During the early efforts of Human Colonization of space, there existed those that wished to see their colonization of Neptune succeed within their lifetimes.

To do this, they turned to Machine-governance, Genetic Augmentation, and Cryosleep.

Their bodies were altered to better survive long term stasis, only to be awoken by the machine when they were needed. When their work was done they would return to the long sleep. Hundreds of years would pass for them in the blink of an eye, allowing them to see the results of their work. These colonists and the decedents now make up the Neptunian Colonial Authority, and their near-monopoly on Fusion Fuels within the Sol System has made them very wealthy.

Their augmentations have managed to start spreading out into the genepool as traffic to the orbital colonies of Neptune has increased.

Neptunian has a dual-meaning.

Anyone who lives in a Neptunian colony, or has the Neptunian genetic modifications (by birth or by upgrade) is a Neptunian in the most literal sense. The genetic modifications themselves provide a much greater resistance to cold and better acclimates an individual to long and repeated stays in long term stasis

Neptunian more commonly is used to refer to any Human that is unmoored from conventional time. While a day in the life of most humans takes 24 hours, there are many humans who's 24 hours can last months or even years in normal time which can lead to a different and more distant way of thinking about the world. The colonists of Neptune who use Hypersleep for extended periods of time are included in this group, as are those who travel at high speeds for extended periods of time to experience time dilation.