Pluto

A formerly dwarf planet that after the Blackout was converted into a full-blown planet by Sidhe redirecting several large asteroids and other pieces of space debris (plus some of its moons) and using their wondrous technology to reforge the resulting mass into Solar System's one and only Cyber Planet.
Today it is one of the most alien places in the Known Space and remains a center of Sidhe's civilization while also acting as Mankind's gateway into the greater Cosmos.

Geography

Geography of Pluto is dependent almost entirely on the conscious decision of its inhabitants. Sidhe, when they were allowed to settle Pluto (at their own insistence, since they saw the dwarf planet as offering nothing of value to humans) they infuse the entire planet with their most potent reality warping technology, which changed its nature completely.
Today, Pluto is less a planet in a conventional sense and more a planet-sized mass of programmable matter that Sidhe can change with much as effort as human programmers alter a world in a videogame they are making.
They want a mountain range? Sure, no problem. They want a lake? A matter of writing a program and/or executing one already existing in the place of their chosing. It's natural, inert form resembles a completely smooth marble ball the size of a planet and a silvery hue.
The main reason why Sidhe limit themselves to only most basic modifications of their habitat is that excess manipulation of a world using a technology that even they no longer fully understand threatens to cause a cascade error in its basic code, which according to Sidhe can easily turn vast swathes of Pluto into oceans of scrapcode that can no longer be controlled and that changes itself randomly while being inimical to every form of life (even cybernetic one).

History

Sidhe didn't arrive in the Solar System - they simply found themselves in it once the Blackout came to pass, having absolutely no clue as to what happened. In fact, they too underwent Blackout, missing several minutes of their life.
As one can expect, the small fleet of alien spaceships that happened to randomly materialize itself close to Neptune Subsystem just as everyone in the Solar System lost consciousness was immediately suspected to be the culprits behind the Blackout. In fact, some still suspect them to this day.
In the end, the completely erratic movement of the fleet in direct aftermath of the Blackout and the eventual First Contact (which involved Sidhe translating human language and sending an open message to everyone in the Solar System that could as well be summarized with words 'ayo, what the fuck?!') managed to convince most of humans that Sidhe were just as confused about their presence in the Solar System as they were.
The discovery of other forms of alien life that was just as confused about the place they found themselves in following Blackout certainly helped as well.
Sidhe - who by their very nature are xeno-equivalent of Virtual transhumans, traveling from species to species and altering themselves to fit the ones they encountered before settling among them to experience their culture and way of being (and get some of them to join their ranks) before leaving - ended up seeing the situation as an opportunity. And quickly reached an agreement with Mankind.
Trading their stock of terraforming nanomachine packets (that Sidhe themselves found before finding themsleves in the Solar System) for the right to settle on Pluto and become part of the Commonwealth. While Pluto does remain their 'homeworld' of sorts, a lot of them are doing their best to spread themselves throughout Mankind as a whole, all in order to experience their way of life.

Pluto
Summary
Distance from Sun:
Aphelion: 7376 million km
Semi-major axis: 5906 million km
Perihelion: 4437 million km
Diameter: 4460 km
Solar Constant: 3.779
Aphelion: 0.000813
Semi-major axis: 0.00127
Perihelion: 0.00225
Mass: Variable
Mean Density: Variable
Orbital Period: 248 Earth years
Day Length: 6.387 Earth days
Rotation axial tilt: 57 degrees
Avg. Temperature per Latitude
Variable

Astrography

Object Type: Exotic
Object Subtype: Cyber Planet
Primary Location: Core Sector
Secondary Location: Solar System
Tertiary Location: Pluto Subsystem
Satellites: Styx
Transshipment Station: Styx

Habitation Status

Status: Colonized
Classification: Sidhe World
Population: ~26 mln.
Capital: First Pentagon
Government: Seventeenth Hexagon
Solar Commonwealth Member: Yes.