Terratin is smaller than Earth with a geologically active and unstable surface. The pronounced seismic activity produced a large number of volcanoes and earthquakes. While its atmosphere contained a breathable mixture of oxygen and nitrogen in the mid 2260s and was high in some rare gases, contamination from volcanic eruptions was expected to render the air unbreathable by the end of the century.
A rare type of radiation known as spiroid epsilon radiation was emitted by the planet, which shrank organic tissue over a period of generations. Some lower animal life forms inhabited the surface as well as vegetation. While it appeared to have standard amounts of geologic ores, the planet was high in dilithium crystals. (TAS episode & Log Four novelization: The Terratin Incident)
In the 21st century an Earth generational ship landed on the planet and named it Terra 10, which over time became Terratin. The settlers became significantly reduced in size due to radiation exposure, with the change affecting their DNA and becoming permanent over time. They were able to calculate the extent of their shrinkage and build a city to accommodate them. Eventually they also developed a means of amplifying the radiation into a compaction beam as a planetary defensive weapon. (TAS episode & Log Four novelization: The Terratin Incident, TOS reference: Spaceflight Chronology)
In 2269, the USS Enterprise orbited Terratin to investigate a mysterious transmission. When the colony's communications equipment was destroyed, the colonists had no other way of drawing attention except to use the compaction beam, which shrank the ship's personnel. Captain James T. Kirk was able to transplant Terra 10 to the planet Verdanis in the Beta Cephei system, which was 10 days' travel from Terratin. (TAS episode & Log Four novelization: The Terratin Incident)
Following their visit, Kirk expected Terratin could become an important dilithium mining source for the Federation. With their settlement rights, the Terratin people would benefit greatly from it. (TAS - Log Four novelization: The Terratin Incident)
In 2298, Ambassador Sarek and Spock discussed Terratin and its radiation. Sarek and Amanda Grayson were planning to visit the transplanted colony on Verdanis. (TOS - Crucible novel: The Fire and the Rose)