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Layout of the Thessian Star System
The Thessian star system consists of a single Population I star, a small red dwarf, generally known as Thessia or Astra Thessiae. Due to heavy metal contaminants it emits an unusually bluish light, creating an output spectrum not unlike that of a yellow dwarf. Thet and the pockets of space around it appear to have devolved from a single, much larger universe, possibly large enough to contain at least one spiral galaxy, and as a result of the inconsistent time-flows between Thet and other regions, as well as exotic physical phenomena caused by planar fissures, it is difficult to exactly quantify the health or nature of the star in terms comparable to the Milky Way. To the Tletketti, Astra Thessiae was known simply as "Ama."

Orbiting Astra Thessiae at a distance of 0.45 AU is the planet Thessia Major. It is also known as Thessia, through synecdoche as Thet, and in Lilitic as Kelí Thessía. The world has much in common with Earth: it is a small, rocky world with an iron mantle and a mostly-silicon and carbon crust. Its atmosphere is 35% oxygen, 12% nitrogen, and 53% argon, with trace amounts of carbon dioxide, water vapour, and ammonia vapour. About 60% of the surface is covered in liquid water oceans. The planet supports a wide range of autotrophs and heterotrophs, and alien species from a number of biospheres have been shown to integrate successfully into the ecosystem, albeit only in the loosest sense, as nutrients are rarely passed between biologies except at the most basic autotrophic level. The planet is approximately 0.93 Earth masses in size. Its orbital period is approximately 312 Earth days, which is close enough to 0.375 Lyran years that the Lyrisclensian calendar was adopted for wide use, albeit counting from the year of the arrival of the Lyrisclensiae. The surface gravity averages 96% of Earth's surface gravity, and the day is 1.12 times the length of the Terran day.

Thessia Major was the native homeworld of the Tletkettoyi, extremely intelligent and mysterious creatures who superficially resembled trilobites. Much of the planet's surface is covered with the ruins of the access hatches to their underground tunnel network, where most of their society existed. They called the surface "Tlekt," and the underworld "Dhazma." It is from Tlekt that the modern word "Thet" and its derivatives come.

Thessia Major has four natural satellites, named (in order of size) Gleméa, Reséa, Haplenía, and Súa by the Lilitai. The Tletketti names were Ekotl, Kintarz, Rentli, and Tlen. The Cossipai and Telai call them Sagan, Tyson, Aldrin, and Dirac, and the Lyrisclensiae draw a compromise, naming them Luna Sagani, Luna Sarthiae, Luna Aldrini, and Luna Suae. None of them are habitable, although the third moon has a thin atmosphere which is sufficient to support a form of primitive autotroph that the Tletketti previously farmed, giving it a distinctive beige colour. The other three moons are silvery-grey, like Earth's own. Small enclosed research facilities exist on the surfaces of all four.

The Thessian system has a second planet, Thessia Minor (Lil. Lezí Thessía), which is completely uninhabitable and noteworthy primarily as a haven for artificial satellites and space stations, and a waypoint for those leaving through planar breach drives at the edge of the brane. It was called Klot by the Tletketti and has two moons, which modern astronomers have named (unanimously) Dyson (Daisonía, Luna Dysoni) and Bohr (Bhora, Luna Borae). No Tletkettoyic names for Klot's moons are known. Thessia Minor orbits at a distance of 2.8 AU and weighs 0.4 Earth masses. It is mostly ferrous, rotates only once every 52 days, and requires 13.4 years (5.88 KY) to orbit its sun.

There are several pockets of asteroids on the furthest reaches of the system, which appear to be remnants of two distinct and much larger planets. However, the fragments that exist are insufficient to account for more than a hundredth of the mass of each world, suggesting that the rest was lost during a planar sundering event. These light up like stars during the night because they reflect sunlight, and hence played an important part in Tletkettoyic religion as the Kedavevoyi, or Ten Thousand Souls. This name has been translated into other languages directly.