Planet Venus is a world in the Sol system.
The atmosphere of Venus is home to numerous aero-cities, the first of which was established in 2065, a decade before the colonisation of Mars began.
Although the surface of Venus is extremely inhospitable, above the clouds can be found the most welcoming environment in the entire Sol system, outside of Earth itself. Gravity is roughly similar to that of Earth. The atmosphere is heavy in carbon dioxide, meaning there is both carbon and oxygen, and gravity stands comfortably at ninety-percent of Earth gravity. At that height, even the temperature is within the acceptable range.
Life support systems in the aero-cities are able to separate oxygen from atmospheric carbon dioxide. And incredibly, the breathable air of the aero-cities is also the very thing that keeps them afloat. Venus is an extremely rich planet. In the late twenty-first century, it was used by several early astro-mining companies as a base from which to explore and extract resources from Sol's asteroids. (Although Venus is further from the asteroid belt than the Earth, the planet's high orbital velocity and astrological position means that the journeys are actually much easier.)
By 2214, the aero-cities had really become a political force to be reckoned with. The following year, they hosted an important summit of the Galactic Union.