Photograph of the planet Neptune and its moon Triton, taken by Voyager 2 as it entered the outer Solar System.

From its beginnings denoting the "wandering stars" of the classical world, the definition of "planet" has been fraught with ambiguity. In its long life, the word has meant many different things, often simultaneously. Over the millennia, use of the term was never strict and its meaning has twisted and blurred to include or exclude a variety of different objects, from the Sun to moons to asteroids. As knowledge of the universe grew, the word "planet" grew and changed with it, casting off old meanings and adopting new ones, though never arriving at a single, concrete definition. Image File history File links Voyager_2_Neptune_and_Triton. ... Image File history File links Voyager_2_Neptune_and_Triton. ... For other uses, see Neptune (disambiguation). ... Triton (trye-tÉ™n, IPA: , Greek Τρίτων), or Neptune I, is the planet Neptunes largest moon. ... Trajectory Voyager 2 is an unmanned interplanetary spacecraft, launched on August 20, 1977. ... The outer solar system (as opposed to the outer planets) is that part of the Solar System which begins at roughly the orbit of Neptune and terminates at maximum orbit distance, approximately one Light Year from the sun in terms of orbital measurements. ... This article is about the astronomical term. ...

By the end of the 19th century, the word "planet" had, without being defined, settled into a comfortable working term. It only applied to objects in the Solar System; a number small enough that any differences could be dealt with on an individual basis. After 1992 however, astronomers began to discover many additional objects beyond the orbit of Neptune, as well as hundreds of objects orbiting other stars. These discoveries not only increased the number of potential planets, but also expanded their variety and peculiarity. Some were nearly large enough to be stars, while others were smaller than Earth's moon. These discoveries challenged long perceived notions of what a planet could be. This article is about the Solar System. ... For other uses, see Neptune (disambiguation). ... An extrasolar planet, or exoplanet, is a planet beyond the Solar System. ... This article is about the astronomical object. ... This article is about Earths moon. ...

The issue of a clear definition for "planet" came to a head in 2005 with the discovery of the trans-Neptunian object Eris, a body larger than the smallest then-accepted planet, Pluto. In its 2006 response, the International Astronomical Union (IAU), recognised by astronomers as the world body responsible for resolving issues of nomenclature, released its decision on the matter. This definition, which applies only to the Solar System, states that a planet is a body that orbits the Sun, is large enough for its own gravity to make it round, and has "cleared its neighbourhood" of smaller objects. Under this new definition, Pluto does not qualify as a planet. The IAU's decision has not resolved all controversies, and while many scientists have accepted the definition, some in the astronomical community have rejected it outright. A trans-Neptunian object (TNO) is any object in the solar system that orbits the sun at a greater distance on average than Neptune. ... Absolute magnitude: −1. ... For other uses, see Pluto (disambiguation). ... IAU redirects here. ... The final definition left the solar system with eight planets. ... Sol redirects here. ... In the end stages of planet formation, a planet will have cleared the neighbourhood of its own orbital zone, meaning it has become gravitationally dominant, and there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence. ...

History

See also: Geocentric model, Heliocentrism, Celestial spheres, and Naked eye planet

This article is about the historical term. ... Heliocentric Solar System Heliocentrism (lower panel) in comparison to the geocentric model (upper panel) In astronomy, heliocentrism is the theory that the sun is at the center of the Universe and/or the Solar System. ... The celestial spheres relate to Johannes Keplers work Harmonia Mundi in which he drew together theories from the world of music, architecture, planetary motion and astronomy and linked them together to form an idea of a harmony and cohesion underlying all world phenomena and ruled by a divine force. ...

Planets in antiquity

The philosopher Plato

While knowledge of the planets is common to most human civilisations and likely predates history, the word "planet" itself dates to ancient Greece. The Greeks believed the Earth to be stationary and at the centre of the universe in accordance with the geocentric model, and that the objects in the sky, and indeed the sky itself, revolved around it. Greek astronomers employed the term asteres planetai, "wandering stars",^[1][2] to describe those starlike lights in the heavens that moved over the course of the year, in contrast to the asteres aplanis, the "fixed stars", which stayed motionless relative to one another. The five bodies currently called "planets" that were known to the Greeks are Mercury, Venus, Mars, Jupiter, and Saturn. Image File history File links Platon-2. ... Image File history File links Platon-2. ... For other uses, see Plato (disambiguation). ... The term ancient Greece refers to the periods of Greek history in Classical Antiquity, lasting ca. ... This article is about the historical term. ... This article is about the planet. ...

Graeco-Roman cosmology commonly considered seven planets, with the Sun and the Moon counted among them (as is the case in modern astrology); however, there is some ambiguity on that point, as many ancient astronomers distinguished the five starlike planets from the Sun and Moon. As the 19th century German naturalist Alexander von Humboldt noted in his work Cosmos, Planets in astrology have a different meaning to the modern astronomical understanding of what a planet is. ... An 1859 portrait of Alexander von Humboldt by the artist Julius Schrader, showing Mount Chimborazo in the background. ...

Of the seven cosmical bodies which, by their continually varying relative positions and distances apart, have ever since the remotest antiquity been distinguished from the "unwandering orbs" of the heaven of the "fixed stars", which to all sensible appearance preserve their relative positions and distances unchanged, five only -Mercury, Venus, Mars, Jupiter and Saturn- wear the appearance of stars- "cinque stellas errantes"- while the Sun and Moon, from the size of their disks, their importance to man, and the place assigned to them in mythological systems, were classed apart.^[3]

The planets as understood before the acceptance of the heliocentric model.

