Tei Tenga is a fictional moon or planet where Doom was originally supposed to take place during its first and third episodes, according to Tom Hall's preliminary design document for the game. The retail version of Doom still has images and references to this planet, as well being mentioned as an off-world site in Doom (2016).
In Official Sources
The original Doom's manual states that the UAC is a 'multi-planetary' conglomerate. This is a reference that the corporation controls way more planets than simply Mars (and it's moons).
Tei Tenga's first appearance in Doom is in the Phobos Hangar on view screens in the main entry way of the Hangar, hanging from the ceiling. Each showing various destinations and landmarks the UAC is concerned with.
Main UAC destinations or monitored regions include to be the Sun, Earth, Tei Tenga, and what appears to be a portion of a nebula.
This Tei Tenga imagery appears on a several computer screens in the various Doom, Doom II, and Final Doom maps. Other images on the computers include the Sun, Earth, etc. Other computers appear to show data being transmitted from UAC Base Tei Tenga. Tei Tenga can later be seen on screens throughout other UAC installations as well.
See Tei Tenga (2016)
Characteristics of the Tei Tenga system
The celestial body is tidally locked and thus the same hemisphere always faces the star it orbits; therefore it does not have day and night cycles. Because of the synchronous rotation, it is divided into two sections, a continually bright hemisphere called the Lightside, and a complementary sunless hemisphere called the Darkside. The anomalies that are being studied are located at Tei Tenga's magnetic poles. There are two main bases on Tei Tenga; the primary military research base is on the Lightside, the secondary base on the Darkside. The secondary base is also the main location of the game's protagonist.
The Planet is littered with fossils of giant animals, suggesting that long ago before the events of the Doom Bible, the planet was green and covered in life much like that of Earth's Sahara desert during the last Ice Age.
The computer screens in the game, as well as the sky textures, indicate Tei Tenga has a brownish red hue. From this, we can easily deduce Tei Tenga has a soil rich in oxides and iron. One sky texture hints that the Deserts covering the planet are so large and expansive, they could cover an entire view of a sky as being akin to that of a sandy cave.
Further, the sky textures, the soil oxidation and the lack of any discernible breathing apparatus indicates Tei Tenga has an oxygen rich atmosphere. The presence of magnetic poles also indicate Tei Tenga has an active magnetosphere, which in turn is evidence of a molten, liquid core, consistent with the presence of an atmosphere. Magnetospheres create a protective sheath against solar winds, which tend to ionize and thus deplete any present atmosphere, as is the case with Mars--which has neither magnetosphere nor sufficient atmosphere.
Moon or planet?
Although it is initially referred to as the giant moon Tei Tenga, it is called a planet in a few other instances. If the celestial body is a planet, it can itself have no moons of substantial size, as these would make tidal lock with the sun an impossibility. The earliest data files with any reference to the celestial body--simply stylized graphics giving the position of the anomalies--names the pictures MOONx0, where x is a letter from A to H.
This would, however, be inconsistent with gravitational physics. Moons can only be tidally locked with their planet, not the central star of the system.
Tei Tenga as scenario
The bases on Tei Tenga used to be so called 'glory posts', but at the time of the game the bases have fallen into disrepair due to lack of funds, partly due to UAC cost cutting and the lack of research progress. UAC is a military contractor and this explains the military presence at the bases.
According to Tom Hall's Doom Bible, the mining operations on Tei Tenga focused on what is only referred to as "Fire Dust", a volatile combustible mined from the celestial body's crust. No further details are given about the substance, and its chemical composition is unknown. The anomalies are located where Fire Dust was first found and excavated. The bases that monitor the two anomalies on Tei Tenga's poles were initially set up by the military -- initially called UAAF (United Aerospace Armed Forces) -- to research the substance; this research, and the money associated with it, is what had caused the bases to experience their 'glamour days'. At the time of the game, the researchers were focusing their efforts on scrutinizing the polar anomalies.
