Wow, this is impressive!
The stellar nebulae you've put in the background look stunning, the planet looks great (I especially like the fact that you've got the night side pointing away from the sun!) and the planetary descent was fantastic! (I came in once on the night side, just rolling round into daylight, and I got the sun gleaming over the horzion like in "2001". It's a pity I didn't hit print screen.)
I also
loved the Saturn style rings, and the soft blend from low-res to high-res ground textures was superb - how on earth (no pun intended

) did you manage it?.
The frame rate varies a great deal for me, I'm averaging 59 frames a second when in space, but as the planet comes more and more into view it falls to around 30 - possibly something to do with how you generated the planet shadow?
On a couple of
really pedantic notes, though, (and I'm not sure if anyone but physicists would even care) the spaceship is ignoring Newton's Laws and the physics of space. Once you're moving at a set speed in space, you will keep moving at that speed forever because there's no friction to slow you down. So when you engage the afterburner you'll just keep speeding up (at least until you get above I think about 0.1c when relativity comes into play). Also, given that the engines seem to be permanently on (lovely thruster glow, by the way) you should be experiencing a constant acceleration - which means you'd wind up moving towards the planet without having to press any buttons at all.
I'm not saying you ought to change the spaceship physics, but I thought I'd mention it in case you were interested (I know I'm being incredibly picky, and it doesn't really matter anyway).
Apart from that I think this is absolutely fantastic, and I can't wait to play the next demo!
"I wish I was a spaceman, the fastest guy alive. I'd fly you round the universe, in Fireball XL5..."