In his Timaeus, written in roughly 360 BC, Plato mentions, "the Sun and Moon and five other stars, which are called the planets".^[4] His student Aristotle makes a similar distinction in his On the Heavens: "The movements of the sun and moon are fewer than those of some of the planets".^[5] In his Phaenomena, which set to verse an astronomical treatise written by the philosopher Eudoxus in roughly 350 BC,^[6] the poet Aratus describes "those five other orbs, that intermingle with [the constellations] and wheel wandering on every side of the twelve figures of the Zodiac."^[7] Image File history File linksMetadata Download high resolution version (6001x5022, 4200 KB) Summary See also, the original picture. ... Image File history File linksMetadata Download high resolution version (6001x5022, 4200 KB) Summary See also, the original picture. ... In astronomy, heliocentrism is the theory that the Sun is at the center of the Universe and/or the Solar System. ... Timaeus (Greek: Τίμαιος, Timaios) is a theoretical treatise of Plato in the form of a Socratic dialogue, written circa 360 BC. The work puts forward speculation on the nature of the physical world. ... For other uses, see Plato (disambiguation). ... For other uses, see Aristotle (disambiguation). ... On the Heavens (or De Caelo) is Aristotles chief cosmological treatise: it contains his astronomical theory. ... Not to be confused with Eudoxus of Cyzicus. ... Aratus (Greek Aratos) (ca. ...

In his Almagest written in the 2nd century, Ptolemy refers to "the Sun, Moon and five planets."^[8] Hyginus explicitly mentions "the five stars which many have called wandering, and which the Greeks call Planeta."^[9] Marcus Manilius, a Latin writer who lived during the time of Caesar Augustus and whose poem Astronomica is considered one of the principal texts for modern astrology, says, "Now the dodecatemory is divided into five parts, for so many are the stars called wanderers which with passing brightness shine in heaven."^[10] Almagest is the Latin form of the Arabic name (al-kitabu-l-mijisti, i. ... This article is about the geographer and astronomer Ptolemy. ... Gaius Julius Hyginus, (c. ... Marcus Manilius (fl. ... The famous statue of Octavian at the Prima Porta Caesar Augustus (Latin:IMP·CAESAR·DIVI·F·AVGVSTVS) ¹ (23 September 63 BC–19 August AD 14), known to modern historians as Octavian for the period of his life prior to 27 BC, is considered the first and one of the most... Hand-coloured version of the anonymous Flammarion woodcut (1888). ...

The single view of the seven planets is found in Cicero's Dream of Scipio, written sometime around 53 BC, where the spirit of Scipio Africanus proclaims, "Seven of these spheres contain the planets, one planet in each sphere, which all move contrary to the movement of heaven."^[11] In his Natural History, written in 77 AD, Pliny the Elder refers to "the seven stars, which owing to their motion we call planets, though no stars wander less than they do."^[12] Nonnus, the 5th century Greek poet, says in his Dionysiaca, "I have oracles of history on seven tablets, and the tablets bear the names of the seven planets."^[9] For other uses, see Cicero (disambiguation). ... The Dream of Scipio (Latin, Somnium Scipionis) is a dream-vision by the Roman philosopher Cicero in which Scipio Aemilianus Africanus meets his grandfather by adoption, Scipio Africanus Major (236 BC - 184 BC), hero of the Second Punic War against Hannibals Carthage. ... Publius Cornelius Scipio Africanus Major (Latin: P·CORNELIVS·P·F·L·N·SCIPIO·AFRICANVS¹) (235–183 BC) was a general in the Second Punic War and statesman of the Roman Republic. ... Naturalis Historia, 1669 edition, title page. ... Pliny the Elder: an imaginative 19th Century portrait. ... The Greek epic poet Nonnus (Greek Nonnos), a native of Panopolis (Akhmim) in the Egyptian Thebaid, probably lived at the end of the 4th or the beginning of the 5th century AD. He produced the Dionysiaca, an epic tale of the god Dionysus, a paraphrase of the Gospel of John... Nonnus, Greek epic poet, a native of Panopolis (Akhmim) in the Egyptian Thebaid, probably lived at the end of the 4th or the beginning of the 5th century AD. His principal work is the Dionysiaca, an epic in forty-eight books, the main subject of which is the expedition of...

