Earth Two

Earth Two was the inhabited third planet in its system. Classified as M-class, it was the homeworld of the Earth Two natives, a pre-warp humanoid species, that had been reduced to a subset known as the "Onlies". 

Planetary data
The planet was a spheroid body with a circumference of 24,874 miles, a mass of 6 tons, a mean density of 5.517 g/cm3, and an atmosphere of nitrogen and oxygen. Although located hundreds of light years from Earth, the planet was a prime example of Hodgkin's Law of Parallel Planetary Development. It appeared to be an exact duplicate of the planet Earth in composition. 

History
The entire adult population of the Onlies' homeworld (referred to as "Grups" by the Onlies) was killed in a failed microbiological life prolongation experiment in the mid-1960s. While tracing an old Earth SOS distress signal to its source, the USS Enterprise (NCC-1701) discovered this planet in 2266. A landing party from the Enterprise beamed into a deserted, dilapidated street on the planet's surface. An initial survey of the planet, thereafter conducted by the landing party, revealed that the surviving population was composed of children who referred to themselves as the "Onlies". 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. 

Background information
This planet is never given a name in the episode. In the final draft script of "Miri", however, it was repeatedly referred to as "Earth Two", including in the script sets list.

In the same script, this planet was initially described thus; "One green tinted planet in the center of the screen seems to pinpoint itself by its position and growth as THE planet [....] And what we see looks to be Earth... clearly... incredibly... an exact duplicate of our planet...." In the same script, the planet's mean density was said to be 5.552, rather than 5.517. North America was also identified in the script, and Hong Kong (or the planet's equivalent) was mentioned too. The street which the landing party beams into, when they first arrive on the planet, was described, in the scripted stage directions, as "what once was an important street in a great city."

In the original version of the episode, this planet lacked any cloud cover or weather patterns. In the 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.

Star Trek Maps
According to the reference work Star Trek Maps (p. 31), the planet became known as "Miri's World". The planet orbited a yellow dwarf star, which 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 thirteen million Onlies.

The Worlds of the Federation
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 planet's 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.

Star Trek: Star Charts
According to Star Trek: Star Charts (p. 54), "Miri" was the third planet in the FGC-347601 system, in the Beta Quadrant. 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.

The Federation
According to FASA's The Federation, Miri's homeworld was the third planet in the Beta Persei system.

Apocrypha
In his first volume of  episode adaptations, James Blish had no idea of the "duplicate Earth" premise of the television episode  (at the time of writing his first Star Trek adaptation book, he said that – probably because of the distance separation between those making Star Trek, and his residence in England at the time – he did not have access to the scripts that were used in shooting; he had only been given access to the original stories, which were often quite different from the TV shows). Blish 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 and 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 of 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 novel The Cry of the Onlies, this planet was called "Juram Five".

According to the 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. It is one of three duplicates of Earth that have been discovered by 2375.

According to the Star Trek: Department of Temporal Investigations novel Forgotten History by Christopher L. Bennett, the planet was the Earth from an alternate parallel timeline that had become folded into the prime timeline due to temporal instability in the region.