Page "United Federation of Planets" Paragraph 10

Later, in Star Trek: Enterprise the actual founding of the Federation can be seen in the episodes " Zero Hour " and " These Are the Voyages ...," and early negotiations that lead to it in " Demons " and " Terra Prime.

" Alpha Centauri is not mentioned as part of the founding, which is explicitly said to be between Humans, Vulcans, Andorians, and Tellarites.

This leaves open the possibility of the Alpha Centauri colony becoming an independent polity some time between " Terra Prime " and " These Are the Voyages ...," and then helping to form the Federation as a separate member.

However, Alpha Centauri is only ever mentioned in passing as an Earth colony on screen.

In the alternate timeline seen in the DS9 episode " Past Tense ," where the Federation was never formed, Alpha Centauri is a Romulan colony instead.