Ayato: Tokyo Jupiter?
Haruka: Yup. The place that was your whole world until this morning. (Episode 3)
Tokyo Jupiter is technically Tokyo, Japan encased in a sphere. This sphere creates an alternate dimension in which time passes six times slower than in the outside world (i.e., one year Tokyo Jupiter time is six years real-time).
This is where the series starts, and where Ayato, Asahina, Mamoru, the Shitow family, and Maya are orginially from. When the barrier was created/appeared, the Shitow family was trapped outside the barrier, causing Haruka and Megumi to age faster than Ayato and the others.
It is known that people from the 'real world' can get through the barrier, such as Haruka at the beginning of the series, along with Ayato later on.
Tokyo Jupiter gets its name because the visual barrier resembles the planet Jupiter.