(covers information from several alternate timelines)
In the Federation standard system of planetary classification, some barely habitable planets, planetoids, and moons were classified as class L. A class L planet had an oxygen/argon atmosphere. A class L planet contained vegetation, but no animal life. (VOY: "The 37's")
The USS Olympia crashed on a class L planet in the Rutharian sector in 2371. While the sole survivor was able to survive on the planet's surface, she needed regular tri-ox injections to prevent carbon dioxide poisoning. (DS9: "The Sound of Her Voice")
A riddle presented by Neelix to Tuvok in 2376 contained the story of "a lone Ensign finds himself stranded on a class L planetoid with no rations. His only possession, a calendar. When Starfleet finds him twelve months later, he's in perfect health. Why didn't he starve to death?". Tuvok suggested that such a planetoid might contain hot water springs. (VOY: "Riddles")
B'Elanna Torres crash-landed the Delta Flyer on a class L planet in 2376 and was stranded there for several days, but Harry Kim finally reached her after walking 200 km with the part she needed to contact Voyager. It was home to a a pre-warp civilization and contained at least one body of water while the area around the crash site was mountainous and densely forested. (VOY: "Muse")
|D • H • J • K • L • M • N • R • T • Y • Other classes|
The L-class planet shown in "Muse" appeared somewhat atypical of a class L planet as it contained at least one region that supported lush vegetation and even a pre-warp civilization. Although Harry Kim traversed 200 km of the planet's surface by foot without any visible life-sustaining equipment, little was seen of the planet's surface, so perhaps the remainder of the planet was far less hospitable and more standard for a class L planet.
According to the Star Trek: Star Charts, on page 25, class L planets were "marginal" planets. They had an age that ranged from four to ten billion years and a diameter between 10,000 and 15,000 kilometers. Marginal planets were located within the ecosphere of a star system. They were categorized by a rocky and barren surface with little surface water and an atmosphere of oxygen/argon with a high concentration of carbon dioxide. Native lifeforms were limited to plant life, although the majority of class L planets were suitable for humanoid colonization.