Brightness Reef — David Brin

The planet Jijo is forbidden to settlers, its ecology protected by guardians of the Five Galaxies. But over the centuries it has been resettled, populated by refugees of six intelligent races. Together they have woven a new society in the wilderness, drawn together by their fear of Judgment Day, when the Five Galaxies will discover their illegal colony. Then a strange starship arrives on Jijo. Does it bring the long-dreaded judgment, or worse--a band of criminals willing to destroy the six races of Jijo to cover their own crimes?