|Affiliation:||United Federation of Planets|
Classified as M-class, Andoria was an inhabited moon. This was the homeworld for the Andorians, a warp capable humanoid species, and the Aenar. Andoria orbited a ringed gas giant in the Andorian planetary system. This system was located in the Alpha Quadrant. The moon was a founding member of the United Federation of Planets.
Andoria was located in a neighboring system to Vulcan, which led to a series of conflicts between the Andorian Empire and the Vulcans. In 2154, the Vulcan High Command deployed a fleet of 12 starships to Regulus, which lay just outside Andorian sensor range, preparing to attack Andoria. Andoria was also relatively close to the Sol system. In the 24th century, Andoria was located near to the Cardassian border. (DS9: "Tears of the Prophets")
Andoria was inhabited by the blue skinned Andorian species of humanoids, and the Aenar, a subspecies of the Andorians. There was contact between the two sentient species on Andoria in the distant past, but eventually contact was lost and both species only vaguely remembered the other in myths and stories. It was not until around 2104 that contact was re-established between the Andorians and the Aenar. (ENT: "The Aenar")
In designing her new solarium, Yanas Tigan used tile she imported from Andoria, which she then had hand-painted by this charming old man she met on a trip to the Hovarian Cluster. (DS9: "Prodigal Daughter")
Andoria was an ice world, with a human-breathable oxygen-nitrogen atmosphere. Andorian cities were underground and took their energy from geothermal activity. The cities were connected to each other by thousands of kilometers of tunnels.
Only during rare heatwaves would the temperature on Andoria rise above freezing, and even then only for a few weeks at a time. During mid-summer, a temperature reading of -28° was not uncommon. (ENT: "The Aenar")
Points of interest:
According to writers Judith and Garfield Reeves-Stevens, in a special features segment in ENT Season 4 DVD, the establishment of the Andorian homeworld as a moon orbiting a gas giant was devised to help explain the contradiction of why it was sometimes called Andor and sometimes it was called Andoria, the gas giant being Andor and the moon Andoria.
The Last Unicorn Games Star Trek RPG and Samuel John Ross used information about Andoria, a frozen world first published in Star Trek: The Role Playing Game, for the work The Andorians: Among the Clans. When Mike Sussman and Manny Coto saw this book they "were taken by the striking cover image of an ice-covered Andoria" and went with that interpretation of the planet when it was finally seen in the ENT episode "The Aenar". 
The Andorian ice caves were actually a redress of the caverns built for Enterprise's Vulcan trilogy – simply repainted white.
According to Star Trek: Star Charts, on page 36, Andoria was located in a binary star system. This system consisted of a F-type main-sequence star orbited by an A-type main-sequence star. On page 52, Andoria was identified as the eighth planet in the Procyon star system. This planet was Class M. On page 52, Andoria was identified as the capital of the Andorain Empire, and was a founding member of the United Federation of Planets in 2161. The capital of Andoria was Andor, and 38.2 billion people lived on Andoria. Andorians were warp capable in 1154 A.D. On page 19, Andoria was located in the Andorian Sector (Sector 006). This sector was the sixth explored by Earth ships. In his final voyage in 2120, Zefram Cochrane visited Andoria. (page 60)
In Worlds of Star Trek: Deep Space Nine, Volume 1, Thirishar ch'Thane and Prynn Tenmei note the differences between the names "Andor" and "Andoria." While Tenmei grew up thinking of ch'Thane's homeworld as "Andoria," ch'Thane grew up knowing it as "Andor." Conversely, ch'Thane grew up thinking of Tenmei's homeworld as "Terra," only to discover that Terrans call it "Earth."