How do you create a planet-specific cryptocurrency that will ignore longer block chains from other planets? I'm sure that cryptocurrencies will play a huge part of the economy across the solar system, when we have colonized many different planets, moons, and asteroids. But there are problems with using a single coin for the entire solar system. Let's just start with the example of a Mars colony. Long distances mean long delays in communication so it would take a very long time to get confirmations of Bitcoin transactions that originate on Mars. That could be problematic. Additionally, people on Mars could never hope to mine bitcoins because the average communications delay between Earth and Mars is 20 minutes one way, which is twice as long as the ten-minute blocks of Bitcoin. So I believe that a different coin would be best for each planet. But here's the crazy part. Even if Mars had its own coin (let's call it Marscoin), Earth would most likely have more computer processing power, so miners on Earth could download Marscoin's block chain and extend it faster than the miners on Mars. Then the miners on Mars would no longer be able to mine Marscoin. Then we would be back to the same problem of using Bitcoin on Mars. So, I think someone would have to create a coin that can somehow be planet-specific, so that longer block chains from other planets will always be rejected.
Simply limit the block target (i.e. 10 minutes) to the speed of light (300,000 km/s).

Unless there is some wormhole or other unknown to reduce this limit, Bitcoin is safe from honest mining from any planet outside of 180,000,000 kilometers away (300000km/s * 60 sec * 10 minutes) since any miner can't keep up with the blockchain and would get any block at about the time the next block comes out. 

Someone with more advanced technology / more hashing power could create a 51% attack and submit it to disrupt the Bitcoin economy even outside of this radius.