How do the soldier outposts and invasion defense work exactly? i just built 11 of them on a planet.
Also, i have never gotten galaxy points fro mdaily reward but my friends have, why is that? Is it just badl luck? How long has galaxy points been in daily reward?

As for the GP, it's not your bad luck it's their good luck, I've been playing since April or so and I've not gotten any from Daily Reward.

Also, as far as I know it's just +90 defense, but it only applies when they're actually invading instead of when they're attacking.

Ya GP's in the daily reward has been in since the start as far as I know. I've been playing about 8 months now and just got my first one this last month here. Its only 5 so theres not much you can do with them.

True, it's mostly good if you can't buy GP and have a planet you'd like to abandon, though that can be a very good thing if you took some bad planets at the start.

True I didn't even think about using it for that. Good thought.

oh, i see now. how is invasion defense scaled compared to a player attack defense. say the invaders attack/defense are equal to the invasion defense. what is the invasion chance?

If they're equal, probably about 50/50, with diminishing returns the more defense you have vs their attack. Basically your defense on a planet reduces they're chance to succeed on the invasion roll.

Then what is the point of even having invasion defense? why not just build up defense and then prevent them lowering defense and protect form invasion?

I'm not entirely sure, though a lot of the Invasion Defense artifacts provide more defense for each space, so if you didn't want to put much population on you'd be better off with really good invasion defense than some regular defense.

"Invasion defense" is exactly that. It's defense that only counts when they hit the invasion button. If you look at the colony tab, you'll notice that invasion defense isn't counted under the defense column in that tab. The reson for the invasion defense, such as those soldier outposts, is that they give more defense aginst the invasion role per structure point used (30) than even the highest tech levels of defense structures do. The only difference is that normal defense also reduces their damage when they attack the planet if you have enough of it. However, with the 20 damage cap on planet pop and and sheer amount of attack high end ships have, the only real good defense is good for is for reducing their chances of passing an invasion roll.

What about galaxy point? how often can i expect them? I am yet to get any and yet my rank 100 friend (i'm rank 170) has gotten enough to abandon several planets, get a bunch of energy refills AND purchase 3 planet fluxes.

Either he buys points or does the offers to get them, there's very little chance he's gotten that many from daily reward. And I've been playing since April and not gotten any, so don't expect any at all.