Cat hearder
This is like herding cats. At first its manageable, but as the levels progress you REALLY need to have some thing that separates out the planets. Even a larger planet that you could place would have helped out. As it was, every time I wend to circle a group of planets "4 or more" as you say, I'd invariably "clip" one that happened to lazily drift into my line.
as I see it there are several solutions to this problem:
1) Make it so the line REPELS planets as it is being drawn. This could help with herding as it would make singling out groups a bit more manageable.
2) as I said above, include one or more objects in the star field that can be placed by the player that the planets bounce off of.
3) Make the cursor either repel and/or attract the planets. It doesn't have to be a whole lot either, just enough to alter their trajectory slightly.
4) Purchasable power ups would be a good idea as well. Perhaps the ability to make temporary negative gravity wells where ever the player double clicks, or some kind of planet "type magnet" would be good examples. You could even make the above ideas purchasable.