I seem to recall theories that Jupiter used to be far more massive than it is even now, but shed a great deal of it somehow, creating the 'ice skater' effect? That's sort of how Pulsars get started, so I wouldn't be surprised if it was something along those lines...
In fact, it does make one stop to wonder what a Pulsar would look like after several Billion years, as it loses momentum and heat, it would become a large, slightly warm body which rotates rapidly and is a heavy source of X-Rays, it would have thrown heavier elements out early in its life, and therefore consist mostly of light elements such as Hydrogen, Helium etc...
