Earth
Earth is the home planet of the Terrans. It is a planet in the Sol System (our solar system), and is believed to be the only one that had a station orbiting it, unlike the other eight planets, and was the only planet that could support life.
Once a Homeworld
Earth was once the homeworld of Terran race. It is the planet where the Terrans lived in and thrived, before migrating out to the rest of the galaxy. It is believed that a Grey had given them the technology to travel among the stars. The planet, along with the system, has been lost.
Alice mentioned Earth in GoF2 (”… We’d all still be on Earth, picking nits from each other’s hair.”), so it's clear that Earth is still remembered by the Terran population, but it may have been destroyed during the Vossk Wars or kept a secret to protect it from invasion, to the point that everyone forgot where it was.