40 Eridani is a nearby trinary (three-star) system in the constellation Eridanus and is also known as Omicron 2 Eridani or Keid. 40 Eridani A is an orange main-sequence K star some 4 billion years old. 40 Eridani B is a white dwarf, the first such star to be discovered. 40 Eridani C is a red dwarf and also a flare star with the variable star designation DY Eridani. Components B and C orbit around each other and, as a pair orbit the primary, which lies about 400 astronomical units away. The system is 16 light years (5.025 parsecs) away from Earth.

In Star Trek, 40 Eridani A is the star around which the Vulcan homeworld orbits. Although there is no canon reference to Vulcan being located in the 40 Eridani system, it is widely considered as a fact used by Star Trek creator Gene Roddenberry. In the Star Trek: Enterprise episode "Daedalus", Vulcan is mentioned as being slightly over 16 light years from Earth, the same distance between Earth and 40 Eridani A. When three astronomers (Sallie Baliunas, Robert Donahue, and George Nassiopoulas) who had been studying the system at Mount Wilson Observatory published a letter stating "that [the star] 40 Eridani A could support a planet with Earth-like life" and that it would "have the Sun's brightness for a planet 50 million miles away", they theorized that Vulcan could be in orbit of this star, a theory Gene Roddenberry himself supported by signing their letter.

Astronomers have recently considered if this star might actually have any Earth-like planets and, if so, they might be capable of supporting life. The first problem to consider is whether an Earth-mass planet could have formed around 40 Eridani A, which has two stellar companions tugging at it. The answer to this seems to be "yes". On the Jet Propulsion Laboratory's "PlanetQuest" website, planetary theorist Sean Raymond of the University of Colorado, Boulder, commented: "Since the three members of the triple star system are so far away from each other [hundreds of astronomical units], I see no reason why an Earth-mass planet would not be able to form around the primary star, 40 Eridani A."