Very nice concept, unfortunately I couldn't figure out how to switch planets so I wasn't able to progress very far, but what I was able to play was rather fun and more polished than I expected.
I kinda wish there was something to indicate how the astronaut is being affected by gravity, either through directly showing the gravitational pull of the planets, or adding some kind of path in front of the astronaut showing how the planets are affecting him.
Also, it would have been fun to see planets affecting each other through gravity, but I can imagine that 48 hours isn't nearly long enough to program something like that.
Overall, it's a great entry that I had a lot of fun with!