Star Trek has been visiting Class M planets since the series began, but it was Star Trek: Enterprise that revealed what Class M means.
Star Trek: Enterprise season 1 solved the Class M planet mystery that lasted for over 35 years. As a prequel to every other Star Trek series, Enterprise seized the opportunity to explain many of the fundamental building blocks of Star Trek as Captain Jonathan Archer (Scott Bakula) and the NX-01 Enterprise launched Starfleet's first voyages of deep space exploration. However, Enterprise is also a sequel to Star Trek: First Contact and revealed how influential the Vulcans were as Starfleet took its first baby steps into the stars.
Enterprise season 1, episode 4, "Strange New World" - no relation to Star Trek: Strange New Worlds, which premiered 21 years later - saw Captain Archer's NX-01 Enterprise visit its first uninhabited planet. Before Archer and a landing party boarded a shuttlepod, Subcommander T'Pol (Jolene Blalock) pointed out that the Captain might want to consider protocol before venturing onto a world they know nothing about. A Vulcan ship would send a series of probes from orbit to scan the planet before putting boots on the ground, a procedure the overeager Captain Archer and Commander Trip Tucker (Connor Trinneer) scoffed at. But this is standard operating procedure for Vulcans, who coined the term Class M planets are named for.
Enterprise Revealed The Origin Of Star Trek's Class M Planet Designation
Enterprise retconned Star Trek's Class M planet classification to mean "Minshara-Class." Ensign Hoshi Sato (Linda Park) translated "Minshara-Class" as "suitable for humanoid life." A Minshara-Class planet has the environmental conditions of Earth, or Vulcan, which humans are capable of living in, so it has a breathable oxygen-nitrogen atmosphere. In fact, the environments of starships like Enterprise are kept at Class M levels, although most of the 22nd century Starfleet hadn't heard of the term "Minshara-Class" or used the Class M designation until after T'Pol and Hoshi explained it in Enterprise episode 4.
Star Trek has used the term Class M to describe planets with Earth-like conditions since the original pilot for Star Trek, The Cage." The term Class M has been widely used in every Star Trek series since then, but Enterprise was the first to reveal its origin as "Minshara-Class" and that it came from the Vulcans. This is, of course, logical. Enterprise establishes that after the Vulcans formally landed on Earth in Star Trek: First Contact, they were the guiding hand in Starfleet's development, although humans also grew to resent the Vulcans for "holding them back." But it makes sense that Vulcan procedures and terminology would become interwoven with Starfleet, although the humans redesignated Minsahra-Class into Class M to make the designation their own, and likely, to downplay its Vulcan origin.
Enterprise Episode 4 Had 2 More Star Trek Firsts
Along with Captain Archer and his crew visiting their first Minshara-Class planet, Enterprise episode 4, "Strange New World," also had a couple more firsts for Star Trek. Captain Archer brought his beloved beagle, Porthos, to the planet with him, and Commander Tucker noted Porthos is the first dog to go "where no dog has gone before." Porthos is indeed a pioneer for all canines as the first dog to explore a Minshara-Class planet. This was a rare occasion for Porthos to leave Captain Archer's cabin on Enterprise. Thankfully, no calamity befell Porthos as it did the members of the landing party who remained on the planet overnight.
Further, Enterprise episode 4 was the first time T'Pol used the Vulcan nerve pinch. One of the most famed abilities of Vulcans, the nerve pinch was invented by Leonard Nimoy on Star Trek: The Original Series. Nimoy objected to a scripted instance of Mr. Spock resorting to fisticuffs, and he created the nerve pinch as an elegant and nonviolent way for Vulcans to render an enemy unconscious. Not only did Star Trek: Enterprise episode 4 solve the 35-year-old mystery of Class M's meaning, but T'Pol got to echo Spock's famous nerve pinch while also predating his use of it by a century.