Zeronus is a presumably ISA controlled colony that is not directly mentioned in the Killzone games, but it appears on advertisements in the Vektan Slums. 

In the posters, Zeronus is portrayed as a tourist world like Gyre, but on the posters it says:

• 'Guaranteed Alien Free'
• 'The only planet with extra oxygen for a real relaxed vacation'
• 'Sun 24 hours a day'

On the poster is a background of the planet's surface which appears to be covered in large, beautiful beaches and jungles; separating the beaches and the jungles are cliffs, making Zeronus a large tourist destination. The quote of 24 hours of sunlight implies that Zeronus is either a gimmick or the colony is tidally locked to its star.