Planets in the Middle Ages

Copernicus

Medieval and Renaissance writers generally accepted the idea of seven planets. The standard medieval introduction to astronomy, Sacrobosco's De Sphaera, includes the Sun and Moon among the planets,^[13] the more advanced Theorica planetarum presents the "theory of the seven planets,"^[14] while the instructions to the Alfonsine Tables show how "to find by means of tables the mean motuses of the sun, moon, and the rest of the planets."^[15] In his Confessio Amantis, 14th century poet John Gower, referring to the planets' connection with the craft of alchemy, writes, "Of the planetes ben begonne/The gold is tilted to the Sonne/The Mone of Selver hath his part...", indicating that the Sun and the Moon were planets.^[16] Even Nicolaus Copernicus, who rejected the geocentric model, was ambivalent concerning whether the Sun and Moon were planets. In his De Revolutionibus, Copernicus clearly separates "the sun, moon, planets and stars";^[17] however, in his Dedication of the work to Pope Paul III, Copernicus refers to, "the motion of the sun and the moon... and of the five other planets."^[18] Image File history File links Download high resolution version (556x648, 99 KB) de: Nikolaus Kopernikus (Portrait aus Thorn - Beginn des 16. ... Image File history File links Download high resolution version (556x648, 99 KB) de: Nikolaus Kopernikus (Portrait aus Thorn - Beginn des 16. ... Johannes de Sacrobosco or Sacro Bosco (John of Holywood, c. ... De sphaera mundi (Latin meaning Of the Spheres of Worlds, sometimes rendered The Sphere of the Cosmos; the Latin title is also given as Tractatus de sphaera, or simply De sphaera) is a medieval astronomy textbook written by Johannes de Sacrobosco c. ... The Alfonsine tables were astronomical tables drawn up at Toledo by order of Alfonso X in 1252 to correct the anomalies in the Ptolemaic tables; they divided the year into 365 days, 5 hours, 49 minutes, 16 seconds. ... Confessio Amantis (The Lovers Confession) is a 33,000-line Middle English poem by John Gower, which uses the confession made by an ageing lover to the chaplain of Venus as a frame story for a collection of shorter narrative poems. ... John Gower shooting the world, a sphere of earth, air, and water (from an edition of his works c. ... Extract and symbol key from 17th century alchemy text. ... Copernicus redirects here. ... Nicolai Copernici Torinensis De Revolutionibus Orbium Coelestium, Libri VI (On the Revolutions of the Heavenly Spheres, by Nicolaus Copernicus of Toruń, Six Books: title page of 2nd edition, Basel, 1566). ...

Modern planets

William Herschel, discoverer of Uranus

Eventually, when Copernicus's heliocentric model was accepted over the geocentric, Earth was placed among the planets and the Sun and Moon were demoted, necessitating a conceptual revolution in the understanding of planets. As the historian of science, Thomas Kuhn, noted in his book, The Structure of Scientific Revolutions:^[19] Image File history File linksMetadata William_Herschel01. ... Image File history File linksMetadata William_Herschel01. ... In astronomy, heliocentrism is the theory that the Sun is at the center of the Universe and/or the Solar System. ... The geocentric model (in Greek: geo = earth and centron = centre) of the universe is a paradigm which places the Earth at its center. ... This article is about Earth as a planet. ... Science is a body of empirical and theoretical knowledge, produced by a global community of researchers, making use of specific techniques for the observation and explanation of real phenomena, this techne summed up under the banner of scientific method. ... Thomas Samuel Kuhn (July 18, 1922 – June 17, 1996) was an American intellectual who wrote extensively on the history of science and developed several important notions in the philosophy of science. ... To meet Wikipedias quality standards, this article or section may require cleanup. ...

The Copernicans who denied its traditional title 'planet' to the sun ... were changing the meaning of 'planet' so that it would continue to make useful distinctions in a world where all celestial bodies ... were seen differently from the way they had been seen before... Looking at the moon, the convert to Copernicanism ... says, 'I once took the moon to be (or saw the moon as) a planet, but I was mistaken.'

It could be said, therefore, that Earth was the first planet of the modern era.

In 1781, the astronomer William Herschel was searching the sky for elusive stellar parallaxes, when he observed what he termed a comet in the constellation of Taurus. Unlike stars, which remained mere points of light even under high magnification, this object's size increased in proportion to the power used. That this strange object might have been a planet simply did not occur to Herschel; the five planets beyond Earth had been part of humanity's conception of the universe since antiquity. However, unlike a comet, this object's orbit was nearly circular and within the ecliptic plane. Before Herschel announced his discovery of his "comet", his colleague, British Astronomer Royal Nevil Maskelyne, wrote to him, saying, "I don't know what to call it. It is as likely to be a regular planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any coma or tail to it."^[20] The "comet" was also very far away, too far away for a mere comet to resolve itself. Eventually it was recognised as the seventh planet and named Uranus after the father of Saturn. For other persons named William Herschel, see William Herschel (disambiguation). ... Parallax (Greek: παραλλαγή = alteration) is the change of angular position of two stationary points relative to each other as seen by an observer, due to the motion of said observer. ... Comet Hale-Bopp Comet West For other uses, see Comet (disambiguation). ... Taurus (IPA: , Latin: , symbol , ) is one of the constellations of the zodiac. ... Astronomer Royal is a senior post in the Royal Household of the Sovereign of the United Kingdom. ... Nevil Maskelyne. ... For other uses, see Uranus (disambiguation). ...

Gravitationally induced irregularities in Uranus's observed orbit led eventually to the discovery of Neptune in 1846, and presumed irregularities in Neptune's orbit subsequently led to the search which ultimately located Pluto in 1930. Initially believed to be roughly the mass of the Earth, observation gradually shrank Pluto's estimated mass until it was revealed to be a mere five hundredth as large; far too small to have influenced Neptune's orbit at all.^[21] In 1989, Voyager 2 determined the irregularities to be due to an overestimation of Neptune's mass.^[22] For other uses, see Neptune (disambiguation). ... For other uses, see Pluto (disambiguation). ... Trajectory Voyager 2 is an unmanned interplanetary spacecraft, launched on August 20, 1977. ...

