From The Rio Grande web site:
This first expansion for Race for the Galaxy adds cards, a 5th player, and other ways to expand your universe. Each expansion adds one more player, more cards, plus some goodies besides cards including objectives for players to compete over. The first expansion includes solitaire rules. It also includes some blank cards and a contest card for players to submit a card idea to RGG. Two slots have been left open in the second expansion for the winners.
The information that is known is summarized by the designer. For a more detailed narrative from the designer, himself, please see the following article at Boardgamenews.com:
Game Preview: Race for the Galaxy: The Gathering Storm