The Counter-Earth is a hypothetical body of the Solar System hypothesized by the pre-Socratic Greek philosopher Philolaus (c. 470 – c. 385 BC) to support his non-geocentric cosmology, in which all objects in the universe revolve around an unseen "Central Fire" (distinct from the Sun which also revolves around it). The Greek word Antichthon (Greek: Ἀντίχθων) means "Counter-Earth".
In modern times a hypothetical planet always on the other side of the Sun from Earth has been called a "Counter-Earth", and has been a recurring theme in UFO claims, as well as in fiction (particularly science fiction).
However, Burch argues Philolaus must have thought it orbited on the other side of the Fire from Earth. Since "counter" means "opposite", and opposite could only be in respect to the Central Fire, it follows that the Counter-Earth must be orbiting 180 degrees from Earth.
A planet orbiting the Sun so that it was always on the other side of the Sun from Earth could (in theory) have such an orbit because it was the same distance from the Sun and had the same mass as Earth. Thus, what would make it undetectable to astronomers (or any other human beings) on Earth would also make it habitable to beings at least similar to humans. With the same size and distance from the Sun as Earth, it could have the same (or very similar) surface environment—gravity, atmospheric pressure, and surface temperature range. At the same time such a planet could have the same orbiting velocity and path as Earth, so that if it was positioned 180 degrees from Earth, it would remain behind the Sun being blocked from view from Earth indefinitely.
Phaeton (or Phaëton) was the hypothetical planet theorized by the Titius–Bode law to have existed between the orbits of Mars and Jupiter, the destruction of which supposedly led to the formation of the asteroid belt (including the dwarf planet Ceres). The hypothetical planet was named for Phaethon, the son of the sun god Helios in Greek mythology, who attempted to drive his father's solar chariot for a day with disastrous results and was ultimately destroyed by Zeus.
According to the now-discredited Titius–Bode law, a planet was believed to exist between Mars and Jupiter. After learning of the regular sequence discovered by the German astronomer and mathematician J.D. Titius (1729–1796), astronomer Johann E. Bode urged a search for the fifth planet corresponding to a gap in the sequence. Ceres, the largest asteroid in the asteroid belt (now considered a dwarf planet), was serendipitously discovered in 1801 by the Italian Giuseppe Piazzi and found to closely match the "empty" position in Titius' sequence, which led many to believe it to be the "missing planet". However, in 1802 astronomer Heinrich W.M. Olbers discovered and named the asteroid Pallas, a second object in roughly the same orbit as Ceres.
In 1927, Kugler wrote a short book (56 pp.) titled ''The Sybilline Battle of the Stars and Phaeton Seen as Natural History''. The central idea in Kugler’s book is that the myth of Phaethon was based on a real event: Making use of ancient sources, Kugler argued that Phaeton had been a very bright celestial object that appeared around 1500 BC which fell to Earth not long afterwards as a shower of large meteorites, causing catastrophic fires and floods in Africa and elsewhere.
3. Planet X
Separately, in 1908, Pickering announced that, by analysing irregularities in Uranus's orbit, he had found evidence for a ninth planet. His hypothetical planet, which he termed "Planet O" (because it came after "N", i.e. Neptune), possessed a mean orbital radius of 51.9 AU and an orbital period of 373.5 years.Plates taken at his observatory in Arequipa, Peru, showed no evidence for the predicted planet, and British astronomer P. H. Cowell showed that the irregularities observed in Uranus's orbit virtually disappeared once the planet's displacement of longitude was taken into account. Lowell himself, despite his close association with Pickering, dismissed Planet O out of hand, saying, "This planet is very properly designated "O", is nothing at all." Unbeknownst to Pickering, four of the photographic plates taken in the search for "Planet O" by astronomers at the Mount Wilson Observatory in 1919 captured images of Pluto, though this was only recognised years later. Pickering went on to suggest many other possible trans-Neptunian planets up to the year 1932, which he named P, Q, R, S, T and U; none were ever detected.
Vulcan is a small hypothetical planet that was proposed to exist in an orbit between Mercury and the Sun. Attempting to explain peculiarities of Mercury's orbit, the 19th-century French mathematician Urbain Le Verrier hypothesized that they were the result of another planet, which he named "Vulcan".
In 1840, François Arago, the director of the Paris Observatory, suggested to the French mathematician Urbain Le Verrier that he work on the topic of the planet Mercury's orbital motion around the Sun. The goal of this study was to construct a model based on Sir Isaac Newton's laws of motion and gravitation. By 1843, Le Verrier published his provisional theory on the subject, which would be tested during a transit of Mercury across the face of the Sun in 1843. As it turned out, predictions from Le Verrier's theory failed to match the observations.
Le Verrier postulated that the excess precession could be explained by the presence of a small planet inside the orbit of Mercury, and he proposed the name "Vulcan" for this object. In Roman mythology, Vulcan was the god of beneficial and hindering fire, including the fire of volcanoes, making it an apt name for a planet so close to the Sun. Le Verrier's recent success in discovering the planet Neptune using the same techniques lent veracity to his claim, and astronomers around the world attempted to observe a new planet there, but nothing was ever found.
5. Planet Nine
As of September 2019, no observation of Planet Nine had been announced. While sky surveys such as Wide-field Infrared Survey Explorer (WISE) and Pan-STARRS did not detect Planet Nine, they have not ruled out the existence of a Neptune-diameter object in the outer Solar System. The ability of these past sky surveys to detect Planet Nine were dependent on its location and characteristics. Further surveys of the remaining regions are ongoing using NEOWISE and the 8-meter Subaru Telescope. Unless Planet Nine is observed, its existence is purely conjectural. Several alternative theories have been proposed to explain the observed clustering of TNOs.
Theia is a hypothesized ancient planet in the early Solar System that, according to the 'giant-impact hypothesis', collided with Gaia (the early Earth) around 4.5 billion years ago. According to the hypothesis, Theia was an Earth trojan about the size of Mars, with a diameter of about 6,102 km (3,792 miles). Geologist Edward Young of the University of California, Los Angeles, drawing on an analysis of rocks collected by Apollo missions 12, 15, and 17, proposes that Theia collided head-on with Earth, in contrast to the previous theory that suggested a glancing impact. Models of the impact indicate that Theia's debris gathered around Earth to form the early Moon.
According to the giant-impact hypothesis, Theia orbited the Sun, nearly along the orbit of the proto-Earth, by staying close to one or the other of the Sun–Earth system's two more stable Lagrangian points (i.e. either L4 or L5). Theia was eventually perturbed away from that relationship by the gravitational influence of Jupiter and/or Venus, resulting in a collision between Theia and Earth
Tyche is a hypothetical gas giant located in the Solar System's Oort cloud, first proposed in 1999 by astrophysicists John Matese, Patrick Whitman and Daniel Whitmire of the University of Louisiana at Lafayette. They argued that evidence of Tyche's existence could be seen in a supposed bias in the points of origin for long-period comets. More recently, Matese and Whitmire re-evaluated the comet data and noted that Tyche, if it existed, would be detectable in the archive of data that was collected by NASA's Wide-field Infrared Survey Explorer (WISE) telescope. In 2014, NASA announced that the WISE survey had ruled out any object with Tyche's characteristics, indicating that Tyche as hypothesized by Matese, Whitman, and Whitmire does not exist.