artist's depiction of the extrasolar planet HD 209458b orbiting its star

A planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion in its inner core, and has cleared its neighbouring region of planetesimals.^[1][2] Artists impression of Pluto (background) and Charon (foreground). ... Look up planet on Wiktionary, the free dictionary. ... Image File history File links Metadata Size of this preview: 750 × 600 pixelsFull resolution‎ (3,000 × 2,400 pixels, file size: 477 KB, MIME type: image/jpeg) source An artists impression of the planet HD 209458b File historyClick on a date/time to view the file as it appeared... Image File history File links Metadata Size of this preview: 750 × 600 pixelsFull resolution‎ (3,000 × 2,400 pixels, file size: 477 KB, MIME type: image/jpeg) source An artists impression of the planet HD 209458b File historyClick on a date/time to view the file as it appeared... An extrasolar planet, or exoplanet, is a planet beyond the Solar System. ... HD 209458b is an extrasolar planet that orbits the Sun-like star HD 209458 in the constellation Pegasus, some 150 light years from Earths solar system. ... The final definition left the solar system with eight planets. ... IAU redirects here. ... Two bodies with a slight difference in mass orbiting around a common barycenter. ... This article is about the astronomical object. ... Projected timeline of the Suns life In astronomy, stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. ... Gravity is a force of attraction that acts between bodies that have mass. ... In physics, nuclear fusion (a thermonuclear reaction) is a process in which two nuclei join, forming a larger nucleus and releasing energy. ... 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. ... In cosmogony, planetesimals are objects thought to exist within solar nebulae. ...

The term planet is an ancient one having ties to history, science, myth, and religion. The planets were originally seen as a divine presence; as emissaries of the gods. Even today, many people continue to believe the movement of the planets affects their lives, although such a causation is rejected by the scientific community. As scientific knowledge advanced, the human perception of the planets changed over time, incorporating a number of disparate objects. Even now there is no uncontested definition of what a planet is. In 2006, the IAU officially adopted a resolution defining planets within the Solar System. This definition has been both praised and criticized, and remains disputed by some scientists. Hand-coloured version of the anonymous Flammarion woodcut (1888). ... This article needs additional references or sources for verification. ... The final definition left the solar system with eight planets. ... This article is about the Solar System. ...

The planets were initially thought to orbit the Earth in circular motions; after the development of the telescope, the planets were determined to orbit the Sun, and their orbits were found to be elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes and shared such features as ice-caps and seasons. Since the dawn of the space age, close observation by probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics and even hydrology. Since 1992, and the discovery of hundreds of extrasolar planets, scientists are beginning to observe similar features across the galaxy. The Space Shuttle takes off on a manned mission to space. ... Infrared Image of a possible extrasolar planet (lower left) in the Constellation Taurus, taken by the Hubble Space Telescope. ...

Under IAU definitions, there are eight planets in the Solar System (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and also at least three dwarf planets (Ceres, Pluto, and Eris). Many of these planets are orbited by one or more moons, which can be larger than small planets. There have also been more than two hundred planets discovered orbiting other stars.^[3] Planets are generally divided into two main types: large, low-density gas giants and smaller, rocky terrestrials. This article is about the planet. ... For other uses, see Venus (disambiguation). ... This article is about Earth as a planet. ... Adjectives: Martian Atmosphere Surface pressure: 0. ... For other uses, see Jupiter (disambiguation). ... This article is about the planet. ... For other uses, see Uranus (disambiguation). ... For other uses, see Neptune (disambiguation). ... Artists impression of Pluto (background) and its satellite Charon (foreground). ... Spectral type: G[8] Absolute magnitude: 3. ... For other uses, see Pluto (disambiguation). ... Absolute magnitude: −1. ... A natural satellite is an object that orbits a planet or other body larger than itself and which is not man-made. ... An extrasolar planet, or exoplanet, is a planet beyond the Solar System. ... This article does not cite any references or sources. ... The inner planets, Mercury, Venus, Earth, and Mars, their sizes to scale. ...