Satellites

Christiaan Huygens, discoverer of Titan

When Copernicus placed the Earth among the planets, he also placed the Moon in orbit around the Earth, making the Moon the first natural satellite to be discovered. When Galileo discovered his four satellites of Jupiter in 1610, they lent weight to Copernicus's argument, since if other planets could have satellites, then the Earth could too. However, there remained some confusion as to whether these objects were "planets"; Galileo initially intended to name them the "Medicean planets", in honour of his patrons, the Medicis.^[23] Similarly, Christiaan Huygens, upon discovering Saturn's largest moon Titan in 1655, employed many terms to describe it, including "planeta", (planet) "stella" (star) "Luna" (moon), and the more modern "satellite".^[24] Giovanni Cassini, in announcing his discovery of Saturn's moons Iapetus and Rhea in 1671 and 1672, described them as Nouvelles Planetes autour de Saturne ("New planets around Saturn").^[25] However, when the "Journal de Scavans" reported Cassini's discovery of two new Saturnian moons in 1686, it referred to them strictly as "satellites".^[26] When William Herschel announced his discovery of two objects in orbit around Uranus in 1787, he referred to them as "satellites" and "secondary planets".^[27] All subsequent reports of natural satellite discoveries used the term "satellite" exclusively.^[28] Image File history File links Christiaan_Huygens-painting. ... Image File history File links Christiaan_Huygens-painting. ... A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. ... Galileo redirects here. ... Jupiters 4 Galilean moons, in a composite image comparing their sizes and the size of Jupiter (Great Red Spot visible). ... The Medici family was a powerful and influential Florentine family during the Renaissance, whose wealth and influence initially derived from the textile trade guided by the guild of the Becoming first bankers, and later politicians, clergy and nobles, the Medici attained their greatest prominence during the 15th through 17th centuries... Christiaan Huygens (pronounced in English (IPA): ; in Dutch: ) (April 14, 1629 – July 8, 1698), was a Dutch mathematician, astronomer and physicist; born in The Hague as the son of Constantijn Huygens. ... Titan (, from Ancient Greek Τῑτάν) or Saturn VI is the largest moon of Saturn and the only moon known to have a dense atmosphere. ... Giovanni Domenico (Jean-Dominique) Cassini Giovanni Domenico Cassini (June 8, 1625 - September 14, 1712) was an Italian-French astronomer and engineer. ... Iapetus (eye-ap-ə-təs, IPA , Greek Ιαπετός) is the third-largest moon of Saturn, discovered by Giovanni Domenico Cassini in 1671. ... Atmosphere none Rhea (ree-a, Greek ‘Ρέα) is the second largest moon of Saturn and was discovered in 1672 by Giovanni Domenico Cassini. ...

Minor planets

Giuseppe Piazzi, discoverer of Ceres

One of the unexpected results of William Herschel's discovery of Uranus was that it appeared to validate Bode's law, a mathematical function which generates the size of the semimajor axis of planetary orbits. Astronomers had considered the "law" a meaningless coincidence, but Uranus fell at very nearly the exact distance it predicted. Since Bode's law also predicted a body between Mars and Jupiter that at that point had not been observed, astronomers turned their attention to that region in the hope that it might be vindicated again. Finally, in 1801, astronomer Giuseppe Piazzi found a miniature new world, Ceres, lying at just the correct point in space. The object was hailed as a new planet.^[29] Download high resolution version (696x885, 182 KB) This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... Download high resolution version (696x885, 182 KB) This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... For other persons named William Herschel, see William Herschel (disambiguation). ... The Titius-Bode law (or Bodes law) is the observation that orbits of planets in the solar system closely follow a simple geometric rule. ... In geometry, the semi-major axis (also semimajor axis) a applies to ellipses and hyperbolas. ... Two bodies with a slight difference in mass orbiting around a common barycenter. ... Giuseppe Piazzi. ... 1 Ceres (IPA , Latin: ) is a dwarf planet in the asteroid belt. ...

Then in 1802, Heinrich Olbers discovered Pallas, a second "planet" at roughly the same distance from the Sun as Ceres. That two planets could occupy the same orbit was an affront to centuries of thinking; even Shakespeare had ridiculed the idea ("Two stars keep not their motion in one sphere").^[30] In 1804, another world, Juno, was discovered in a similar orbit.^[29] In 1807, Olbers discovered a fourth object, Vesta, at a similar orbital distance.^[a] Heinrich Wilhelm Olbers. ... 2 Pallas (pal-us, Greek Παλλάς) was the first asteroid discovered after 1 Ceres. ... Shakespeare redirects here. ... Juno (IPA: ), designated 3 Juno in the Minor Planet Center catalogue system, was the third asteroid to be discovered and is one of the largest main belt asteroids, being the second heaviest of the stony S-type. ... 4 Vesta (ves-ta) is the second most massive asteroid in the asteroid belt, with a mean diameter of about 530 km and an estimated mass 12% the mass of the entire asteroid belt. ...

Herschel suggested that these four worlds be given their own separate classification, asteroids (meaning "starlike" since they were too small for their disks to resolve and thus resembled stars), though most astronomers preferred to refer to them as planets. This conception was entrenched by the fact that, due to the difficulty of distinguishing asteroids from yet-uncharted stars, those four remained the only asteroids known until 1845.^[31] Science textbooks in 1828, after Herschel's death, still numbered the asteroids among the planets. With the arrival of more refined star charts, the search for asteroids resumed, and a fifth and sixth were discovered by Karl Ludwig Hencke in 1845 and 1847.^[31] By 1851 the number of asteroids had increased to 15, and a new method of classifying them, by affixing a number before their names in order of discovery, was adopted, inadvertently placing them in their own distinct category. Ceres became "(1) Ceres", Pallas became "(2) Pallas", and so on. By the 1860s, the number of known asteroids had increased to over a hundred, and observatories in Europe and the United States began referring to them collectively as "minor planets", or "small planets", though it took the first four asteroids longer to be grouped as such.^[29] To this day, "minor planet" remains the official designation for all small bodies in orbit around the Sun (whether asteroid or not), and each new discovery is numbered accordingly in the IAU's Minor Planet Catalogue.^[32] For other uses, see Asteroid (disambiguation). ... This article is about the astronomical object. ... Karl Ludwig Hencke (April 8, 1793 – September 25, 1866) was a German astronomer. ... Minor planets, or asteroids or planetoids, are minor celestial bodies of the Solar system orbiting the Sun (mostly Small solar system bodies) that are smaller than major planets, but larger than meteoroids (commonly defined as being 10 meters across or less[1]), and that are not comets. ... This is a list of numbered minor planets, nearly all of them asteroids, in sequential order. ...

