If you havent researched Colony Cluster 1 or Colony Cluster 2, or level 3
Do that first
If you have, you will cluster you lowest 5 that are not a U. Type planet, you go to projects, colony upgrade then do the cluster you want
5 planets - 1 c1
5 c1s - 1 c2 =25 planets
5 c2s - 1 c3 = 125 planets
Thats it :D