This is a two part idea!
Currently planets have the following differences:
- Picture in the map
- Bonus (iron, end, oct, food and science)
- Size
- Max Population
I am recommending the addition of a new type. Defensive Boost.
As we all know, terrain can have a major impact on a battle in the real world. A smaller army can even fend off a larger army if the terrain is perfectly suited and the defender has proper control of it. So I am recommending that some planets get a defensive boost (lets say 1-20%) in that anyone attacking it now has to deal with the potential of terrain as well as fleet sizes.
This would push attackers especially to explore defensive planets much more so than others but also play key roles in share wars where maybe a player has a strong defensive planet in a major share but the attacker with a larger fleet can not afford to hit it due to similar fleet sizes.