Although as far as I know, this has not been formalized in any kind of a universal treaty, historically you "own" something (whether you are a nation or a person) when a sufficient number of people or nations "recognize" that ownership.

Using the Middle East as an example again, Israel has not been so "recognized" by enough nations to keep it from having to constantly fight for legitimacy. Jordan, although it was basically pulled out of Britain's hat in 1947 exactly like Israel was, seems to have been "recognized" as a "real" nation. No one (important) questions the right of Jordan to exist.

For the record, Jordan "occupied" the controversial West Bank, touted as the ancient Palestinian Homeland, from 1950 until 1967, although this annexation was "recognized" only by Britain and Pakistan. In 1970, Jordan massacred thousands of Palestinians and expelled the rest ("Black September") after Arafat attempted to take over. Yet somehow this occupation and abuse of the Palestinians has never been considered sufficient reason to question the Jordanian "right to exist."

The loophole in the treaty seems to allow private ownership, but not government ownership in space. This may actually end up being a good thing someday, setting a precedent for individual property rights over collective ownership.