Sol

Most everyone knows Sol is the birthplace of humanity. The Contractor Wars timeline diverged rather dramatically from the real one when the Greys revealed themselves to humanity in 2020 when their numerous city/ships ripped free of the Earth's crust, where they'd been buried, steadily converting minerals and metals into useful material until they were ready to terraform all of Earth into something the Greys could live in.

Despite suffering nearly 80% casualties across the globe, Humanity remained resilient. Major population centers such as China, India, and South America suffered catastrophic losses as scientists and engineers attempted to salvage what they could from the few city-ships humans had managed to destroy. The more wreckage they recovered, the more they learned, until they'd destroyed all but one of the city-ships. Soldiers in bodged-together mecha assaulted the ship, delivered by stealth bombers to the upper surface of the giant, floating cities. They fought their way to the center of the ship, battling automated turrets, cyborg'd wolves, jungle cats, and silverback gorillas, and discovered that the city-ship was home to just a single Grey alien. When they Grey was finally killed, the assaulting forces took the ship intact and scientists began reverse-engineering the tech on board. This gave Earth the bootstrap it needed to head out and colonize space.

In current era, Sol is mostly of interest as a place of historical significance, with tourists from the stars coming to visit what surviving landmarks still exist from before the War for Terra, like going to visit the Parthenon, or the Coliseum, or the Great Pyramids.