The Shadow of Yesterday is the roleplaying game where Solar System was first used, although the name of the rules engine is not mentioned in the game itself.
To quote Clinton R. Nixon's homepage for the game, “The Shadow of Yesterday is pulpy romantic sword-and-sorcery at the end of one world and the beginning of the next. This game melds the best of standard fantasy role-playing and a hard-charging narrative engine. The revised version of this game, out now, focuses even more on mechanics that make your story zing off the table and into your imagination. Set in a post-apocalyptic fantasy world where the creation of a moon has ravaged cultures and thrown the fate of the earth into question, TSOY is like a Weird Tales contribution written by Emily Bronte.”
You can buy the game in book form here or read all of it for free here