It is common today for which to be used with both non-restrictive and restrictive clauses, especially in informal contexts:
- Who ate the cake that I bought this morning?
- Who ate the cake which I bought this morning?
It is, however, incorrect even in informal contexts to use that for a non-restrictive or parenthetical clause. For example, these sentences would be considered incorrect:
- This computer, that I have never liked, is very slow.
- The blue desk, that my father gave me.
Even though the usage of which has been relaxed to some extent, it is still better to keep your writing as clear as possible by using which for only non-restrictive clauses, and that for restrictive ones.
So, to return to Justin’s example:
- “To our knowledge, it is the only body in the solar system which currently sustains life, although several other bodies are under investigation.”
- “To our knowledge, it is the only body in the solar system that currently sustains life, although several other bodies are under investigation.”
_from English Grammar 101_