So I'm a bit new, I'm rank 19 at the moment, and I'm preparing to rank up, which will allow me to attack other player's planets. I have a few questions:

Do all the structures on the planet stay there when i take it over? Meaning, even if I can't get a theory engine yet, if there are theory engines on the planet, will they stay there?

Will i have to pay to colonize the planet (meaning energy and credits) or when it hits 0 population does it just become mine?

Any other tips for me?

Yes all the structures will remain, however, you won't be able to use any structures that you have yet to research.

Also any structures that are on the planet that are obtained through mission blueprints will also be non-functional. So you will need to demolish those structures if you want to utilize the planet fully.

Yes. You will pay 15 Energy, and invasion costs are double that of a regular colonization. So if it will cost you 1 million to colonize it will cost you 2 million to invade.

yes, existing structures remain on the planet (unless the person is online and paranoid and starts using Mylarai build extractors on his gemstone buildings)

yes, you will have to pay to invade, which costs double your current colonization price plus 15 energy instead of 10..... you can only attempt an invade once per day (unless you use a paradox to reset the timer) and only pay the credits if successful, i think

before clicking invade, equip 8 of your highest attack weapon currently researched... and use any attack boosting artifacts you have (ancient crystal foci, crimson obelisk, crimson amplifier, etc).... higher attack increases chance to invade....

when you attack a planet you need at least double the attack of its combined attack and defences to have close to a 90% chance. check out the planetary invasion formula thread.

when you capture a planet all the structures will remain on it, but if you havent researched them yet they will provide no bonuses for you.

when you drop the population down to zero, it will cost you double the price in credits than it does to colonize a planet. the highest chance you can get is 90%, and even then you have a chance to fail and have a crew member die (permanently) and yes, you can go negative on crew.

and also, when you use artifacts on a planet or capture it you are alerted to the owners log, where they will see you for 48 hours. if you flux it, then you will be disabled constantly for the next 48 hours. if the player is higher than you, then do not expect to hold onto it. i can easily take any planet that has 3000 combined defense on it. at your rank its actually better to maximize production and put no defense on it whatsoever, because if anyone over 100 wants the planet theyll take it.

my advice would be if its truely a good planet, wait till you have singularity minefields or planetary shields to take it. if its an ok planet, then go for it. the owner will probably be happy you stole it. and if the owners inactive, then it dosnt matter whether its good, bad, or just ok. you can steal it without any reprocussions. but just remember, you need at least double the attack of the planets combined defense to get over a 90% chance, otherwise youll most likely fail, have to wait 24 hours, and may have a crew member die. ive lost maybe 5 crew to failed invasions, and another 2 more to failed raids. so be wary of these problems.

The planet has absolutely no defenses on it, the player isn't in a legion and doesn't appear to have changed within the last week, and it has some structures on it that i can use like the spy up link. I'm relatively sure i won't have a problem taking it, i just wanted to know what would happen, but thanks for the extra information. And it'd decent, not amazing or anything, but it's good enough for me right now.

I have a bit of advice for after you take it.

Defend it just enough. At your level, you cannot possibly defend a planet from anyone even a few dozen levels higher than you, which is almost everyone in the game. Even if you put nothing but attack and defense structures on it at level 19, most of the people in the game could sneeze and knock those defenses down.

Instead, put a minimal defense on so it's not FREE for someone to take (as it seems to be for you), and then put production into place. I also wouldn't bother with artifacts for production right now unless you get it from an artifact pull... any limited mission artifacts are just candy topping on a planet sundae right now.

As you get more attack and defense, upgrade your planets.