Earth is no more. That's not quite accurate, but to the survivors of the Federation, it may as well be no more. What happened?
Wars happened. A nuclear winter brought Earth to ruin. No one is sure who fired the first shot. But it was suspected that it was the Chinese that fired the opening salvos. By 3907AD, hyper-globalization had taken effect. Little of nature remains. The air quality is much better than it was in its history.
Technology has taken over all aspects of life and what remains of nature are protected in large biodomes designed to export clean air. Around the world there're large filters that clean the air but even then, this wasn't helping.
So then there was the introduction of large flying airships built with gravity drives designed to work in-atmosphere that held large forests. This innovation was what saved the Earth from catastrophic famine and terrible air quality. There are 50 such vessels, evenly split up between continents.
But even then, that wasn't enough to keep Earth safe. No one is sure what exactly set the Chinese off but it is suspected that an assassination of the Premier's wife was the cause.
The project to save what was left of humanity and the animals was thus born, along with the gene sequencing of plants and animals stored in the Svalbard Global Seed Vault on Earth. After that, the initiative to clean up the Earth and return it to nature was born as well.
Five Worldships were built and launched. It was decided to keep the Worldships together rather than sending them out along randomised paths and creating a disparity between humans scattered amongst the stars.
Old Earth now has markers surrounding the planet warning of the consequences of attempting to return to the world that's being returned to nature. There is a Worldship-sized gate nearby that helps enforce this rule as well as automated security platforms designed to enforce the ruling.