Gatecrashing is the fantastic Eclipse Phase supplement for extrasolar adventures. In this particular setting, transhumanity gained access to the stars via five mysterious gates scattered around the solar system, mechanisms that somehow--the best guesses involve stable wormholes--create instantaneous connections to other gates scattered across the Milky Way Galaxy. It's widely believed that these gates were manufactured by the TITANs, the superintelligent AIs that wrecked the Earth and fled for points unknown, but even the suspicion that the TITANs might be monitoring the network doesn't prevent their use. The obvious survival benefits of rapidly spreading transhumanity beyond a single planetary system into the wider galaxy aside, it's only through the study of other worlds and other civilizations that transhumanity can learn the truth about what's really going on. Where is everybody, for starters?

This is an excellent hard SF book. The science is real-world and crunchy, the worldbuilding (of destinations and transhuman societies/economies both) interesting, and the hooks many. Here is an ancient Dyson sphere, a vast artifact with a surface area of millions of Earths; there, a promising colony world with an extinct native sophont species set to be revived; through this gate, a world with a mysteriously integrated ecology centered around a species of tree; through that gate, a planet of hallucinogenic plant life settled by hedonists and run by, among others, Lady Gaga. (Trust me, this works in its context.)