Outposts are buildings that Polaris has in place of Ports. Outposts are unlocked with the Frostwork technology. Outposts cost 5 stars, provide one population, and can only be built on Ice tiles. Additionally, Outposts can form city connections over Ice (similarly to how Ports form city connections over water).
Connecting a city to the capital gives one population to the connected city and the capital. Outposts must be adjacent to a city or connected to a city by Roads to be used to form a city connection. Outposts can form city connections with other Outposts that have three or fewer Ice tiles between them.
When Polaris was initially released, Outposts provided 2 population (but could only be built on Ice tiles adjacent to land). Since Polaris can build Outposts on Turn 0 and Outposts are very cheap (compared to Ports), this lead to Polaris being very overpowered. Therefore, Outposts were nerfed on July 5, 2019, in update 1.14.1. Outposts now provide only 1 population (but can be built on any Ice tile).