I think its not hard to implement, just use simple rule and don't add complication with spells or etc.
* Stars are still visible in map, so everyone can recognize there is a start or not in that coord (x,y)
* if a star/system "in range 'm' from home" or "in range 'n' from nearest planet" so each planets info (owner or unexplored) visible, if not then only showing "unknown".
* Spell p-list only give planet list of targetted player, but visibility is not changed.
* Ops (planet infra) to a planet will give result planet info too (owner or unowned), so anyone can try to blindly check to distant to make sure owner of the planet or even only to explore.
* sending explorer to unknown planet is possible, but at risk eship returned/destroyed if already owned.