Platonius

Platonius was an inhabited planet colonized by the Platonians, a humanoid species, from Sahndara.

This planet was rich in kironide deposits.

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 republic based on the writings of the philosopher Plato, a disciple of Socrates. They named the planet after Plato.

In 2268, the USS Enterprise (NCC-1701) 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 SS Kogin. (, okudagram)

Background
The classified Platonius as a class M planet.

According to FASA's The Federation sourcebook, Platonius is the fourth planet in the Helios system.

According to Decipher's Worlds sourcebook, Platonius is 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. 

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