During the development of Doom the scenario was neither Phobos or Deimos, as is the case with retail Doom. Therefore, certain maps were known by different names; Phobos Anomaly, for instance, was known as simply Anomaly.
According to a tweet from Tom Hall  the name of Tei Tenga is based on the style of names used in Star Trek, Star Wars and other 1970s-1990s Science Fiction Films.
What is Tei Tenga?
Since Tei Tenga displays characteristics that are incompatible with planetary and gravitational physics numerous conjectures, theories and hypothesis have been proposed as to what the real nature of Tei Tenga is.
Evidence suggests Tei Tenga is not a normal celestial body, but that it is a planet sized autonomous entity. The celestial body has an atmosphere and extensive cloud formations without any percievable body of water, making underground aquifers the only possible source of the required moisture. Fossils of giant creatures have been found, pointing to a past teeming with life. These two facts are inconsistent with each other, abiogenesis theory, and developmental and evolutionary theories. Further, Tei Tenga's massive magnetosphere is evidence of a huge dynamo at the core, which is inconsistent with Tei Tenga's tidally locked orbit.
The only hypothesis that can explain these inconsistencies is also the only one to gain somewhat wide acceptance amongst specialists on the subject; the hypothesis states Tei Tenga is not an inert celestial body but a life form, based not on carbon but on iron. The entities encountered in the form of demons are in fact entities created by Tei Tenga itself to drive off the beings that are parasiting it, being the humans that are mining Tei Tenga. Being a planet sized, iron based life form and operating on electricity, it would be a trivial matter for Tei Tenga to scan the brain waves of the "messianic" scientists and extract blueprints for the innermost demons from their subconsciousness. It would further be a trivial matter for Tei Tenga to recreate these various demonic entities with magnetic fields and matter from the crust, the so called Fire Dust, and to then use these agents to drive off the current parasites. It has been suggested this has happened before and the fossils found on Tei Tenga are, in fact, evidence of this, however due to the presence of traveling to Hell in the original Episode 2, and apparently a Episode set on the Moon of Earth, seem to quash this theory.
A similar scenario was explored by author Stanislaw Lem in his classic book Solaris.
The Inspiration for Tei Tenga seems to be drawn from Darwin IV from Wayne Barlowe's Science Fiction book Expedition, both of the planets (can be considered a life form or Moon in Tei Tenga's case) in their weakening state, both long dying and once being wet and full of life. Both also orbit weak stars (Binary Brown Dwarfs for Darwin IV and A Red Dwarf in Tei Tenga's case) and have large active atmospheres, low concentrations of water and a Mars-like Geography. However unlike Tei Tenga, Darwin IV still supports a wide variety of life (Unknown in Tei Tenga's case)
Tom Hall replied to a tweet asking if Darwin IV from Expedition inspired he replied stating: "Nope! Just popped into my head. It may be a combo of of Star Trek, Star Wars, other sci fi novel-type planet naming style." 
Tei Tenga does still appear in the current Doom games, although somewhat obscurely, being named in some of the computer console display textures. Additionally, some fan-made PWADs expand on the Tei Tenga concept.
The planet makes other appearances as well. In another 3D game--Terminal Velocity by 3D Realms--the planet is encountered as an episode setting. The second episode of the game starts on Tei Tenga. According to the documentation the planet is the second orbiting body around the Ross 154 star; there are no references to any other low-mass celestial bodies in the system. The planet is described as "a desert planet, littered with the fossils of long extinct giant animals". As Doom designer Tom Hall co-produced Terminal Velocity, it is safe to assume the thinly veiled and very apparent similarities between the two celestial bodies are more than an accident. It is argued they are, in fact, the same planet but captured at different points in time.
The actual Ross 154 system is located 9.6 light years from Earth, in the Sagittarius constellation.
- The image of Tei Tenga appears to be taken from Tethys one of the moons of Saturn.