attacking, its the best and cheapest way to get more planets choose targets with similar planet count and lower or same pr that will mostly be those with little or no ships.
Otherwise just make a bunch of minerals while researching for a while, then sell as much of them as you can, and be willing to sit a day at least for 16 hours, preferably over two days simply exploring with no more than 3600 scouts, will use more turns, but cost less and be virtually indistinctable at that planet count to if you had more scouts.