Pluto

Clyde Tombaugh, discoverer of Pluto

The long road from planethood to reconsideration undergone by Ceres is mirrored in the story of Pluto, which was named a planet soon after its discovery by Clyde Tombaugh in 1930. Uranus and Neptune had been declared planets based on their circular orbits, large masses and proximity to the ecliptic plane. None of these applied to Pluto; a tiny, icy world in a region of gas giants with an orbit that carried it high above the ecliptic and even inside that of Neptune. In 1978, astronomers discovered its largest moon, Charon. This allowed astronomers to determine Pluto's size. It was found to be much tinier than expected, smaller even than the Earth's Moon. However, it was, as far as anyone could tell, unique. Then, beginning in 1992, astronomers began to detect large numbers of icy bodies beyond the orbit of Neptune that were similar in composition and size to Pluto. They concluded that they had discovered the long-hypothesised Kuiper belt (sometimes called the Edgeworth-Kuiper belt), a band of icy debris that is the source for "short-period" comets—those with orbital periods of up to 200 years.^[33] Image File history File links Clyde_Tombaugh_image. ... Image File history File links Clyde_Tombaugh_image. ... For other uses, see Pluto (disambiguation). ... An image of Clyde Tombaugh Clyde William Tombaugh (February 4, 1906 – January 17, 1997) was an American astronomer who discovered the dwarf planet Pluto in 1930. ... This article does not cite any references or sources. ... The plane of the ecliptic is well seen in this picture from the 1994 lunar prospecting Clementine spacecraft. ... Charon (pronounced SHAIR-É™n, or KAIR-É™n as in Greek: ), discovered in 1978, is, depending on the definition employed, either the largest moon of Pluto or one member of a double dwarf planet with Pluto being the other member. ... The Kuiper belt, derived from data from the Minor Planet Center. ...

Pluto's orbit lay right in the middle of this band and thus its planetary status was thrown into question; the precedent set by Ceres in downgrading an object from planet status because of a shared orbit led many to conclude that Pluto must be reclassified as a minor planet as well. Mike Brown of the California Institute of Technology suggested that a "planet" should be redefined as "any body in the Solar System that is more massive than the total mass of all of the other bodies in a similar orbit."^[34] The eight planets over that mass limit would be referred to as "major planets". There was outcry at the prospect of Pluto's "demotion", and in 1999 the International Astronomical Union clarified that it was not at that time proposing to change Pluto's status as a planet.^[35][36] Michael (Mike) E. Brown (born c. ... The California Institute of Technology (commonly referred to as Caltech)[1] is a private, coeducational research university located in Pasadena, California, in the United States. ... IAU redirects here. ...

The relative sizes of the largest trans-Neptunian objects as compared to Earth.

The discovery of several other trans-Neptunian objects approaching the size of Pluto, such as Quaoar and Sedna, continued to erode arguments that Pluto was exceptional from the rest of the trans-Neptunian population. On July 29, 2005, Mike Brown and his team announced the discovery of a trans-Neptunian object confirmed to be larger than Pluto,^[37] named Eris.^[38] Image File history File links Download high-resolution version (2750x1995, 1859 KB) Summary Comparison of the eight largest TNOs, based on the public domain NASA image: Image:2006-16-d-print. ... A trans-Neptunian object (TNO) is any object in the solar system that orbits the sun at a greater distance on average than Neptune. ... Quaoar redirects here. ... you are abunch of bull | bgcolour=#FFFFC0 | name=90377 Sedna | image= | caption= Sedna is located in the center of the green circle | discovery=yes | discoverer=M. Brown, C. Trujillo, D. Rabinowitz | discovered=November 14, 2003 | mp_name=90377 Sedna | alt_names= | mp_category=Trans-Neptunian object | epoch=September 26, 1990 (JD 2448160. ... is the 210th day of the year (211th in leap years) in the Gregorian calendar. ... Year 2005 (MMV) was a common year starting on Saturday (link displays full calendar) of the Gregorian calendar. ... Eris (IPA or ), officially designated 136199 Eris, is the largest known dwarf planet in the solar system. ...

In the immediate aftermath of the object's discovery, there was much discussion as to whether it could be termed a "tenth planet". NASA even put out a press release describing it as such.^[39] However, acceptance of Eris as the tenth planet implicitly demanded a definition of planet that set Pluto as an arbitrary minimum size. Many astronomers, claiming that the definition of planet was of little scientific importance, preferred to recognise Pluto's historical identity as a planet by "grandfathering" it into the planet list.^[40] This article does not cite its references or sources. ... In the United States, a grandfather clause is an exception which allows something pre-existing to remain as it is, despite a change to the contrary in the rules applied to newer situations. ...

