Persephone

Persephone is a new addition to the Solar System. It didn't exist before the Blackout - after it happened, it suddenly was there, orbiting Mercury as its new natural(?) satelite. With the world not resembling anything ever seen in the Solar System (and inhabited by aliens, very confused by their suddent appearane in the Solar System), its existence is one of the most major mysteries of the modern Solar System.
Its inhabitants (the Sidhe has become a de facto member-state of the Mercurial Confederation ever since, reinforcing the armies and fleets of Mercury with their own soldiers and ships.

Geography

Persephone is mostly composed of Sidhe' living metal. This is responsible for its relatively high gravity compared to size. Its inhabitants are capable of sculpting said planet to their will, resulting in majority of geographic features being temporary and manmade. The exception to that are the scrapyards - large parts of the moonthat slipped out of Sidhe's control, going berserk due to faulty programming codes (or something similar, the closest translation seems to be 'scrapcode', but it's hard to say what Sidhe mean by that).
The scrapseas are areas where the living metal is unstable. The results can be summed as alien geometries, perpetually shifting geographic features, occasional changes of the solid surface into literal seas, and occasionally creation of some hostile entities. It also has the influence on the climate of the moon (which is terraformed, but this mostly due to Sidhe being able to - to a degree - influence their world as easily as humans can do a computer program.

History

No one knows how did Persephone moved into the Solar System, not even its inhabitants. When the Blackout happened, Persephone existed thousands of light years away from the Solar System. About five years later, its inhabitants woke up to discover them to be orbiting a completely unfamiliar (and inhabited) world.
To this day, the Sidhe (and the humans) found no explanations for that). Following a rather turbulent first contact, the Sidhe has established close ties with the inhabitants of Mercury, taking full advantage of the Mercury's society being de facto a group of isolationist communities wearing a trenchcoat. Sidhe inserted themselves as another such community (albeit less isolationist), and has continued existing there until today.
Many wonder, however, if they aren't lying about their lack of understanding of what Blackout was and how did they move into the Solar System.

Persephone
Summary

Distance from Sun: 57 mil. km
Distance from Mercury: 0.678 mil. km
Diameter: 1312 km
Solar Constant: 12.179
Mass: 0,11 Earths
Mean density: ???
Mercury's period: 20 years
Day length: 17 Earth days
Rotation axial tilt: 18 degrees to the ecliptic

Avg. Temperature per Latitude
N/A

Astrography

Type: Moon [Sidheformed]
Location: Mercury Subsystem
Satelites: None
Status: Colonized
Population: ~2,5 mln.

Governments
Mercurial Confederation
Seventeenth Hexagon