[Da Bilge Troll]

I'd always thought that, since binary star systems are so common, Jovian planets were simply stars that never achieved enough mass to light off.

[SunkenCiv]

:')

[anymouse]

Or giant blobs of the host star that got ripped out by the dynamic stellar formation process. Kind of like a volcanic eruption spewing a mini-sun out far enough to not be drawn back into the star by gravity. The mini-sun/proto-gas giant reaches "orbital velocity" and eventually settles into a stable orbit around the star.

As it does it eventually cools down, but due to it's stellar heritage it has a relatively hot core, which causes significant radiation. The large mass and associated gravitation force captures roving asteroids and comets to form its moons.