Scilion

Scilion is a primordial world orbiting Alpha Centauri B.

Planetary habitability
Scilion has a type of bacteria in it's oceans that is slowly converting its thick CO2 and Argon atmosphere to a more breathable one of Oxygen and Argon. Before it was turned into a poisonous atmosphere full of noxious fumes after the Scilion War

Post Scilion War
After the war the atmosphere slowly started to regenerate to it's former glory, but it's several thousands of years from recovering.