'The Jennerit Art of Sustainment' uses the their sun harvesting planet, Tempest to transfer immortalising energy into the Sustained few. A character only has to go through the act of sustainment once to become immortal, once they have they are eternally sustained, so long as they stay close to home. Sustainment may be the reason many of the Jennerit are so pale, it may also come at somewhat of a 'cost' of some kind.
'The Act of Sustainment' is a privileged activity; one that only the upper classes got to partake in. This probably means the lower classes live shorter lives. The only way currently revealed allow the underclasses to rise to the sustained caste is through becoming a champion of the gladiator pits. There might also be a case for assuming they had slaves (or at least forcibly 'indentured' servants), since the cultures the Jennerit seem to be based on had them as integral parts of their societies.
The Act
To become Sustained, to live forever, is an alluring prospect. Tens of thousands of years ago, Empress Lenore discovered “Sustainment,” a process by which the Jennerit can expend enormous amounts of power to make a single being all-but-immortal. The Sustained do not age and do not grow ill from natural illnesses. There are Sustained Jennerit who are nearly 20,000 years old in the universe. This process is extremely energy-expensive, consuming stored energy on a planetary scale just to alter one being. Therefore, Sustainment isn’t done often, and candidates for sustainment are vetted through a rigorous process, finally approved by the Empress herself. What’s even rarer, and quite unusual, is when a member of a non-Jennerit race is allowed to become Sustained.