IAU definition

Main article: 2006 definition of planet

The discovery of Eris forced the IAU to act on a definition. In October 2005, a group of 19 IAU members, which had already been working on a definition since the discovery of Sedna in 2003, narrowed their choices to a shortlist of three, using approval voting. The definitions were: The final definition left the solar system with eight planets. ... Absolute magnitude: −1. ... IAU redirects here. ... you are abunch of bull | bgcolour=#FFFFC0 | name=90377 Sedna | image= | caption= Sedna is located in the center of the green circle | discovery=yes | discoverer=M. Brown, C. Trujillo, D. Rabinowitz | discovered=November 14, 2003 | mp_name=90377 Sedna | alt_names= | mp_category=Trans-Neptunian object | epoch=September 26, 1990 (JD 2448160. ... On an approval ballot, the voter can vote for any number of candidates. ...

• A planet is any object in orbit around the Sun with a diameter greater than 2000 km. (eleven votes in favour)
• A planet is any object in orbit around the Sun whose shape is stable due to its own gravity. (eight votes in favour)
• A planet is any object in orbit around the Sun that is dominant in its immediate neighbourhood. (six votes in favour)^[41][42]

Since no overall consensus could be reached, the committee decided to put these three definitions to a wider vote at the IAU General Assembly meeting in Prague in August 2006,^[43] and on August 24, the IAU put a final draft to a vote, which combined elements from two of the three proposals. It essentially created a medial classification between "planet" and "rock" (or, in the new parlance, "small Solar System body"), called "dwarf planet" and placed Pluto in it, along with Ceres and Eris.^[44][45] The vote was passed, with 424 astronomers taking part in the ballot.^[46] Two bodies with a slight difference in mass orbiting around a common barycenter. ... For other uses, see Prague (disambiguation). ... is the 236th day of the year (237th in leap years) in the Gregorian calendar. ... A Small Solar System Body (SSSB) is a term defined in 2006 by the International Astronomical Union to describe objects in the Solar System that are neither planets nor dwarf planets: [1] This encompasses: all minor planets apart from the dwarf planets, : the classical asteroids, (except for 1 Ceres, the... Artists impression of Pluto (background) and Charon (foreground). ... For other uses, see Pluto (disambiguation). ...

The IAU also resolved that "planets and dwarf planets are two distinct classes of objects", meaning that dwarf planets, despite their name, would not be considered planets.^[47]

On September 13, 2006, the IAU placed Eris, its moon Dysnomia, and Pluto into their Minor Planet Catalogue, giving them the official minor planet designations (134340) Pluto, (136199) Eris, and (136199) Eris I Dysnomia.^[48] is the 256th day of the year (257th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... This is a list of numbered minor planets, nearly all of them asteroids, in sequential order. ...

Acceptance of the definition

Plot of the current positions of all known Kuiper belt objects, set against the outer planets

Among the most vocal proponents of the IAU's decided definition are Mike Brown, the discoverer of Eris, and Steven Soter, professor of astrophysics at the American Museum of Natural History. Image File history File links Size of this preview: 611 × 599 pixelsFull resolution (2708 × 2656 pixel, file size: 314 KB, MIME type: image/png) Legend Red = The Sun Aquamarine = Giant Planet Green = Kuiper Belt Object Orange = Scattered Disk object or Centaur Pink = Trojan of Jupiter Yellow = Trojan of Neptune Axes... Image File history File links Size of this preview: 611 × 599 pixelsFull resolution (2708 × 2656 pixel, file size: 314 KB, MIME type: image/png) Legend Red = The Sun Aquamarine = Giant Planet Green = Kuiper Belt Object Orange = Scattered Disk object or Centaur Pink = Trojan of Jupiter Yellow = Trojan of Neptune Axes... Michael (Mike) E. Brown (born c. ... Steven Soter is an astronomer who argued for a distinction between dwarf planets and the other eight planets based on their inability to clear the neighborhood around their orbits. This article belongs in one or more categories. ... Main Lobby in the Theodore Roosevelt Memorial. ...

In an article in the January 2007 issue of Scientific American, Soter cited the definition's incorporation of current theories of the formation and evolution of the Solar System; that as the earliest protoplanets emerged from the swirling dust of the protoplanetary disc, some bodies "won" the initial competition for limited material and, as they grew, their increased gravity meant that they accumulated more material, and thus grew larger, eventually outstripping the other bodies in the Solar System by a very wide margin. The asteroid belt, disturbed by the gravitational tug of nearby Jupiter, and the Kuiper belt, too widely spaced for its constituent objects to collect together before the end of the initial formation period, both failed to win the accretion competition. January 2007 is the first month of that year. ... Scientific American is a popular-science magazine, published (first weekly and later monthly) since August 28, 1845, making it the oldest continuously published magazine in the United States. ... The theories concerning the formation and evolution of the Solar System are complex and varied, interweaving various scientific disciplines, from astronomy and physics to geology and planetary science. ... Protoplanets are moon-sized planet embryos within protoplanetary discs. ... A protoplanetary disc (also protoplanetary disk, proplyd) is an accretion disc surrounding a T Tauri star. ...

When the numbers for the winning objects are compared to those of the losers, the contrast is quite striking; if we accept Soter's concept that each planet occupies an "orbital zone,"^[b] then the least orbitally dominant planet, Mars, is larger than all other collected material in its orbital zone by a factor of 5100. Ceres, the largest asteroid, is only larger by a factor of 0.33; Pluto's ratio is even lower, at 0.07.^[49] Mike Brown asserts that this massive difference in orbital dominance leaves "absolutely no room for doubt about which objects do and do not belong."^[50]

Ongoing controversies

Despite the IAU's declaration, a number of critics remain unconvinced. The definition is seen by many as arbitrary and confusing, and a number of Pluto-as-planet proponents, in particular Alan Stern, head of NASA's New Horizons mission to Pluto, have circulated a petition among astronomers to alter the definition. His claim is that, since less than 5 percent of astronomers voted for it, the decision was not representative of the entire astronomical community.^[51][52] The issue of what constitutes a planet will likely remain contentious at least until 2009, when the IAU holds its next Congress in Rio de Janeiro.^[53] Even with this controversy excluded however, there remain several ambiguities in the definition. For other uses, see Pluto (disambiguation). ... This page may meet Wikipedias criteria for speedy deletion. ... For other uses, see NASA (disambiguation). ... For other uses, see New Horizons (disambiguation). ... This article is about the Brazilian city. ...

