference to the movie, Evolution). Its fragments can be found throught the stage containing cell parts. Some could argue that this asteroid was in fact a Comet.
Asteroids appear in two instances in the creature stage. Occasionally, a large meteor (about the size of the players creature) will crash into the planet, and is just eye-candy. The large asteroid will not affect the creature stage at all. It will be followed by loads of smaller meteors which can stun creatures. When exploring in the creature stage, the player may come across a crashed asteroid in the ground. This, too, is merely eye-candy.
As in the creature stage, in the tribal stage you may come across crashed asteroids in the ground. Again, they are merely eye-candy.
Like the previous two stages, the player can find crashed asteroids, which are merely eye-candy.
In the Space stage, asteroids are much more common. Occasionally, orbiting a star may be an asteroid belt. These cannot be interacted with. Asteroids also appear as a terraforming tool, which raises the temperature and lowers the atmosphere. On Rocky planets, meteors are common hazards.
In GA, asteroids appear in two instances. The first is as a prop that the player can use. The second instance is when the player turns the adventures atmosphere down, and has hazards switched on.