Surface of the Xyrillian homeworld

The Xyrillian homeworld was an inhabited planet and the homeworld of the space-faring Xyrillians. Thera was one of its cities.

In 2151, Xyrillian engineer Ah'len created a holographic projection of her homeworld for Trip Tucker. The simulation included Thera. (ENT: "Unexpected")

In the episode, Ah'len calls her home "Thera". It is not explained on-screen if this was the name of the planet, or just the name of the city seen in the hologram. However, the script for "Unexpected" specified it was the name of the city.
The informal designation for this planet was from the Star Trek Encyclopedia (4th ed., vol. 2, p. 389).