1. Planet Defense Rating
You know how on some missions they tell you that you need x number of marines to be successful in the mission? well that number is modified by the defense rating of the planet which you can see on their colony page when you are on the planet. Said defense rating also modifies how powerful the defense of the planet is without player interference.
2. Fleet Size
This one is also on the colony page, Fleet size modifies the size of your AI defense fleets as well as their ship and weapon load outs. the bigger this number the better your defender ships are and the better they are at space battle against raiders/invaders before they actually reach your planets. Don't worry tho, if they die they'll just cost you some ships&weapons commodity and credits but not affect defense rating, they respawn every 14 days unless you modify the numbers in the settings.
3. Ship Quality
Now this here is a bit funky, the higher the quality bonus the more s-mod your defense fleet will have, the lower they are the more d-mod they'll have. You cannot control what s/d-mod they'll have, which in some cases can be fatal or very powerful combinations.
4. Planetary Invasion
Now to the main topic! The invasion itself has two party, the attacker and the defender. The attacker units are the ones shown above the defender units which is on the bottom, next to the units on the map is their combat strength. Each buildings you have on your planet became what I call control points(CP) and your objective as the attacker is to clear every CP of any defender while as a defender you want to clear every CP of every attacker.
5. Combat
I don't remember if it's called heavy armament or heavy weaponry but i'm sticking to armament. each armament require 1 marine to be used. Imagine 1 marine = 1 combat strength while 1 heavy = 10 combat strength, you want to have as many heavy armament as you can get but remember that these bad boys also eats up cargo space that might or might not be essential for your supplies and fuel. To bomb a planet be it just tactical bombardment or using bombing abilities you need fuel while to deploy units you need supplies. Note that the more supplies you have the more units you can deploy per turn. your deployment point are 10% (I think?) of your supplies, if you have 10000 then your deployment point will be 1000 (at least in my experience). Note that every turn your dp is recalculated so the more you use your supplies to maintain your ship and deploy units, the less DP you'll have next turn.
You will want a mix between marines and heavy to maximize your combat strength but remember, you can recall units and cut your losses anytime if you think you cannot win the battle!
Now how do you do that you ask?
****ATTACKER****
Spoiler
Assuming you have units such as the marines and heavy armaments, you sent out your unit onto the planet by deploying them through the action button or if you're lazy just use q.move and choose which site you'll deploy them on. Now you have a simple yet complicated objective which is to overpower your enemy combat strength through overwhelming numbers, superior firepower (heavy armaments), and fleet support and abilities.
Your fleet gives additional combat strength based on the amount of units are still on your ships and some bonus from ships with the build in mod "atmospheric capable". the more units you have on the ground the more spread out the bonus will be and the less effective they are so be tactical in your deployment!
Abilities are a major thing in invasion use it to your advantage, it can make or break your invasion attempt.
I advise bombing a planet before you invade them as this reduce not only their defense rating which means less defender but it also disable their defensive bonus from buildings and stop those pesky turrets from shooting your ships and unit as they drop onto the planet.
There are actions such as merging units but unless you have manpower to spare or want to be tactical it is not really something you need to worry about. Oh and final note, watch your unit morales because the lower they are the lower their combat strength will be. It is essentially your ground unit combat readiness, just like your ships they perform better when the CR is high.
****DEFENDER****
Spoiler
Assuming you are in the area and can help the invaded planet, you can first go to the planet and same as attacking you sent out units but this time they increase the defender numbers and combat strength. In my opinion this mode is harder because unlike attacking where you can go wild you need to be careful of enemies bombing your planet and units while also being around to support your people.
It is in your best interest to take down enemy invasion fleets orbiting the planet as quickly as you can while maintaining the stalemate on the ground as killing the fleet would lower the attacker combat strength by a lot and stop their reinforcement from pouring down. heck you might even get heavy armament to further help your defense.
As the defender, you want to avoid using tactical and saturation bombardments as they can and will also harm your own units, using the bombardment ability is fine tho but be ready to disable your own targeted CP building. In a sense you will want to play cat and mouse, fighting like a guerilla fighter by moving out lower morale unit and sending in higher morale unit and replenish their numbers when they are resting. you also want to keep your turrets (ground defense or heavy batteries) manned as they don't fire unless a friendly unit control it.