Humans left earth long ago to colonise other planets, but never quite let go of their homeworld. They created earth anew wherever they went, but gave each colony world a different name until they found this one.
The year was five billion and the original earth, humanity’s birthplace, was gone, burnt to a crisp as the sun expanded. But this planet was so like the one they’d just lost that they named it New Earth. Why not?
That was more or less how the Doctor explained it.
But to Rose, it was nothing like earth. It was much too clean.