Pretty straightforward - just need to visit every planet in a cluster, ignore the "unexplored" part. I did this in Omega's cluster and picked up a ton of resources, but there's probably a smaller cluster where this would be easier. ME2 is nicer than ME1 because you can tell if you've visited a system before since it shows the % explored. If you've not visited a planet then its name isn't displayed, so it should be pretty easy to tell what you're missing.