M-Class planets in the Star Trek universe are the habitable worlds that humans can walk around on without a space suit. They have an Earth-like atmosphere, survivable temperatures, and gravity similar to Earth's. M-Class planets also have clean water sources and are typically found in the Goldilocks Zone of their solar system, ensuring they can support life. The term 'M-Class' was introduced in the original Star Trek pilot 'The Cage' in 1966, but it wasn't until 'Star Trek: Enterprise' in 2001 that it was explained that 'M' stands for 'Minshara', a classification invented by the Vulcans. Starfleet adopted the term and shortened it to 'M-Class', creating a new planetary taxonomy. M-Class planets are the most common habitable worlds in the Star Trek universe, but they are not the only ones. L-Class planets can support human life and have some vegetation, while K-Class worlds are not habitable but can be made so with artificial biospheres. The classification of planets in the Star Trek universe is complex and often inconsistent, with different sourcebooks providing conflicting information. For example, A-Class planets are described as gas giants in one sourcebook and molten, volcanic worlds in another. Despite this, M-Class planets have always been the habitable ones, and their characteristics have remained consistent throughout the franchise.
