This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
Terralysium was a planet in the Beta Quadrant, located over 50,000 light years from the Federation. It was inhabited by Humans transplanted there by time traveler Gabrielle Burnham, who they revered as the Red Angel. (DIS: "New Eden")
It had a mean radius of 3958.8 mi, an equatorial radius of 3963.2 mi, and a polar radius of 3949.9 mi. Its circumference was 24901.461 mi, and it had a surface area of 196,940,000 sq mi. Its density was 5.514 g/cm3 and its gravity was 9.807 m. Its surface temperature ranged from -69.2 to 56.9 degrees Celsius. (DIS: "The Sound of Thunder")
In 2053, the planet became home to several hundred Human refugees transported from Earth, who had been saved from World War III by a mysterious angelic being. Believing that Earth had been destroyed in the war, the inhabitants developed a religion cobbled together from the primary faiths of Earth and also incorporated their enigmatic savior. They were technologically limited and eventually lost the use of electricity for a period.
Over the next two centuries, the planet's population grew to roughly 11,000, spread across ten settlements including New Eden, and northern territories. In 2257, the USS Discovery visited Terralysium while investigating a red burst. Though the burst itself had disappeared by the time they arrived, they were able to prevent radioactive debris from one of the planet's rings from causing a nuclear winter on the surface, which would have constituted an extinction level event.
Captain Christopher Pike decided that, despite their being Human, the inhabitants of Terralysium constituted a pre-warp civilization and were subject to General Order 1. He thus did not reveal the existence of Discovery to them, save for one man named Jacob who had already learned the truth. (DIS: "New Eden")
In 3186, Gabrielle Burnham built herself a home base on the planet, being careful to not use any pre-existing technology so that Control could not find her there. She found that the Humans she had transplanted to the planet were thriving, from which she concluded that the future could be changed. (DIS: "Perpetual Infinity")