In the

history of astronomy Astronomy is the oldest of the natural sciences, dating back to antiquity, with its origins in the religious, mythological, cosmological, calendrical, and astrological beliefs and practices of prehistory: vestiges of these are still found i ...

, a handful of

Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...

bodies have been counted as the fifth planet from the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared rad ...

. Under the present definition of a

planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a ...

Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandt ...

is counted as the fifth.

Hypotheses

There are three main ideas regarding hypothetical planets between Mars and Jupiter.

Asteroids

During the early 19th century, as

asteroids An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...

were discovered, they were considered planets.

became the sixth planet with the discovery of Ceres in 1801. Soon, three more asteroids,

Pallas Pallas may refer to: Astronomy * 2 Pallas asteroid ** Pallas family, a group of asteroids that includes 2 Pallas * Pallas (crater), a crater on Earth's moon Mythology * Pallas (Giant), a son of Uranus and Gaia, killed and flayed by Athena * Pal ...

(1802),

Juno Juno commonly refers to: * Juno (mythology), the Roman goddess of marriage and queen of the gods * ''Juno'' (film), 2007 Juno may also refer to: Arts, entertainment and media Fictional characters *Juno, in the film ''Jenny, Juno'' *Juno, in the ...

(1804), and

Vesta Vesta may refer to: Fiction and mythology * Vesta (mythology), Roman goddess of the hearth and home * Vesta (Marvel Comics), a Marvel Comics character * Sailor Vesta, a character in ''Sailor Moon'' Brands and products * Lada Vesta, a car from ...

(1807) were discovered. They were counted as separate planets, despite the fact that they share a single orbital spacing given by

Titius–Bode law The Titius–Bode law (sometimes termed just Bode's law) is a formulaic prediction of spacing between planets in any given solar system. The formula suggests that, extending outward, each planet should be approximately twice as far from the Sun as ...

. Between 1845 and 1851, eleven additional asteroids were discovered and Jupiter had become the twentieth planet. At this point, astronomers began to classify asteroids as

minor planets According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''minor ...

. Following the reclassification of the asteroids in their own group, Jupiter became the fifth planet once again. With the redefinition of the term ''planet'' in August 2006, Ceres is now considered a

dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to ...

Disruption theory

The disruption theory suggests that a planet which was positioned between

Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin atmos ...

and

was destroyed, resulting in the

asteroid belt The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...

between these planets. Scientists in the 20th century dubbed this

hypothetical planet Various unknown astronomical objects have been hypothesized throughout recorded history. For example, in the 5th century BCE, the philosopher Philolaus defined a hypothetical astronomical object which he called the " Central Fire", around whic ...

" Phaeton". Today, the Phaeton hypothesis, superseded by the accretion model, has been discarded by the scientific community; however, some fringe scientists regard this theory as credible and even likely.

Planet V theory

Based on simulations, NASA space scientists John Chambers and

Jack J. Lissauer Jack Jonathan Lissauer (born 1957) is an American research scientist who has worked for NASA's Ames Research Center since 1996. He is a science co-investigator on the Kepler space telescope mission. Biography Lissauer received a PhD in mathemat ...

have proposed the existence of a planet between Mars and the asteroid belt, going in a successively eccentric and unstable orbit, 4 billion years ago. They connect this planet, which they name Planet V, and its disappearance with the

Late Heavy Bombardment The Late Heavy Bombardment (LHB), or lunar cataclysm, is a hypothesized event thought to have occurred approximately 4.1 to 3.8 billion years (Ga) ago, at a time corresponding to the Neohadean and Eoarchean eras on Earth. According to the hypoth ...

episode of the

Hadean The Hadean ( ) is a geologic eon of Earth history preceding the Archean. On Earth, the Hadean began with the planet's formation about 4.54 billion years ago (although the start of the Hadean is defined as the age of the oldest solid material ...

era. Chambers and Lissauer also claim this Planet V most probably ended up crashing into the Sun. Unlike the disruption theory's fifth planet, "Planet V" is not credited with creating the asteroid belt.

Fifth planet in fiction

The concept of a fifth planet which had been destroyed to make the asteroid belt, as in the Disruption Theory, has been a popular one in fiction.

Notes

References

* {{cite book, title=Catastrophism and the Old Testament: The Mars-Earth Conflicts , first= Donald W. , last= Patten , place= Seattle, WA , publisher= Pacific Meridian , date= 1988 , oclc= 18757674 Ancient astronomy Early scientific cosmologies Hypothetical bodies of the Solar System Hypothetical planets Ceres (dwarf planet)