The Cetra's Promised Land was the Lifestream. The Promised Land is not any one particular place. It varies for each person.

The Cetra were not capable of space travel. They stayed on Gaia and never left it. I am absolutely positive that there is no in-game dialogue stating, or even hinting, to the concept that the Cetra were capable of colonizing other planets.

There are no heaven or hell realms in FF7. There is only the Lifestream. It's made even clearer in The Maiden Who Travels the Planet, since both "bad" and "good" consciousnesses are present there.