The main reason caps get a free pass is because they are also VITAL combat ships, and don't really have time to stick around in a dedicated siege role. That's why the egg is SO good, because drain planet can facerape a planet before a battle, and then finish the job afterwords, without seriously impacting its combat power.
the first is that planets, unlike ships, actually do lose usefulness as they get attacked in the form of population loss leading to revenue loss. a ship, on the other hand, fights just as well at 1% health as it does at 100% health. this doesn't necessarily mean a damage mitigation system for planets couldn't work, but it would have to be built up to account for the population as well as the planet health.
Actually, the more valuable part of a planet is NOT the population, but the planet upgrades. A planet is a BIG investment, with players putting in thousands of resources to increase income, orbital slots, planet health, or durability. Check it out sometime; a planet costs a ton of resources to build up. That planet keeps the upgrades until it dies, so a planet with 1% health holds the same value of investments as a planet with 100% health.
One interesting thing about planets is they are almost always the target of focus fire. The whole point of mitigation is to penalize players for focusing directly on one unit. This makes skirmishes faster while large scale battles bump the mitigation to max, and keeps initial casualties from steamrolling against a player.
makes it more complicated so i'd like to see some more details on how the system would work.
There's really not much to it. It's just a mitigation system. As the incoming damage increases, more damage gets reduced. Population gets the same treatment, you can't kill people that are already dead. Percentage based attacks will be affected by mitigation, with resource drain also getting hurt. If you want numbers, then I could make some up.
At zero damage, mitigation is at a minimum, and after say 1000 damage it goes to max. This means a planet without health upgrades is still an easy kill. At max mitigation, incoming damage is cut by at least half. This means that siege ships can do a nasty initial punch, but need to stick around twice as long to make the kill. This makes it more difficult for players to siege rush a world, since the primary goal of a quick kill is directly countered by mitigation.
Mitigation will drop relatively quickly, so that a planet under a long term siege doesn't simply max out. The siege ships are just sitting back and crushing every new attempt at government until they give up. This rewards players who have "beaten all resistance", and are just keeping a few siege frigates out back to finish the job.
One possibility is to have the maximum mitigation increase with infrastructure levels. This makes upgraded planets even more reslilient to a hard hitting siege attack, the most critical of course being the homeworld. More upgrades means increased protection against percentage based attacks like hysteria and drain planet, and increased resistance to attacks like resource drain. Planets without infrastructure upgrades (usually most of them) will still be highly vulnerable.
Advent planets will get the mitigation bonus from culture. TEC have the planetary shield generator (which may need tweaking). Vasari have phase gates to respond to ANY distress call. Separate abilities, but all of them protect planets in their own way.
The main idea is to dramatically slow down a siege raid designed to steamroll across your territory. In 1.00 this was a serious issue, and players had little recourse as world after world gets crushed in seconds. But with mitigation, even a buffed siege frigate will find it twice as difficult to pull off.
the second reason i don't really like it is less relevant to gameplay and more based on fluff and feel. its simply that mitigation in Sins is supposed to be the result of Shields and Armor. planets have neither of those things. they can gain some of them from research and tactical structures (hardened cities, disaster recovery, nano trauma medicine, planetary shield generator, etc.) so that provides a good fluff justification.
Justifying fluff is pretty easy. You can say that the initial government structures are easy targets, but that it becomes very difficult to target the emergency facilities. With the planet lit up like a roman candle, sensors won't be able to find vital targets, so most shots will be firing blind, with reduced effectiveness. Smaller facilities like asteroids could have internal force fields which act the same way.