Etymology

The gods of Olympus, after whom the Solar System's planets are named

In ancient times, astronomers noted how certain lights moved across the sky in relation to the other stars. The lights were first called "πλανήται" (planētai),^[4] meaning "wanderers", by the ancient Greeks, and it is from this that the word "planet" was derived.^[5][6] The Greeks gave the planets names: the farthest was called Phainon, the shiner, while below it was Phaethon, the bright one. The red planet was known as Pyroeis, "fiery", while the brightest was known as Phosphoros, the light bringer, and the fleeting final planet was called Stilbon, the gleamer. However, the Greeks also made each planet sacred to one of their pantheon of gods, the Olympians: Phainon was sacred to Kronos, the Titan who fathered the Olympians, while Phaethon was sacred to Zeus, his son who deposed him as king. Ares, son of Zeus and god of war, was given dominion over Pyroeis, while Aphrodite, goddess of love, ruled over bright Phosphoros, and Hermes ruled over Stilbon.^[7] Image File history File links Olympians. ... Image File history File links Olympians. ... Mount Olympus (Greek: ; also transliterated as Mount Ólympos, and on modern maps, Óros Ólimbos) is the highest mountain in Greece at 2,919 meters high (9,576 feet)[1]. Since its base is located at sea level, it is one of the highest mountains in Europe, in real absolute altitude... Twelve Olympians, also known as the Dodekatheon (Greek: Δωδεκάθεον < δωδεκα, dodeka, twelve + θεον, theon, of the gods), in Greek religion, were the principal gods of the Greek pantheon, residing atop Mount Olympus. ... Cronus is not to be confused with Chronos, the personification of time. ... This article is about the race of Titans in Greek mythology. ... For other uses, see Zeus (disambiguation). ... This article is about the ancient Greek god; for other uses, see Ares (disambiguation). ... The Birth of Venus, (detail) by Sandro Botticelli, 1485 For other uses, see Aphrodite (disambiguation). ... For other uses, see Hermes (disambiguation). ...

The Greek practice of grafting of their gods' names onto the planets was almost certainly borrowed from the Babylonians, a contemporary civilisation in what is now Iraq, from whom they had begun to absorb astronomical learning, including constellations and the zodiac, by 600 BCE.^[8] The Babylonians named Phosphoros after their goddess of love, Ishtar, Pyroeis after their god of war, Nergal, and Phaethon after their chief god, Marduk.^[9] There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately.^[7] There does, however, appear to have been some confusion in translation. For instance, the Babylonian Nergal was a god of war, and the Greeks, seeing this aspect of Nergal's persona, identified him with Ares, their god of war. However, Nergal, unlike Ares, was also a god of the dead and a god of pestilence.^[9] Babylonia was an ancient state in Iraq), combining the territories of Sumer and Akkad. ... The name Nergal (or Nirgal, Nirgali) refers to a deity in Babylonia with the main seat of his cult at Cuthah represented by the mound of Tell-Ibrahim. ... This article is about the ancient Greek god; for other uses, see Ares (disambiguation). ...

Early printed rendition of a geocentric cosmological model.

Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods; however, because of the influence of the Roman Empire and, later, the Catholic Church, they are known by their Roman (or Latin) names, rather than the Greek. The Romans, who, like the Greeks, were Indo-Europeans, shared with them a common pantheon under different names but lacked the rich narrative traditions that Greek poetic culture had given their gods. During the later period of the Roman Republic, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable.^[10] When the Romans studied Greek astronomy, they gave the planets their own gods' names. To the Greeks and Romans, there were five known planets; each presumed to be circling the Earth according to the complex laws laid out by Claudius Ptolemy in the 2nd century. They were, in increasing order from Earth (according to Ptolemy): Mercury (Hermes), Venus (Aphrodite), Mars (Ares), Jupiter (Zeus), and Saturn (Kronos). Although strictly the term "planetai" referred only to those five objects, the term was often expanded to include the Sun and the Moon.^[11] When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained: Uranus (Ouranos) and Neptune (Poseidon). The Greeks still use their original names for the planets. Image File history File links Ptolemaicsystem-small. ... Image File history File links Ptolemaicsystem-small. ... Twelve Olympians, also known as the Dodekatheon (Greek: Δωδεκάθεον < δωδεκα, dodeka, twelve + θεον, theon, of the gods), in Greek religion, were the principal gods of the Greek pantheon, residing atop Mount Olympus. ... For other uses, see Roman Empire (disambiguation). ... The name Catholic Church can mean a visible organization that refers to itself as Catholic, or the invisible Christian Church, viz. ... Proto-Indo-European Indo-European studies The existence of similarities among the gods and religious practices of the Indo-European peoples suggests that whatever population they actually formed had some form of polytheistic religion. ... A head of Minerva found in the ruins of the Roman baths in Bath Roman mythology, the mythological beliefs of the people of Ancient Rome, can be considered as having two parts. ... The bust of Zeus found at Otricoli (Sala Rotonda, Museo Pio-Clementino, Vatican) Greek mythology is the body of stories belonging to the Ancient Greeks concerning their gods and heroes, the nature of the world and the origins and significance of their own cult and ritual practices. ... This article refers to the state which existed from the 6th century BC to the 1st century BC. For alternate meanings, see Roman Republic (18th century) and Roman Republic (19th century). ... This article is about the historical term. ... This article is about the geographer and astronomer Ptolemy. ... This article is about the planet. ... For other uses, see Hermes (disambiguation). ... For other uses, see Venus (disambiguation). ... The Birth of Venus, (detail) by Sandro Botticelli, 1485 For other uses, see Aphrodite (disambiguation). ... Adjectives: Martian Atmosphere Surface pressure: 0. ... This article is about the ancient Greek god; for other uses, see Ares (disambiguation). ... For other uses, see Jupiter (disambiguation). ... For other uses, see Zeus (disambiguation). ... This article is about the planet. ... Cronus is not to be confused with Chronos, the personification of time. ... For other uses, see Uranus (disambiguation). ... For other uses, see Neptune (disambiguation). ... Neptune reigns in the city of Bristol. ...

