Aegir, known scientifically as Epsilon Eridani b, is the first planet in the Epsilon Eridani system and among the first worlds colonized by humanity in the 22nd century. It is roughly ten and a half light years away from the Earth. Aegir is a gas giant a little bit larger than Jupiter, and possesses eleven natural satellites, two of which are considered to be of Garden-level habitability. Aegir was discovered and named in the early 21st century, and was settled starting in the 2260s. After orbital habitats were assembled and jump-gates were completed, waves of colonists arrived and established the first settlement in 2266, and incorporated the next year as the Trust Territory of Eridanus. Settlement expanded across the other larger moons. Unrest began to grow, however, as a result of heavy domestic taxation to subsidize the provision of basic resources and services while the colonies grew. A booming population, lack of complex industrial economy, and resulting high unemployment and poverty sparked the Eridanus Uprising in 2303. This would be the first conflict in the Colonial Wars, and Aegir would be the flashpoint of many further conflicts during the Eridanus Free Forces terrorist campaign of the middle 24th century and the later Orionese War of Independence. Aegir remained a part of the Terran Federation, however, and was rewarded with full membership in the federation as the Commonwealth of Aegir in 2381. It would be designated a province in the Terran Empire and reincorporated as a Crown County in 2900. In 3275, it was admitted to the new Republic as a member world, the Aegir Free State.

Aegir's primary center of habitation is the moon Eldir, a pluvial Type IVa Garden world which boasts a population of 400 million people and contains an indigenous biosphere of sprawling rivers, marshland, and vast rainforest. Its second largest moon, Kolga, is an tundrapelagic Type VIa Garden world, with much of its 88% ocean cover locked up in glaciers and sea ice, though supporting a rich native oceanic ecology. As such, Kolga supports a smaller population of 202 million. Its other moons have harsher native conditions, ranging from proximal pre-Garden to icy Europan moons, but all have settler populations ranging from 30 million to 2 million people. A network of orbital habitats house a large population of half a billion as well. The planet's lunar settlements and orbital habitats, taken together, have a population of 2.35 billion people.

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