Earth Two Earth Two was the inhabited third planet in its system . Classified as M - class , it was the homeworld of the Earth Two native s , 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 mile s , a mass of 6 ton s , a mean density of 5 . 517 , and an atmosphere of nitrogen and oxygen . Although located hundreds of light year s 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 " Grup " s 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 discovered this planet in 2266 . A landing party from the " Enterprise " beam ed 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 child ren 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 teacher s and advisor s to be sent to this planet so they could aid the Onlies in rebuilding their civilization . ( ) Appendices . 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 orbit ed 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 . Scientist s 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 " " ( 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 adaptation s , 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 continent s connected by island s . 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 " " 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 .