Yeah, that totally blew the plausibility for me too.
The opening scene spaceship's control console clearly indicated that the gas giant was well within the frost line DUH.
JAMES CAMERON DOES NOT FUCK AROUND!!!!!!!
DON'T YOU UNDERSTAND GAS GIANTS DON'T ORBIT MAIN SEQUENCE STARS INSIDE THE FROST LINE THEIR HYDROGEN COMPOUNDS ARE BLOWN OUT INTO SPACE JAMES CAMERON IS THE SAME MUTHERFUCKER WHO FORCED ME TO SIT THROUGH 3 HOURS OF KATE WINSLETT AND LEONARDS DECAPRICO!!! FUCK AVATAR AND LIKE THOSE ISLANDS COULD REALLY DEFY GRAVITY I MEAN WHAT WAS UP WITH THAT WAS IT EM, STRONG FORCE, WEAK FORCE OR WHAT HE NEVER FUCKING EXPLAINS IT EXCEPT IT IS SOME MYTHICAL VORTEX GIMME A FUCKING BREAK AND THIS IS THE 23RD CENTURY AREN'T THEY SUPPOSED TO BE USING LAZER BEAMS AND SHIT THEY'RE STILL USING BULLETS FOR GOD'S SAKE!!!