Some Romans, following a belief imported from Mesopotamia into Hellenistic Egypt,^[12] believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts began with Jupiter and worked inwards; as a result, a list of which god had charge of the first hour in each day became Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn, i.e. the usual weekday name order.^[13] Sunday, Monday, and Saturday are straightforward translations of these Roman names. In English the other days were renamed after Tiw, (Tuesday) Wóden (Wednesday), Thunor (Thursday), and Fríge (Friday), Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus respectively. Ancient Rome was a civilization that grew from a small agricultural community founded on the Italian Peninsula circa the 9th century BC to a massive empire straddling the Mediterranean Sea. ... Mesopotamia was a cradle of civilization geographically located between the Tigris and Euphrates rivers, largely corresponding to modern-day Iraq. ... The conquests of Alexander the Great brought Egypt within the orbit of the Greek world for almost 900 years. ... Týr, depicted here with both hands intact, is identified with Mars in this illustration from an 18th century Icelandic manuscript. ... This is the article about the belief in Odin among West Germanic peoples, for other uses see Woden (disambiguation), Wotan (disambiguation). ... For other uses, see Thor (disambiguation). ... Frige (Anglo-Saxon, Friia (Germany) or Frea (Langobard)) was the love goddess of Germanic mythology, and the wife of Wotan (Odin). ... The Anglo-Saxon gods were cognate to the gods of Norse mythology in particular and of Germanic mythology in general. ...

Since Earth was only generally accepted as a planet in the 17th century, there is no tradition of naming it after a god. Many of the Romance languages (including French, Italian, Spanish and Portuguese), which are descended from Latin, retain the old Roman name of Terra or some variation thereof. However, the non-Romance languages use their own respective native words. Again, the Greeks retain their original name, Γή (Ge or Yi); the Germanic languages, including English, use a variation of an ancient Germanic word ertho, "ground,"^[14] as can be seen in the English Earth, the German Erde, the Dutch Aarde, and the Scandinavian Jorde. The same is true for the Sun and the Moon, though they are no longer considered planets. The Romance languages (sometimes referred to as Romanic languages) are a branch of the Indo-European language family that comprises all the languages that descend from Latin, the language of the Roman Empire. ... The Germanic languages are a group of related languages constituting a branch of the Indo-European (IE) language family. ...

Some non-European cultures use their own planetary naming systems. India uses a naming system based on the Navagraha, which incorporates the seven traditional planets (Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn) and the ascending and descending lunar nodes Rahu and Ketu. China, and the countries of eastern Asia subject to Chinese cultural influence, such as Japan, Korea and Vietnam, use a naming system based on the five Chinese elements.^[13] In Hindu astrology, the Navagraha are the nine chief celestial beings: Surya (Sun) Chandra (Moon) Chevaai (Mars) Budhan (Mercury) Guru (Jupiter) Shukran (Venus) Shani (Saturn) Rahu (Head of Demon Snake) Ketu (Tail of Demon Snake). ... The lunar nodes are the orbital nodes of the Moon, that is, the points where the orbit of the Moon crosses the ecliptic (which is the apparent path of the Sun across the heavens against the background stars). ... In Hindu mythology, Rahu is a snake that swallows the sun or the moon causing eclipses. ... In Hindu mythology, Ketu is generally referred to as a shadow planet. ... This article is about the Korean civilization. ... Chinese Wood (木) | Fire (火) Earth (土) | Metal (金) | Water (水) Japanese Earth (地) | Water (水) | Fire (火) | Air / Wind (風) | Void / Sky / Heaven (空) Hinduism and Buddhism Vayu / Pavan — Air / Wind Agni / Tejas — Fire Akasha — Aether Prithvi / Bhumi — Earth Ap / Jala — Water In traditional Chinese philosophy, natural phenomena can be classified into the Five Elements (Chinese: ; Pinyin: ): wood, fire...

