Celtris III
Celtris III was the uninhabited third planet in its star system. This system was located in Cardassian space, near the Federation-Cardassian border in Sector 864 of the Alpha Quadrant.

The lynar, a type of Celtran bat, was indigenous to this planet. 

In 2367, Starfleet Intelligence learned about an installation buried beneath the planet's surface. The information gathered about this installation was used later for the planning of a special operations mission led by Captain Jean-Luc Picard in 2369.

In that year, Starfleet Intelligence began monitoring bursts of theta-band subspace emissions from this installation on Celtris III, which typically indicated a metagenic device in operation. Since such devices were illegal, this led Starfleet to believe that the Cardassians might have 
been operating a secret research lab in this facility. However, Picard's team discovered that the signal had been faked and that they had walked into a trap. It was a ruse by the Cardassians to lure the captain, who was an expert in theta emissions, to the planet. The Cardassians assumed that the , Picard's ship, would be the command ship of Sector 21527 and he would know the defenses of the Minos Korva system. During the team's escape from the planet, Picard was taken into custody and was later interrogated by Gul Madred.

During his negotiations with Captain Edward Jellico, Gul Lemec, using the elements of Picard's confession, alleged that the Federation had launched an unprovoked attack. In the allegation, Lemec said that Picard and his teammates, Dr. Beverly Crusher, and Worf, had assaulted an outpost on this planet, resulting in the deaths of 55 men, women, and children. They were holding Picard as evidence of this assault. The allegation was later dropped and Picard was returned to the Federation. ()

Appendices

Background information 
This planet's quadrant of origin is inferred based on the position of the Cardassian Union in the star chart appearing in the  episode .

"SELL-tris" was the pronunciation for this planet's name from the script pronunciation guide for  "Chain of Command, Part II". 

According to Star Trek: Star Charts (p. 46) and Stellar Cartography: The Starfleet Reference Library ("Stellar Cartography", pp. 18-19; "Federation Historical Highlights, 2161-2385"), the primary of the Celtris system was a M-class star. According to a map of the Cardassian Union – prepared by an anonymous astrography clerk, from the Cardassian Central Archives, in 2364 – Celtris was identified as a Cardassian system.

The  classified Celtris III as a class M planet.

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