I did a search on Wikipedia for LV-426 (ask Mark) and found a truly amazing article. An extract of it is included for your enjoyment:
LV-426, also known as Acheron, is the name of the fictitious moon (frequently but erroneously referred to as a planet) where the Alien was first encountered by humans in the movie Alien (1979) of the Alien Series. It bears a strong resemblence to Saturn's moon, Titan.
LV-426 is a moon in orbit around a gas giant in the Zeta II Reticuli system, which lies beyond the "Outer Rim." LV-426 has .86 Earth gravity and is geologically active, judging from its surface composition (igneous rock, "lava base") and steam vents on its surface. Prior to terraforming, the atmosphere consisted mostly of nitrogen, crystalised carbon dioxide, and methane, and its temperature was well below the Celsius line. The source materials differ widely on the diameter of this world: figures of 1,200 m, 1,200 km and 12,201 km are given in different sources. Out of these, 12,201 km (from the Aliens Colonial Marines Technical Manual) is the only one that is possible given the surface gravity.
Most of the details in the passage above are not supplied in any of the Alien movies, and a check into the "Aliens Colonial Marines Technical Manual" tells me it is a piece of reasonable fan-fiction. What drives these people to invest so much time and effort, and possibly money, into a wholly fictional reality extending it with their own ideas?
Aliens isn't the end of this madness either, go have a look at a few Star Wars and Star Trek related entries on Wikipedia.
I think Wikipedia need to start looking into these articles a bit closer. The way they read seem to represent the fictional reality as a real one. Especially the entry for Alderaan, so much history and backstory that was written by fans has made its way into the article. If the Wikipedia Foundation wish to be a real player in the encyclopedia business they need to start making sure their content is factual and when it is not, clearly label it as so.