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Tarchannen III was the third planet in the Tarchannen system. The planet was home to the Tarchanne.

The planet was characterized by perpetual twilight, frequent sandstorms and its sparse vegetation, including a variety of dark, spindly plants. The planet's atmosphere stretched up to an altitude of 210 kilometers.

Federation outpost

In the mid-24th century, the United Federation of Planets was unaware of the indigenous population. Starfleet built an outpost consisting of five buildings and manned it with 49 people. They had been infected by the aliens through a parasite which planted its DNA in living hosts to reproduce itself; the hosts then transformed into the primitive reptile-like species. In 2362, the transformation manifested itself and they left the outpost. The USS Victory arrived to investigate, and sent an away team led by Lieutenant Susanna Leijten. The outpost showed no signs of an attack or a natural disaster – the people had just vanished. Without further evidence, they abandoned the planet.

Graphic of a plant native to the planet

It was not until 2367 that this mystery could be solved. Three members of the former away team suddenly deserted and attempted to return to Tarchannen; one of them, Paul Hickman, was killed in this attempt when his shuttlecraft exploded during atmospheric entry. The USS Enterprise-D was investigating these incidents with Leijten on board as an observer, tailing him. Both she and the last away team member, Geordi La Forge, underwent the same transformation. Leijten was kept in sickbay, while La Forge managed to transport down to the planet, close to Building One. The Enterprise-D crew discovered the nature of the transformation and was able to reverse it. However, there was no hope for the others and they had no option but to live the rest of their lives as the aliens on the planet.

Following this discovery, warning beacons were placed in orbit and on the surface of the planet to prevent similar incidents in the future. (TNG: "Identity Crisis")

According to the script, the pronunciation for Tarchannen was "tar-CHAN-in". [1]
The script thrice describes the planet as being a world of "perpetual twilight"; it is unclear whether this refers to tidal locking or merely possessing a dim sun.
According to the Star Trek Encyclopedia (4th ed., vol. 2, p. 371) and, Tarchannen III was a M-class planet. [2]