Treason by Orson Scott Card

Treason - Three thousand years ago, the Republic exiled several families to the planet of Treason.  The families could only return to the Republic once they build their own space ships.  However, there is no metal on Treason, and the only way to obtain iron is to trade with the Republic.  The Mueller family trades human body parts.  When their children go through puberty,  they grow extra arms, legs, etc which are then removed.  When they obtain maturity, their bodies cease to grow extra parts.  However, some people, the radical regenerates (rads), never stop.  The rads are penned for the rest of their lives and their extra body parts are harvested to trade for iron.

When Lanik Mueller, heir to the throne, turns 15, he is discovered to be a rad.  Therefore, he is illegible to ascend to the throne.  His father, instead of condemning him to the pens, sends him on an emissary mission.  Throughout his mission, Lanik learns about the history of Treason as well as the special abilities of the other exiled families.