Alpha Centauri

Star Trek: Deep Space Nine
Episode: DS9 457 - Past Tense, Part I

Sol's closest stellar neighbor, a trinary that is only 4.3 lightyears away from Earth. One of its components' habitable planets was alreay colonized by Earth humans in 2154--possibly the planet where warp-drive inventor Zefram Cochrane took up residence later in life.

In the future altered by the premature death of Gabriel Bell, the lack of an Earth-fostered Federation left the Romulans free to expand as far as Alpha Centauri. One of its components is Alpha Proxima.