Control a majority of flats on the planet. Thought it's also good to own the PGs and Cities to do so, but even if you do control it, if others have cities they too can build on it and overtake your own flats and take control of a planet.
it's not easy to control a planet on ones own, without a faction or at the very least w/o a shield grid to prevent building. I know most people simply give control of the planet to their faction to prevent and control the planet with easier means.
I'd add if you want to own a planet on your own, you can't control it as an individual, only factions can, so its helpful to have it fully slabbed and own all cities. That way nobody beside you and people you give permission to can build on it.
Well credit wise im no where near doing any of that. So owning slabs or shielding planets not in my plans but thank you. I ask only cause i seen a system unowned by a goverment or faction. I thought i could take it or own it for myself or faction. Of course me being 4 months old still have alot to learn. :)