Talos I was the first planet in the Talosian planetary system. This small planet was located over one astronomical unit from the binary stars Talos A and Talos B. (TOS: "The Cage", "The Menagerie, Part I")
Background
The planet was never mentioned in dialog; this name came from a map of the Talosian system.
The Explored Galaxy
This chart was first seen, chronologically, in 2293. (Star Trek VI: The Undiscovered Country) This chart was also seen in several Star Trek: The Next Generation and Star Trek: Deep Space Nine episodes set in the 24th century, from the year 2364 to 2370. These were: TNG: "Conspiracy", "The Measure Of A Man", "The Emissary", "The Mind's Eye", "The Game" and DS9: "In the Hands of the Prophets", "Cardassians".
Star Trek: Star Charts
Talos I was classified as a Class B planet.
The Talos system was a quadrinary star system. Talos Prime (Talos A) was a Class M star with a magnitude of +1, which was a hundred times brighter than Sol. The spectral class of this star was M0IV, and its absolute magnitude was 0.1. Talos B, Talos C, and Talos D were Class M stars. The distance of the stellar companions of Talos Prime ranged from 175 to 650 astronomical units.
Note: The book stated that there were five planets in the system. This was contradicted by what was established in the canon where eleven planets are said to be in the system. (Pages 34, 37, 40-41)