Addresses can be committed to the contract in exchange for sparks. The contract then holds those addresses, until someone burns a spark in exchange for them.
We're not sure exactly when or precisely how that market will get initialized — but you get the basic idea. Your star ownership will get mirrored in the contracts.
It seems that if you bought a star earlier then Tlon will "[...] write [Ethereum smart-] contracts that help galaxies and stars distribute their stars and planets" such that the "[...] star owner configures the planet sale contract with two parameters: the price in ether of a planet, and the number of planets available."
Basically, star owners will be handed a mechanism to run their own mini-ICO, where planets represent ICO tokens.
Why would you want to buy a planet from star owner X? I presume the answer lies somewhere in the statement: "Galaxies and stars are network infrastructure. Regular users have planets."
We're not sure exactly when or precisely how that market will get initialized — but you get the basic idea. Your star ownership will get mirrored in the contracts.