I feel her head is too small...

First up I'd recommend you at scale this down to at most half it's current size (preferably smaller). Pixelling at such sizes isn't really practical even for a highly skilled pusher, let alone someone who is still early in their learning. It only encourages you to skimp on the things that make pixel art what it is.

The main problem with the mountains is they are drawn as though they are being viewed from near to and perpendicular to the surface.
They are also very large compared to curvature of the planet (the reason one doesn't see mountains like that from Earth orbit is a matter of scale (not atmospheric obstruction as some may tell you). A mountain is just insignificant in size compared to the Earth.

The ring around the secondary planetoid being so squarely horizontal and edge-on really breaks the dynamism of the image.

Lighting is sparse and inconsistent: planets and face show no sighs of lighting; ring of near planet and body are inconsistent.
If the lighting on the body is to be taken, then the side away from the primary light source should receive some light as reflected off the planet.
If she's holding a lantern, might it not cast some light?

I see what you mean, Metaru. As soon as I get some time I'll fix what you said and throw it into the WIP Forum.

oh my, i must encourage you to use references when working with anatomies, and specially when working with such dinamical poses like the one you've tried. her hips are WAY too big(compare it with her head). also, the lantern and background are totally odd(since when planets have such thin atmosphera that you can clearly see the landscape from space?), even the green lartern logo on the girl's breast isn't centered(and i believe it have to). the dithering on her hair is completely out of place... noburo, look, this really have potential to be great, but these and other details completely ruin it.

Looks pretty great. Hope one of these days you'll be able to put the finishing touches on this thing.