Neat game, and I could easily see this being the beginning of something much bigger. Especially if the planets have more unique terrain/features.
Speaking of which, if you do decide to turn this into something bigger, I would suggest deciding on either a mythological or scientific basis for the game. If you go mythological, the planets could have features based on the gods they're named after (you're already on the right track with Mercury giving a speed boost). If you go for a scientific theme, then the gas/ice giants might have to be replaced by their moons, since you won't be landing on the planets themselves any time soon.