Castaways of Tanagar — Brian Stableford

A few starships had set out from Earth before civilization ended. One of them found a new world, created a new home for mankind, an ideal intellectual society on Tanagar. Humane, its deviants and criminals were sent into space in deep freeze forever...

But then came the rediscovery of Earth, a world now far different from that of the star-voyaging days. And only the explorers with guts enough to face that terrifying old world were the very criminals who could not abide Tanagar.

This is the powerful novel of those castaways, outcast from Utopia, who were sent to work out Utopia's dark plots against the struggling descendants of those who remained behind.