The Counter-Earth is a hypothetical body of the Solar system first hypothesized by the presocratic philosopher Philolaus (c. 470 - c. 385 BCE) to support his non-geocentric cosmology, in which all objects in the universe revolve around a Central Fire. The Greek word "Antichthon" means "Counter-Earth."
Greek philosophy 
Necessity of a Counter-Earth 
By 500 BC most contemporary Greek philosophers considered the Earth to be spherical — there was obvious evidence for this from the behavior of objects near the horizon. This meant that all objects on the surface of the Earth had to be attracted to its center in some way; otherwise, they would fall off. It also required other objects, such as the stars and planets, to float above the Earth in relation to its center, because otherwise they would presumably move rapidly away. This argument — reasonable in view of the data available at the time — resulted in a geocentric world view.
When the movements of celestial objects convinced Philolaus that the world must be not only turning on its own axis but revolving around a fixed point elsewhere in space, he was faced with the problem of explaining how a spherical world could move in this way without spilling everything on the surface into space. He came to the conclusion that the directions of up and down do not exist in space, except in that all things must fall towards the center of the universe, around which all things (including the Earth, Sun, and all the planets) must revolve. As such, the Earth would be a flat world and the underside of the world would face this fiery, central point at all times.
This created a contradiction within the Pythagorean school of thought. Since planets, in their understanding, were composed of a fiery or ethereal matter having little or no density, they could quite easily rotate eccentric to the Earth without becoming off balance. However, the Earth was obviously made of the dense elements of earth and water. If there were a single Earth revolving at some distance from the center of space, the universe's center of balance would not coincide with its spatial center. Since this is the point towards which things fall, the Earth must have a counterbalance of the same mass or the universe would be flung apart. This problem led Philolaus to develop the idea of a Counter-Earth, a second, flat Earth, identical but opposite to ours in every way. This conception of the solar system is outlined in the diagram at the right, with Counter-Earth referred to as Antichthon. The upper illustration depicts Earth at night while the lower one depicts Earth in the day. (In order to prevent confusion, the diagram fails to show that Earth and Counter-Earth are flat and point away from the Central Fire). It is likely that Philolaus believed that the whole orbit of Earth was composed of an ethereal sphere, with the Earth and Counter-Earth being local dense points on the surface.
This theory is a very credible attempt to incorporate all known cosmological data at the time, and indicates the sophistication of Classical Greek thinking. The ideas of a flat Earth, Counter-Earth, and Central Fire were all eventually superseded by the theory of Gravitation (which is currently held by the scientific community) that describes a spherical Earth both rotating on its own axis and revolving around the sun. The Counter-Earth is still a popular motif in science fiction and fantasy writing today, usually serving as an allegory for the real Earth.
In the 1st century A.D., after the idea of a spherical Earth superseded the original Counter-Earth theory, Pomponius Mela, a Latin cosmographer, developed an updated version of the idea, wherein a spherical Earth must have a more or less balanced distribution of land and water. Mela drew the first map on which the mysterious continent of Earth appears in the unknown half of Earth — our antipodes. This continent he inscribed with the name Antichthones.
Modern era 
The idea of a hypothetical planet always on the other side of the Sun from the Earth has been a recurring theme in science fiction, fiction, and UFO claims. The idea was particularly popular in 1950s science fiction.
Scientific analysis 
A planet the same distance from the sun and with the same mass as Earth could (according to the theory) have the same orbiting speed/path as Earth, which could allow it to remain in a position 180 degrees from Earth, behind the Sun.[better source needed] With the same size and distance from the sun as Earth it could have the same (or very similar) gravity, atmospheric pressure, and surface temperature range. Thus, what would make the counter-earth planet undetectable to astronomers would also allow it to be habitable to beings at least similar to humans.
However, if such a planet actually existed, the laws of physics and cosmology would make it detectable from Earth for a number of reasons.
A Counter-Earth would have gravitational influence upon the other planets, comets and man-made probes of the Solar System. No such influence has been detected, and indeed space probes sent to Venus, Mars and other places could not have successfully flown by or landed on their targets if a Counter-Earth existed, as it was not accounted for in navigational calculation.
