128 bits is a big number/keyspace but the solar system is no lightweight, either.

    2^128 = 3.4028E38 
    solar mass = 1.9884E30 kg or about 9E56 atoms. 

You don't need to get down to atoms, you could be crunching 128 bit keys on a single AWS dyson1-medium instance.

256 bits is the one where the hosting costs start getting not merely planetary but intergalactic.

    2^256 = 1.1579E77
    atoms in Milky Way = 2.9E76 atoms