I would greatly appreciate some help in understanding how the likelihood of invasion success is calculated.
I invade a planet with a force of 4 and soldiering bonus of +30%. I see that gives me an invader power of 5.2. That's easy to understand (4*1.3 = 5.2).
The defending planet has population 16.2, resistance 51%, planetary defense 0%. That gives defender power of 8.3 (16.2 * .51). So I'm outnumbered, but the "invade planet" screen tells me my likelihood of success is 100% 
So I go for it and win with minimal casualties. Why are my odds so lopsided? Is it the 0% defense? Doesn't the resistance mean anything here?
Many thanks for some instruction here.