The database can be browsed at
http://steamdb.info/, where they compile a mass amount of information on games that are listed internally to appear on Steam. If we're lucky, occasionally we get confirmation on games that haven't even been announced yet.
In this case, Defiance has been announced but not confirmed for Steam. At this time, the only way we know that players will be able to play is through the independent Trion Worlds launcher. However, it looks like that is about to change, according to Steamdb.info, the game may be coming to Steam after all.