The islands' and continents' outlines look good to me, but the surface itself could do some more colour tones I think. Particularly the green looks like TMNT mutagen (or equivalent biohazard waste) spreading over the surface; forests generally reflect less light than plains so they would be darker. Water looks fine, but lighting could improve somewhat. If the star is behind the planet, there would be only a shallow crescent of the planet that well visible, unless there's another star in the system (which is of course possible).
As for what comes to background and the star... I'll keep it diplomatic and say that I wouldn't make them like that.
I prefer my space dark and nebulas subtle... And I think the lens flare effect looks kinda cheesy, but that's just me...
What did you use to make the planet?
->Mad Cat: Thanks for the link, I'll check it out as soon as I'm back home to my own beloved PC. I'll have to convert the instructions to GIMP-jargon though...curse the way they use different terminology in different image editing programs.
I won't even try any of this stuff with my parents' PC with only 512 MB RAM (my own now has 3072 MB RAM... Which reduces the page file HD crunching significantly with high-res layers.