Alternia is a dark gray planet which is the homeworld of the trolls. It has two moons, a pink one with a minor moon of its own, and a green moon. The green moon only exists due to the actions of Doc Scratch and therefore does not exist on the alternate iteration of the planet, Beforus.
The Alternia we observe is part of a post-scratch reality, resulting from a type of "universal reset" known as a scratch, initiated by twelve trolls playing a realityaltering video game called Sgrub. When their game session became impossible to win, they decided to reset their entire universe. Originally the planet had the name Beforus, and was a very peaceful place populated by similarly peaceful trolls. It had only one moon, which was pink. Post-scratch they gained Doc Scratch as a first guardian and he "rewrote" the entire history of the planet, ensuring that the trolls would excel at playing Sgrub this time, by turning them into an aggressive, warlike race. He set up a caste system 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 the rest of the galaxy. As a result Alternia is a planet populated by children.
After the trolls living on the planet were killed by the Reckoning and trolls living off-world were killed by the Vast Glub, the planet and green moon became home to carapacians. The planet was destroyed along with the rest of the universe when Spades Slick killed Snowman whose life had been bound to that of the universe.
The light from Alternia's host star is so intense most trolls can't withstand its light, preferring night and shady places; looking directly into it will cause instant blindness, as experienced by Terezi. Kanaya is mentioned as being one of the few members of the species to enjoy sunlight, although whether this tolerance of sunlight is common among jade-blooded trolls like her is unknown.