Terra 10 was an early Human colony, settled during the 21st century by emigres opposed to the political changes of the era on Earth. (SCE eBook: Out of the Cocoon) The colony was established on the only planet orbiting Cephenes near the Arachna Nebula.
By the 23rd century, the colony had been shrunken due to exposure to natural radiation (spiroid energy waves) present on the planet where it had been established. When that planet started dying, the colonists desperately sought the help of the USS Enterprise, shrinking the crew of that ship in the process. The Enterprise was eventually able to restore its own crew to full size, rescue the colony, and relocate it to a distant Class M planet. (TAS episode & Log Four novelization: The Terratin Incident, ST website: Startrek.com)