The Great Planet Robbery by Craig DiLouie
Lawrence Dobbs and Timothy Muldoon, Colonial Marines and thorough rascals, are the last of a dying breed of adventurers in a Federation that has tamed dozens of wild planets and is increasingly becoming civilized. When an old astronaut offers to sell them a map that will take them to a legendary planet promising rivers of gold, Dobbs and Muldoon recruit a crew of misfits for one last great adventure. They soon realize they haven't just discovered gold, but the very secret of alchemy. To keep it, they just have to fight millions of hostile natives, a team of elite bounty hunters, a combat-assassin android named Bova and the emperor of a dead civilization.