Ignoring the HOLY PHYSICS BATMAN problems with the concept, which could be a doctoral thesis in and of itself...
The gameplay is fun! The timing and aiming mechanic rewards skill while necessitating economy, the stats are clearly and quantifiably explained (which seems like a no-brainer, but soooo many games just don't bother).
The selection of weapons and their different functions was cool. I would would have liked perhaps more player involvement in the recovery and implementation of weapons rather than a "it just happens!" kind of deal. Artifacts were a cool perk system, I'd have liked maybe a more visual component to go with it, at least an item name or what have you.
I found the military/scientist dynamic interesting, but it wasn't explored very much and they essentially provided the same gameplay function, i.e. side quests. The planet as a medium and your position as the commander seems to beg for a more political aspect of the game; an example might be the planet's response to devastation from an attack, or response to a discovery that increases their contentment. Perhaps things like a population mechanic, just spitballing here, but repeated devastating planetary attacks (as the opening indicated they were) would wreak havoc on infrastructure and population, and this might require some hard planetary survival choices, like forced repopulation programmes, totalitarianism vs. civil unrest, propaganda, etc.. Something with continuity. Maybe that's not this game, I dunno, but I think it could be an interesting facet of a sequel.
I think that some of the weapon sounds could be better. The machine gun, for instance, sounds pretty scrawny for a cannon that shoots shells the size of the Chicxulub asteroid. The music is nice and atmospheric, though I think the music could be more climactic for different encounters. It might also be interesting to add moons and planetoids as your smaller encounters, and have more salient artificial aspects of higher-level planets. Perhaps some story-based recognition of the fact that you've annihilated a whole planet might be good too, to remind us that we're not just playing "big gun balls gone rogue!".
Anyway, good stuff, look forward to seeing more.