This is one of the best time wasting games you can play, I clocked in at about an hour and a half on this game. Great game, great creator.
Earth is in the middle, harbouring life, and must be protected. Jupiter's massive gravity attracts asteroids that might otherwise strike Earth. Mercury is close to the Sun, constantly baked by its rays.
More about the game here: camrichards.wordpress.com/clockwork-cosmos