Any planetary sized body 180 degrees from Earth should also have been visible to some space probes, such as NASA's STEREO coronagraph probes (two spacecraft launched into orbits around the sun in 2006, one farther ahead of and one behind the Earth's orbit) which would have seen the Counter-Earth during the first half of 2007. The separation of the STEREO spacecraft from Earth would give them a view of the Lagrangian "L3" point during the early phase of the mission. Given the sensitivity of STEREO's COR2 coronagraph, anything larger than 100 kilometres (62 mi) in diameter should have been detected.
Also, if a Counter-Earth had an electromagnetic energy signature similar to that of Earth's, the Sun's "wobbling" motion around its "barycenter", would prevent it from blocking all signs of that energy from earth, at least for part of the year. The Sun does not remain stationary—relative to its planets—at the center of the solar system because of the gravitational pull of the most massive planet—Jupiter. The Sun wobbles around the "true" center known as a barycenter, which lies just inside the sun. When the Sun's position is 90 degree from the barycenter relative to earth, it comes close to 'unblocking" the sight of a planet 180 degrees from Earth, and the electromagnetic energy signature of that planet would be quite visible to astronomers.
For a Counter-Earth orbiting the same path as Earth to always stay 180 degrees from Earth, the two planets must have circular orbits, but Earth's orbit is elliptical. Following Kepler's second law, a planet revolves faster when it is close to the star, so a Counter-Earth following the Earth on the same orbit with half a year of delay would sometimes be visible from Earth. To be hidden from Earth, the Counter-Earth would have an orbit symmetrical to Earth's, not sharing the second focus or orbit path.
References in culture 
- Gor is the name of the Counter-Earth that is the setting for a series of 32 novels by John Norman.
- The 1969 science-fiction film Doppelgänger depicts the discovery and investigation of another planet sharing Earth's orbit on the opposite side of the sun. The third season of science fiction television series Lexx was set on binary planets Fire and Water which were revealed to be in orbit around the sun on the side opposite to Earth.
- Two 2011 films -- Another Earth and the Lars von Trier's film Melancholia—feature a plot in which a planet emerges from behind the Sun and approaches Earth.
- Several Marvel Comics or spinoffs of comics were set, or had parts of a story set, in a Counter-Earth. These included The High Evolutionary, The Spider-Man Unlimited TV series and the associated comic book, Infinity Crusade: Paradise Omega, The Earth of The New Universe, the Heroes Reborn Earth ("more of a Pocket Dimension than an actual planet"). On The Adventures Of Superman radio series, the planet Krypton is said to be "situated on the other side of the Sun" from the Earth in the first episode.
See also 
- Fictional planets of the Solar System#Counter-Earth
- The Ufo Book of Lists| By STEPHEN J SPIGNESI, Stephen J. Spignesi|
- Harley, John Brian; Woodward, David (1987). The History of Cartography: Cartography in prehistoric, ancient, and medieval Europe and the Mediterranean 1. Humana Press.
- Harley and Woodward, pp. 136-146.
- Pomponius Mela. de Chorographia.
- Another Earth
- How are mass and orbital period related?
- Other planets have an affect on the sun's motion but theirs are not nearly as strong as that of Jupiter
- Andrea Magrath (2011-05-18). "Sunny Kirsten Dunst is picture perfect at the Cannes photocall for her provocative new film Melancholia". Daily Mail (London). Retrieved 2011-05-27.
Further reading 
- The Secret Teachings of All Ages: An Encyclopedic Outline of Masonic, Hermetic, Qabbalistic and Rosicrucian Symbolical Philosophy, by Manly P. Hall, Philosophical Research Society Inc. ISBN 1-58542-250-9
- Book of Earths, by Edna Kenton, Kessinger Publishing. ISBN 0-7661-2856-3
- Burch, George Bosworth. The Counter-Earth. Osiris, vol. 11. Saint Catherines Press, 1954. p. 267-294
- History of the Pythagorean school hosted by Drury University, Springfield, Missouri