Platonius
Platonius was an inhabited planet in the Alpha Quadrant in the vicinity of Xahea that was colonized by the Platonians, a humanoid species originally from . Their colony was a principality. ()

This planet was rich in kironide deposits. 

In 2259, the location of Platonius was labeled on a stellar cartography chart that was seen on the 's ready room viewscreen. This system's symbol had a blue color, indicating that it was affiliated with the . ()

Prior to stardate 5784, this planet was unknown to the Federation. Scans of this planet revealed no intelligent life signs. This ability to hide their life signs was later attributed to the psychokinetic powers of the Platonians.

The Platonians colonized this planet thousands of years prior to its discovery by the Federation. Before they arrived at Platonius, the Platonians spent time on Earth and embraced the ancient Greek civilization. When this civilization ended, they left to create a society based on this civilization. The Platonians established a  based on the writings of the philosopher Plato, a disciple of Socrates. They named the planet after Plato. 

In 2268, the  made first contact with the Platonians. ()

In 2328, this planet was the destination for passengers traveling from Tau Ceti Deep Space Station via commercial transport. Kenneth Ross and Richard Barnett traveled there on the . (, okudagram)

In 2399, the location of Platonius was labeled in a Federation star chart that was in Fleet Admiral Kirsten Clancy's office at Starfleet Headquarters. This planet was located in neutral space. ()

Appendices

Background 
The  classified Platonius as a class M planet.

Apocrypha 
According to FASA's The Federation sourcebook, Platonius was the fourth planet in the .

According to Decipher's Worlds sourcebook, Platonius was the third planet in the Alpha Derridia system.

According to Ron D. Moore, Kirk mislaid the map to Platonious (sic) during a drunken bachelor party he hosted in his quarters.

External links 
 
 

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Colonies
Planets