Interesting idea, but think of the SIZE it would have to be? This is not like the planet creator/destroyer components, which can be thought of as triggering a chain reaction of some sort. This requires raw power to move a planet in one piece, which would require gigantic engine power from the ship as well as power to 'hold on' to the planet with the beam. How about a "Planetary Engine" Facility instead? A facility on a planet is much huger than any ship and if it's ON the planet it doesn't need 'double' power to both hang on to and move the planet - it's already ON the planet!
This would be a cool thing to have, though. Especially if a planet equipped with one could move one square per turn in combat.
Hmm, could you RAM one planet with another?