Re: Earth (spoilers through 4x17)
Yes the Earth they found was the one and only Earth they were looking for and was talked about, but I think there are several Earths. The series finale is called Daybreak so I think it's safe to saw they find a planet and fight John for it. That planet will become our Earth.
"Did you (republicans) fall down,hit your head and think you woke up in the 1950s? Or the 1890s? Should we call for a doctor?" - Senator Elizabeth Warren.