Very good, could've been much better
This game was very easy. Once you know how to play, you can beat the last level in a minute or so without much effort. You've got a good thing going here, but there's a lot missing.
I noticed the enemies never expanded to new planets. This is quite a problem. It means you need to take out a single building, and the enemies can no longer build anything. Additionally, you're at your leasure to expand outwards. The game would be several times more enjoyable and challenging if the enemy left the confines of their base.
The lower quality ships become effectively useless as soon as you can buy better ones. This is a problem because you never use them again after the first level. The ship cap is a large part of this. It would be fixed by adjusting the ship cap to include ship power, so you could for instance, have 20 regular fighters, or 2 of the orbital destroyers.
It's cool that your build speed advances as you get more power, however, the rate is a bit too extreme, especially when the AI does not take advantage of it. Getting +20 power lets you build planet destroyers in about 2 seconds. You should either tone that down a bit, or adjust the AI accordingly.
Speaking of which, the AI is in need of a lot of work. You don't need to make it terribly complicated, just adjust its behavior based on the game lay out. It should attempt to expand fairly early, build lots of power plants, and rush for resources, while attempting to distrupt yours. I generally found they would focus on a single attack structure, when attacking other things would've been very harmful for me. A better AI, with no other changes, would make this an awesome game in my opinion.
If you have trouble making the AI harder, make it cheat. Give it a higher ship cap, make it mine faster (though that was never an issue for me. The cap of ships left me with a whole lot of extra cash)
I'm alright with the bad Interplanetary missile tracking. It makes you want to control better situated planets. You should, however, make it more clear that they can't hit the far side of planets. A sentence in their description would save you a lot of complaints.
You should also allow, if enemies don't expand, for there to be multiple planets with enemies on them. It's very easy to focus your attack on a single target. Prioritizing between multiple bases would add a bit of S to the RTS.
Overall, I think this game is alright, but it has awesome potential. I can't wait to see a sequel.