How do you create a planet-specific cryptocurrency that will ignore longer block chains from other planets?
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.