Clearing the neighborhood

Main article: Cleared the neighbourhood

One of the main points at issue is the precise meaning of "cleared the neighbourhood around its orbit". Alan Stern recently objected that "it is impossible and contrived to put a dividing line between dwarf planets and planets,"^[54] and that since neither Earth, Mars, Jupiter, nor Neptune have entirely cleared their regions of debris, none could properly be considered planets under the IAU definition.^[c] In the end stages of planet formation, a planet will have cleared the neighbourhood of its own orbital zone, meaning it has become gravitationally dominant, and there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence. ... Two bodies with a slight difference in mass orbiting around a common barycenter. ... This page may meet Wikipedias criteria for speedy deletion. ... IAU redirects here. ...

The asteroids of the inner Solar System; note the Trojan asteroids (green), trapped into Jupiter's orbit by its gravity

Mike Brown counters these claims by saying that, far from not having cleared their orbits, the major planets completely control the orbits of the other bodies within their orbital zone. Jupiter may coexist with a large number of small bodies in its orbit (the Trojan asteroids), but these bodies only exist in Jupiter's orbit because they are in the sway of the planet's huge gravity. Similarly, Pluto may cross the orbit of Neptune, but Neptune long ago locked Pluto and its attendant Kuiper belt objects, called plutinos, into a 3:2 resonance, i.e., they orbit the Sun twice for every three Neptune orbits. The orbits of these objects are entirely dictated by Neptune's gravity, and thus, Neptune is gravitationally dominant.^[50] Image File history File links Download high-resolution version (768x768, 188 KB)The inner Solar System, from the Sun to Jupiter. ... Image File history File links Download high-resolution version (768x768, 188 KB)The inner Solar System, from the Sun to Jupiter. ... Image of the Trojan asteroids in front of and behind Jupiter along its orbital path. ... Image of the Trojan asteroids in front of and behind Jupiter along its orbital path. ... In astronomy, a plutino is a trans-Neptunian object that has a 3:2 orbital resonance with Neptune. ...

Whatever definition of "clearing the neighborhood" is ultimately accepted by the IAU, it is still an ambiguous concept. Mark Sykes, director of the Planetary Science Institute in Tucson, Arizona and organiser of the petition, explained the ambiguity to National Public Radio. Since the definition does not categorise a planet by composition or formation, but, effectively, by its location, a Mars-sized or larger object beyond the orbit of Pluto would be considered a dwarf planet, since it would not have time to clear its orbit and would therefore be surrounded by objects of similar mass, whereas an object smaller than Pluto orbiting in isolation would be considered a planet.^[55] NPR redirects here. ...

Proteus, the moon of Neptune, is irregular, despite being larger than many spherical objects.

Image File history File links Size of this preview: 597 × 600 pixel Image in higher resolution (836 × 840 pixel, file size: 41 KB, MIME type: image/jpeg) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Proteus (moon) List... Image File history File links Size of this preview: 597 × 600 pixel Image in higher resolution (836 × 840 pixel, file size: 41 KB, MIME type: image/jpeg) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Proteus (moon) List... Atmospheric pressure 0 kPa Proteus (proe-tee-us, Greek Πρωτέας) is one of Neptunes moons. ... For other uses, see Neptune (disambiguation). ...

Hydrostatic equilibrium

The IAU's definition mandates that planets be large enough for their own gravity to form them into a state of hydrostatic equilibrium; this means that they will reach a shape that is, if not spherical, then spheroidal. This distinction, as opposed to strict sphericity, is mandated by the fact that many large objects in the Solar System, such as the planets Jupiter and Saturn, the moons Mimas, Enceladus and Miranda, and the Kuiper belt object 2003 EL61,^[56] have been distorted into oblate or prolate spheroids by rapid rotation or tidal forces. IAU redirects here. ... Gravity is a force of attraction that acts between bodies that have mass. ... Hydrostatic equilibrium occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. ... In mathematics, a spheroid is a quadric surface in three dimensions obtained by rotating an ellipse about one of its principal axes. ... This article is about the Solar System. ... Mimas (mee-məs or mye-məs, IPA: , Greek Μίμᾱς, rarely Μίμανς) is a moon of Saturn that was discovered in 1789 by William Herschel. ... Apparent magnitude 11. ... Miranda (IPA: ) is the smallest and innermost of Uranus five major moons. ... The Kuiper belt (KYE per) is an area of the solar system extending from within the orbit of Neptune (at 30 AU) to 50 AU from the sun, at inclinations consistent with the ecliptic. ... 2003 EL61 (also written 2003 EL61), nicknamed Santa (non-official designation), is a large and very unusual Kuiper belt object discovered by Mike Brown at Caltech in the United States. ... Comet Shoemaker-Levy 9 after breaking up under the influence of Jupiters tidal forces. ...

