Uncapturable Planet

I've come across this twice.  Once was the Yor and I forget the other.  In both cases, the planet was immune to culture flip, so my only remedy was to encompass the planet in my influence and get the benevolent ideology trait that flips all planets and starbases within your influence.  Nowadays, I keep that one in reserve just for these instances.

I still find it odd the population of the planet I invaded stayed exactly at 15ish billion. Not even a dent after sending in 54 billion invaders.

I would like to see something a little more realistic or at least half a billion casualties per invasion. Weres the death star when you need it?

Were you not able to gas them?

Biological weaponry tends to be made for bugs like this, and is very OP when the AI hasn't had a chance to try and counter it via immunity research.

If you don't need the planet to immediately produce anything and aren't going to suffer from the maintenance expenses of all the buildings, gassing the population seems a LOT faster than farming ideology points.

This sounds very much like you've found a planet with 100+% planetary defense. If you haven't already done so, develop or otherwise acquire the other invasion tactics, and next time you invade, pick one that reduces planetary defense rather than going with Conventional Warfare.

The developers made the (in my opinion rather silly) decision that planetary defense is a guaranteed percentage of the invading force killed before the planetary population and invading force start beating each other up. Any planet with 100+% planetary defense is therefore immune to conventional invasion regardless of how small the planetary population is and regardless of how large the invading force is. You could have a full fleet of Huge-hulled transports containing nothing but fully-loaded troop modules going against a just-settled world with 0.5 population, and your fleet would fail a conventional invasion if the just-settled world somehow has 100+% planetary defense. Information Warfare provides a small reduction in planetary defense, while Biological Warfare, Tidal Disruption, and Core Detonation provide a large reduction in planetary defense. Opponents can become immune to Information and Biological Warfare but not to Tidal Disruption and Core Detonation.

Oddly enough I did try 4 invasion techs from my save with 9 loaded transports. Conventional,biological invasion, bombardment and Tidal disruption.

All with the same results of no planet casualties. The planet is a ghost world with no bonus to defense. I am going to load the save and try researching

core detonation. If that fails ill just start a new game.  

Core detonation had zero effect as well..... There is just no capturing that planet. 

I sent in 10 transports with 6 billion each with core detonation and not one dead drengan.

I think one died from laughing so hard.... Not a warriors death.

Here is the game save if anyone is interested....

https://drive.google.com/open?id=0B7_8qt3GZhp2MGpzQXZOOENPRVU

Thanks for reporting.

It means they have >150% planetary defense. Prolly an elerium shield plus planetary defense dome plus a few other bonuses. Not much you can do really no tech reduces pd by more than 50% so 150 is unbeatable.

Planetary defense should be multiplied by a normally distributed random variable of mean 0.5 in order to ensure that it is not unbeatable. Otherwise, it's simply impossible to invade a planet which is ridiculous. On-ground resistance should have some random factor but be less random.

Or simply always apply some minimum casualties to the defender no matter how high his defense. The super-defense world would still be a near-invulnerable fortress but it would be possible to slowly erode it by pouring in transport after transport after transport in a gigantic 100-to-1 casualty attrition campaign.

In my opinion, planetary defense needs a cap (e.g. 80-95% maximum), a change in stacking mechanics (e.g. 1 - product(1 - x_n, n=1:N), where each x_n is the planetary defense bonus from a specific source - and level bonuses should count as separate from the base improvement, at least if the base improvement bonus is high enough that that plus the level bonus is at risk of hitting 100% or more on its own), or a change in role (e.g. planetary defense now acts as a divisor on invader strength, so invader power is [basic invader power] / (1 + [planetary defense]/100%), reducing the effectiveness of the invader but not destroying invasions before they touch down). The tech tree itself grants you immunity to three of the six possible invasion types, and planetary defense is fairly trivially easy to stack to the point of immunity to a fourth possible invasion type and not terribly difficult to stack to the point of granting immunity to all invasion types for as many planets as you have elerium. If the level bonus were functioning, it would be total invasion immunity to every planet so long as you have the ability to get 30-40 levels on Elerium Defense Shields anywhere in your empire (as defined in ImprovementDefs, the 5% per level planetary defense bonus is empire-wide), but fortunately it seems as though the level bonus simply doesn't work (it doesn't appear to be applied to any planet in your empire, including the one upon which the shield was built).