For its first century of human habitation, it had been a prison planet.

He was released from a prison planet about ten years ago.

Who wants to come to a prison planet in a minor star system?

A prison planet to that one is worse than death.

The prisoners were being sent on a one-way trip to a prison planet.

I wouldn't be surprised if it were standard for prison planets.

"What has this to do with the Emperor's prison planet?"

Ever since the five of us found ourselves on the prison planet, I had been aware that we were far from home.

He's the one who was with us on the prison planet.

"Anyone who wants to go to the Emperor's prison planet obviously needs a little more happiness in his life."