However, there is no one point at which an object can be said to have reached hydrostatic equilibrium. As Soter noted in his article,"How are we to quantify the degree of roundness that distinguishes a planet? Does gravity dominate such a body if its shape deviates from a spheroid by 10 percent or by 1 percent? Nature provides no unoccupied gap between round and nonround shapes, so any boundary would be an arbitrary choice."^[49] Furthermore, objects made of ices,^[d] such as Enceladus and Miranda, assume that state more easily than those made of rock, such as Vesta and Pallas.^[57] Heat energy, from gravitational collapse, impacts, tidal forces, or radioactive decay also factors into whether an object will be spherical or not; Saturn's icy moon Mimas is spheroidal, but Neptune's larger moon Proteus, which is similarly composed but colder because of its greater distance from the Sun, is irregular. This article or section does not cite its references or sources. ... Artists impression of a major impact event. ... Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. ...

Double planets

Main article: Double planet

A telescopic image of Pluto and Charon.

The definition specifically excludes satellites from the category of dwarf planet, though it does not directly define the term "satellite". In the original draft proposal, an exception was made for Pluto and its largest satellite, Charon, which possess a barycenter outside the volume of either body. The initial proposal classified Pluto/Charon as a double planet, with the two objects orbiting the Sun in tandem. However, the final draft made clear that, double or not, both Pluto and Charon would be considered dwarf planets, not planets. Pluto and Charon are sometimes informally considered to be a double (dwarf) planet. ... Image File history File links Download high resolution version (950x494, 11 KB) File links The following pages link to this file: Definition of planet ... Image File history File links Download high resolution version (950x494, 11 KB) File links The following pages link to this file: Definition of planet ... This article does not cite any references or sources. ... For other uses, see Pluto (disambiguation). ... Charon (pronounced SHAIR-ən, or KAIR-ən as in Greek: ), discovered in 1978, is, depending on the definition employed, either the largest moon of Pluto or one member of a double dwarf planet with Pluto being the other member. ... A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. ... For other uses, see Pluto (disambiguation). ... Charon (pronounced SHAIR-ən, or KAIR-ən as in Greek: ), discovered in 1978, is, depending on the definition employed, either the largest moon of Pluto or one member of a double dwarf planet with Pluto being the other member. ... In physics, the center of mass of a system of particles is a specific point at which, for many purposes, the systems mass behaves as if it was concentrated. ...

Under the same definition, the Earth-Moon system is not formally recognised as a double planet, despite the Moon's large relative size, since the barycenter lies within the Earth. As the Moon is slowly receding from the Earth, the Earth-Moon system may eventually become a double planet system on the basis of this barycentric definition. This article is about Earths moon. ... The orbit of the Moon around the Earth is completed in approximately 27. ...

A diagram illustrating the Moon's co-orbit with the Earth.

However, some have suggested that our Moon nonetheless deserves to be called a planet. In 1975, Isaac Asimov noted that the timing of the Moon's orbit is in tandem with the Earth's own orbit around the Sun — looking down on the ecliptic, the Moon never actually loops back on itself, and in essence it orbits the Sun in its own right.^[58] Image File history File links Moon_trajectory1. ... Image File history File links Moon_trajectory1. ... This article is about Earths moon. ... This article is about Earth as a planet. ... Isaac Asimov (January 2?, 1920?[1] – April 6, 1992), pronounced , originally Исаак Озимов but now transcribed into Russian as Айзек Азимов [1], was a Russian-born American author and professor of biochemistry, a highly successful writer, best known for his works of science fiction and for his popular science books. ... The plane of the ecliptic is well seen in this picture from the 1994 lunar prospecting Clementine spacecraft. ...

Also, many moons, even those that do not orbit the Sun directly, often exhibit features in common with true planets. Jupiter's moon Ganymede and Saturn's moon Titan are both larger in terms of diameter (though not mass) than Mercury, and Titan even has a substantial atmosphere, thicker than the Earth's. Moons such as Io and Triton demonstrate obvious and ongoing geological activity, and Ganymede has a magnetic field. Just as stars in orbit around other stars are still referred to as stars, so some astronomers argue that objects in orbit around planets that share all their characteristics could also be called planets.^[59] Indeed Mike Brown makes just such a claim in his dissection of the issue, noting that there is little case for describing an object 400 km across with little internal geological activity as a planet if a 5000 km object with methane lakes, cryovolcanism and storms (i.e. Titan) is called a moon.^[50] This article is about the natural satellite of Jupiter. ... Titan (, from Ancient Greek Τῑτάν) or Saturn VI is the largest moon of Saturn and the only moon known to have a dense atmosphere. ... This article is about the planet. ... Atmosphere Surface pressure: trace Composition: 90% sulfur dioxide Io (eye-oe, IPA: , Greek Ῑώ) is the innermost of the four Galilean moons of Jupiter and, with a diameter of 3,642 kilometers, is the fourth largest moon in the Solar System. ... Triton (trye-tÉ™n, IPA: , Greek Τρίτων), or Neptune I, is the planet Neptunes largest moon. ... For the indie-pop band, see The Magnetic Fields. ... This article is about the astronomical object. ... Image of the south pole of Triton taken by Voyager 2 in 1989. ... Titan (, from Ancient Greek Τῑτάν) or Saturn VI is the largest moon of Saturn and the only moon known to have a dense atmosphere. ...

Extrasolar planets and brown dwarfs

Main articles: Extrasolar planet and Brown dwarf

The IAU's definition of planet applies only to objects within our own Solar System.^[60] The more than 200 extrasolar planets (planet-sized objects in orbit around other stars) were excluded as too complex an issue to be resolved during the congress. However, any future definition will need to include them, as their discovery has widened the debate on the nature of planethood in unexpected ways. Many of these planets are of considerable size, approachi