A phenomenon like this happens once in a blue moon...
Istarius is special, very special... It is the only gaseous moon discovered so far. This second natural satellite of Santri is the third heaviest, but also the third smallest moon in the system. How a celestial that small can be that heavy? Opinions vary but one is sure - Istarius must have a supermassive core - likely made of iron, or something even heavier. Potentially, that is why the moon accreted that much of helium and hydrogen to become what it is today.