Miri's homeworld was the inhabited third planet in its system. Classified as M Class, this was the homeworld of the Onlies, a pre-warp humanoid species. The planet was described as a spheroid-shaped body with a circumference of 24,874 miles, a mass of 6 ×1021, a mean density of 5.517, and an atmosphere of nitrogen and oxygen.
Although located hundreds of light years from Earth, this planet was a prime example of Hodgkin's Law of Parallel Planetary Development. The planet appeared to be an exact duplicate of the planet Earth in composition, but the entire adult population had died of a microbiological experiment gone awry in the mid-1960s. This was a life prolongation experiment.
While tracing an old Earth distress signal to its source, the USS Enterprise discovered this planet in 2266. An initial survey of the planet by a landing party revealed that the surviving population was composed of children. The party learned that the children referred to themselves as the "Onlies". One of the older children of the group encountered was named Miri. After discovering a cure for the virus and gaining the trust of the Onlies, the Enterprise arranged with Starfleet for teachers and advisors to be sent to this planet so they could aid the Onlies in rebuilding their civilization. (TOS: "Miri")
In the original version of this episode, this planet lacked any cloud cover or weather patterns. In the 2006 remastered version of "Miri," several new shots of the planet were incorporated into the episode, this time featuring more realistic atmospheric conditions and cloud cover.
The planet was never given a name in the episode. According to the reference work Star Trek Maps, (pg. 31) the planet became known as Miri's World. The yellow dwarf star the planet orbited had the designation UFC 347601. Not only the planet, but also the entire solar system was an exact duplicate of the Sol system in every measurable detail. The planet had a population of 13 million Onlies.
According to the reference work The Worlds of the Federation by Shane Johnson, the indigenous name of the planet was Earth, but the world was subsequently called Onlies by the Federation. The star had the designation UFC 347601 and the entire solar system was an exact duplicate of the Sol system. After its discovery, the Federation began to colonize the planet. Scientists explained the planets existence by the theory that at some point in the distant past the Sol system moved through a massive tear in the space-time continuum, resulting in replication in the subatomic level. Theologians of the "Judeo-Christian" faiths considered it to be a simultaneous creation by God. Johnson identified the pathogen of the disease that had destroyed the entire adult population as, in quotation of the Star Fleet Medical Reference Manual, a filterable virus, order 2250-67A.
According to Star Trek: Star Charts, (pg. 54) Miri was the third planet in the FGC-347601 planetary system. Also known as Earth, this planet was established as a protectorate of the Federation in 2266. The capital was New York City, and the dominant species were the humanoid Onlies. In 2378, 13.1 million Onlies lived on this planet. Scientists theorized that this world was a terraformed duplicate of Earth created by the Preservers. The adult population was killed by a viral experiment, circa 1966 AD.
In his 1967 first volume of Star Trek: The Original Series episode adaptations, James Blish ignored the "duplicate Earth" premise of the television episode "Miri", and instead wrote that Miri's planet was the fourth planet orbiting the star 70 Ophiucus. He described it as a beautiful Earth-like planet having one large and two smaller continents connected by islands. The planet was located between twelve to fifteen light years from Earth and had been the first planet outside Earth's solar system to be colonized, in this case by refugees from the so-called "Cold Peace", about five hundred years before the events of the episode. These colonists were isolationists who violently repelled the first attempt to contact them by a later expedition from Earth, so no further contact was attempted. As it turned out, the 70 Ophiucus system was in a "backwater" part of the galaxy bypassed in subsequent years of Earth-based space exploration, and so the belligerent colony was easily ignored and almost forgotten. Around three hundred years before the events of "Miri", scientists on the planet developed the experimental life-prolongation project that resulted in the deaths the adult population. Despite their close proximity, the distress signal sent by the colony failed to reach Earth because 70 Ophiucus stood between Earth and the center of the Milky Way, whose radiation created interstellar static that drowned out the SOS signal the colony had directed towards Earth.
According to the 1989 novel The Cry of the Onlies, this planet was called Juram V. According to the 2000 novel Preserver by William Shatner, the planet was called Site 2713 and was revealed to have been duplicated from Earth on a subatomic level by the Preservers. According to the 2012 Star Trek: Department of Temporal Investigations novel Forgotten History, the planet was the Earth from a parallel universe that had become folded into the prime timeline.