According to Startrek.com, Zeon was located in the M34 Alpha system. It was the outer planet of the two planets in the system.  According to the Star Trek Maps (p. 27) and The Worlds of the Federation (p. 122), the name of the system was M43-Alpha. It was a trinary system, Zeon being the fifth planet. According to Star Trek Maps, the total population of the two planets was 16.1 billion.