Eden was an uninhabited planet located along the Romulan side of the Neutral Zone.
Eden had two moons. While the planet's atmosphere was capable of supporting humanoid lifeforms, the planet was completely devoid of animal life. All the plant life, even the grass, was filled with acid, that resulted in severe burns upon touch. This made the world virtually uninhabitable to humanoid life. (TOS: "The Way to Eden")
History
In 2269, the Tiburonian research engineer Doctor Sevrin began a quest to locate the mythical planet Eden, which he believed to be an idyllic, undiscovered planet. Gathering a group of loyal followers, Sevrin stole the space cruiser Aurora and embarked on his search. Sevrin and his followers were eventually located and apprehended by the crew of the USS Enterprise, although they were later on able to gain control of the ship.
Sevrin set the Enterprise on a course for the planet in Romulan space. Spock and Pavel Chekov had discovered the planet earlier by searching for an uncharted celestial body that corresponded with what Sevrin was looking for. They found the world by studying star charts and projecting the orbits of various planets. By mathematical processes, they were able to discern the location of this world by the effect it had on other known planets.
After the ship had crossed the Neutral Zone, the Enterprise arrived at the planet, to the surface of which Sevrin and his followers traveled in a shuttlecraft. Sevrin and the others were distressed to find, however, that the planet was uninhabitable to humanoid life. The party received severe burns, and the deaths of one of Sevrin's followers and Sevrin himself. Spock hoped that Irina Galliulin, one of Sevrin's followers, would not give up the search for Eden. (TOS: "The Way to Eden")