7 Hypothetical Solar System Planets

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7 Hypothetical Solar System Planets

Saturday, October 12, 2019
A hypothetical Solar System object is a planet, natural satellite, moonmoon or similar body in the Solar System whose existence is not known, but has been inferred from observational scientific evidence. Over the years a number of hypothetical planets have been proposed, and many have been disproved. However, even today there is scientific speculation about the possibility of planets yet unknown that may exist beyond the range of our current knowledge.

1. Counter-Earth

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).
Along with the Central Fire, the "mysterious" Counter-Earth (Antichthon) was the other heavenly body not visible from Earth. We know that Aristotle described it as "another Earth", from which Greek scholar George Burch infers that it must be similar in size, shape and constitution to Earth. Some (astronomer John Louis Emil Dreyer) think Philolaus had it following an orbit so that it was always located between Earth and the Central Fire, and a tale of Greek mythology may have placed it in that location to stop man from looking at the throne of Zeus directly.

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.

2. Phaeton

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 1823, German linguist and retired teacher J.G. Radlof called Olbers' destroyed planet Phaëthon, linking it to the Greek myths and legends about Phaethon and others. His ideas were similar to those later advocated by Immanuel Velikovsky. Despite Radlof's precedence, Russian authors of the 20th century claimed that, "The hypothetical planet of Olbers' was left nameless for a century and a half. Only in 1949 did the well-known Soviet astronomer Sergej Vladirimovich Orlov give it the name Phaeton."

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

In 1894, with the help of William Pickering, Percival Lowell, a wealthy Bostonian, founded the Lowell Observatory in Flagstaff, Arizona. In 1906, convinced he could resolve the conundrum of Uranus's orbit, he began an extensive project to search for a trans-Neptunian planet, which he named Planet X, a name previously used by Gabriel Dallet. The X in the name represents an unknown and is pronounced as the letter, as opposed to the Roman numeral for 10 (at the time, Planet X would have been the ninth planet). Lowell's hope in tracking down Planet X was to establish his scientific credibility, which had eluded him due to his widely derided belief that channel-like features visible on the surface of Mars were canals constructed by an intelligent civilization.
Lowell's first search focused on the ecliptic, the plane encompassed by the zodiac where the other planets in the Solar System lie. Using a 5-inch photographic camera, he manually examined over 200 three-hour exposures with a magnifying glass, and found no planets. At that time Pluto was too far above the ecliptic to be imaged by the survey. After revising his predicted possible locations, Lowell conducted a second search from 1914 to 1916. In 1915, he published his Memoir of a Trans-Neptunian Planet, in which he concluded that Planet X had a mass roughly seven times that of Earth—about half that of Neptune — and a mean distance from the Sun of 43 AU. He assumed Planet X would be a large, low-density object with a high albedo, like the giant planets. As a result, it would show a disc with diameter of about one arcsecond and an apparent magnitude of between 12 and 13 — bright enough to be spotted.

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.

4. Vulcan

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 renewed his work and, in 1859, published a more thorough study of Mercury's motion. This was based on a series of meridian observations of the planet as well as 14 transits. The rigor of this study meant that any differences from observation would be caused by some unknown factor. Indeed, there still remained some discrepancy. During Mercury's orbit, its perihelion advances by a small amount each orbit, technically called perihelion precession. The phenomenon is predicted by classical mechanics, but the observed value differed from the predicted value by the small amount of 43 arcseconds per century.

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 

Planet Nine is a hypothetical planet in the outer region of the Solar System. Its gravitational effects could explain the unusual clustering of orbits for a group of extreme trans-Neptunian objects (eTNOs), bodies beyond Neptune that orbit the Sun at distances averaging more than 250 times that of the Earth. These eTNOs tend to make their closest approaches to the Sun in one sector, and their orbits are similarly tilted. These improbable alignments suggest that an undiscovered planet may be shepherding the orbits of the most distant known Solar System objects.
This hypothetical super-Earth-sized planet would have a predicted mass of five to ten times that of the Earth, and an elongated orbit 400 to 800 times as far from the Sun as the Earth. Konstantin Batygin and Michael E. Brown suggest that Planet Nine could be the core of a giant planet that was ejected from its original orbit by Jupiter during the genesis of the Solar System. Others propose that the planet was captured from another star, was once a rogue planet, or that it formed on a distant orbit and was pulled into an eccentric orbit by a passing star.

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.

6. Theia 

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.
Some scientists think the material thrown into orbit originally formed two moons that later merged to form the single moon we know today. The Theia hypothesis also explains why Earth's core is larger than would be expected for a body its size: according to the hypothesis, Theia's core and mantle mixed with Earth's.
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

7. Tyche

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.
Matese, Whitmire and their colleague Patrick Whitman first proposed the existence of this planet in 1999, based on observations of the orbits of long-period comets. Most astronomers agree that long-period comets (those with orbits of thousands to millions of years) have a roughly isotropic distribution; that is, they arrive at random from every point in the sky. Because comets are volatile and dissipate over time, astronomers suspect that they must be held in a spherical cloud tens of thousands of AU distant (known as the Oort cloud) for most of their existence. However, Matese and Whitmire claimed that rather than arriving from random points across the sky as is commonly thought, comet orbits were in fact clustered in a band inclined to the orbital plane of the planets. Such clustering could be explained if they were disturbed by an unseen object at least as large as Jupiter, possibly a brown dwarf, located in the outer part of the Oort cloud. They also suggested that such an object might explain the trans-Neptunian object Sedna's peculiar orbit. However, the sample size of Oort comets was small and the results were inconclusive.