Alternia is a dark, gray planet on which trolls originated. It appears to have two moons, one green, and the other, pink (which has a satellite of its own).
Years sweeps in the future (but not many), it is the setting of the Midnight Crew Intermission.
The Alternia we observe is part of a post-scratch reality. Originally the planet had the name Beforus, and was a very peaceful place populated by similarly peaceful trolls. It had only a pink moon. However, post-Scratch they gained Doc Scratch as a First Guardian and he "rewrote" their entire history, ensuring that the trolls would excel at playing Sgrub by turning them into an aggressive race, as well as ensuring the arrival of his master. He set up a system of oppression based on blood color, which was rebelled against on a large scale on at least two occasions. The second rebellion resulted in the banishment of all adult trolls into outer space, to conquer new planets. As a result Alternia is a planet populated by children.
Alternian sunlight is so intense most trolls can't withstand it, preferring night and shady places; looking directly into the sun causes instant blindness, as experienced by Terezi. Kanaya is mentioned as being one of the few members of the species to enjoy its rays, although whether this is a common feature among jade-blooded trolls is unknown.
Alternia has two moons, one green and one pink. The green moon is home to Doc Scratch and a facility that resembles Felt Manor. This moon first appeared in the aborted, non-canon Felt Forum Adventure created with input from Andrew, and has only appeared as background scenery in canon. It appears that a whole felt city is on the green moon that looks a lot like Real City Streets in Problem Sleuth. Little to nothing has been revealed about the pink moon thus far except that according to Aranea it was the only moon that existed in her life on Beforus (the green one being an addition after the Scratch of her game session), that Meenah lived there while she was in hiding, that it of the pre-scratch Frog Temple, and that it has a wee pink moonlet. She also leads Terezi to it in a dream bubble so they may confront her session's Thief of Life.
Aside from trolls and mother grubs, the lusus population shows us that Alternia is home to a greatly diverse population of terrifying creatures, including dragons; giant spiders; hermaphrodite cow-centaurs; sea-goats; fairy-winged miniature bulls; multi-mouthed felines; bicyclopes; crabstrosities; giant flying whales; giant flying seahorses; and eldritch abominations such as Gl'Bgolyb.
Many types of Alternian wildlife are used in the Fiduspawn game, including fidusuckers and horsaronis.
Feferi, who is quite fond of aquatic life, cares for a great number of cuttlefish, and a few aquatic hoofbeasts, who have fishlike hindquarters and tails, but otherwise are anatomically identical to Earth horses such as Maplehoof from the withers forward, including the forelegs.
Other creatures mentioned include hoofbeasts and musclebeasts, mentioned to roam the planet at night. Both are used interchangeably with the Earth concept of a horse, in phrases such as "get off your hiigh hoofbea2t" and "fuck [him] and the stupid muscle8east he rode in on". Based on the appearance of their aquatic variety, hoofbeasts appear to be roughly identical to Earth horses. Musclebeasts are so strong you don't even know.  Another phrase used by Alternians is "the lion's share", but it is unclear whether they have lions.
Terezi and Nepeta mention cholerbears in their roleplaying, and Karkat refers to one while trolling Jade. Cholerbears are probably similar to the panserbjørne, or armored bears, in that they have thick forearms and may wear armor. When roleplaying, Terezi refers to herself as a dragonyy'yd; however, Terezi's unhatched lusus is referred to merely as a dragon, and it is pointedly made clear that dragons are real. This suggests that the "dragonyy'yd" race is an alternate term only used in roleplaying, or a fabrication of Terezi's entirely, the details of which may not necessarily match up with real dragons. Dragonyy'yds may be reminiscent of Drakonids commonly present in Earth roleplaying games, and thus they may be half-troll, half-dragon creatures, just as Drakonids are half-human and half-dragon creatures to us.
According to Kanaya's introduction page, undead beings rise from the sands at sunrise, stalking the land and feeding on light and the living. Those who hunt the undead use chainsaws as their weapons of choice, although such activity is said to be as dangerous as hunting the nightly musclebeasts.
- During the "name entry" screen the planet was first named "Turdodor Fuckball" using the Daedric Alphabet from the Elder Scrolls series of computer games only rotated 180 degrees. However, this name was promptly rejected. Next, "Trollplanet" was entered and accepted, although it is translated as Alternia.
- The map of Alternia seems to be that of northern Russia turned upside down. The continent in the southern part of the planet would then be Novaja Zemlja ("Newland").
- Beforus, the planet's previous name, can be split into two halves, befor-us; a possible reference to how it came "before" Alternia, if scratching is viewed as putting a session in the past.
- Alternia's musclebeasts are almost certainly based on works from Andrew's time as an art critic depicting incredibly buff centaurs and other absurd animal hybrids.
- Alternia appears to have cherubs' session can be considered to have only a single player, as [S] Act 6 Intermission 3 reveals that Calliope's personality has been completely subsumed into Caliborn's, which fits this pattern as 40 is 1. Another pattern is found if you divide the number of players in the session by decreasing numbers, as followed: 48(created trolls' universe)/4=12, 12(trolls)/3=4, 4(kids)/2=2(cherubs). , implying that the session that created their universe may have consisted of 48 players. This is interesting to note, since 12 (the trolls' session) times 4 (the kids' session) = 48. Additionally, this suggests a series of cascading powers of four (4  players in the kids' session, 16  players in both the kids' and the trolls' sessions, 64  implied players in the kids', trolls' and unknown group's sessions). The
- The image used for Alternia is a stock image of Russia's northern coastline. Images of Mars are used for the pink moon and its wee pink moonlet.
- Alternia was named by Radiation.
- The planet's name "Alternia" was actually from the companion album for the trolls, and not the other way around.