In the full final version of Spore, it has now been shown that while comets are observable in the "solar system" zoom in the Space Stage, they cannot be interacted with. It was theorized during game development that players would be able to colonize comets.
You can, however, "summon" a comet down onto a planet with one of the game's many terraforming tools. For example, "summoning" an Ice Comet onto a planet will increase atmosphere levels and also decrease the temperature, and a Meteor Shower will increase the temperature. Each use will cost you Sporebucks, but they won't use energy.