History

See also: List of Solar System bodies formerly regarded as planets

Heliocentrism (lower panel) in comparison to the geocentric model (upper panel)

As scientific knowledge progressed, understanding of the term "planet" changed from something that moved across the sky (in relation to the starfield), to a body that orbited the Earth (or that were believed to do so at the time). When the heliocentric model gained sway in the 16th century, it became accepted that a planet was actually something that directly orbited the Sun. Thus the Earth was itself a planet,^[15] while the Sun and Moon were not. At the end of the 17th century, when the first satellites of Saturn were discovered, the terms "planet" and "satellite" were at first used interchangeably, although "satellite" would gradually become more prevalent in the following century.^[16] Until the mid-19th century, any newly discovered object orbiting the Sun was listed with the planets by the scientific community, and the number of "planets" swelled rapidly towards the end of that period. The table below lists Solar System bodies formerly considered to be planets: ^ Recently (2006) reclassified as a dwarf planet. ... Image File history File linksMetadata Download high resolution version (1132x1048, 486 KB) Summary Licensing File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Heliocentrism Metadata This file contains additional information, probably added from the digital camera or scanner used... Image File history File linksMetadata Download high resolution version (1132x1048, 486 KB) Summary Licensing File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Heliocentrism Metadata This file contains additional information, probably added from the digital camera or scanner used... A fixed star is a celestial object that does not seem to move (in comparison to the other stars of the night sky). ... In astronomy, heliocentrism is the theory that the Sun is at the center of the Universe and/or the Solar System. ... Sol redirects here. ... This article is about Earths moon. ...

During the 1800s, astronomers began to realize most recent discoveries were unlike the traditional planets. They shared the same region of space, between Mars and Jupiter, and had a far smaller mass. Bodies such as Ceres, Pallas, and Vesta, which had been classed as planets for almost half a century, became classified with the new designation "asteroid." From this point, a "planet" came to be understood, in the absence of any formal definition, as any "large" body that orbited the Sun. There was no apparent need to create a set limit, as there was a dramatic size gap between the asteroids and the planets, and the spate of new discoveries seemed to have ended after the discovery of Neptune in 1846.^[17] For other uses, see Asteroid (disambiguation). ... Adjectives: Martian Atmosphere Surface pressure: 0. ... For other uses, see Jupiter (disambiguation). ... Spectral type: G[8] Absolute magnitude: 3. ... 2 Pallas (pal-us, Greek Παλλάς) was the first asteroid discovered after 1 Ceres. ... 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. ... For other uses, see Asteroid (disambiguation). ... For other uses, see Neptune (disambiguation). ...

However, in the 20th century, Pluto was discovered. After initial observations led to the belief it was larger than Earth, the recently-created IAU accepted the object as a planet. Further monitoring found the body was actually much smaller, but, as it was still larger than all known asteroids and seemingly did not exist within a larger population, it kept its status for some seventy years.^[18] For other uses, see Pluto (disambiguation). ... IAU redirects here. ...

In the 1990s and early 2000s, there was a flood of discoveries of similar objects in the same region of the Solar System. Like Ceres and the asteroids before it, Pluto was found to be just one small body in a population of thousands. A growing number of astronomers argued for it to be declassified as a planet, since many similar objects approaching its size were found. The discovery of Eris, a more massive object widely publicised as the tenth planet, brought things to a head. The IAU set about creating the definition of planet, and eventually produced one in 2006. The number of planets dropped to the eight significantly larger bodies that had cleared their orbit (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus & Neptune), and a new class of dwarf planets was created, initially containing three objects (Ceres, Pluto and Eris).^[19] The Kuiper belt, derived from data from the Minor Planet Center. ... Absolute magnitude: −1. ... This article does not cite its references or sources. ... Photograph of the planet Neptune and its moon Triton, taken by Voyager 2 as it entered the outer solar system. ... This article or section may be confusing or unclear for some readers, and should be edited to rectify this. ... This article is about the planet. ... For other uses, see Venus (disambiguation). ... This article is about Earth as a planet. ... Adjectives: Martian Atmosphere Surface pressure: 0. ... For other uses, see Jupiter (disambiguation). ... This article is about the planet. ... For other uses, see Uranus (disambiguation). ... For other uses, see Neptune (disambiguation). ... Artists impression of Pluto (background) and Charon (foreground). ...

Definition and disputes

Main article: Definition of planet

With the discovery during the latter half of the twentieth century of more objects within the Solar System and large objects around other stars, disputes arose over what should constitute a planet. There was particular disagreement over whether an object should be considered a planet if it was part of a distinct population such as a belt, or if it was large enough to generate energy by the thermonuclear fusion of deuterium. Photograph of the planet Neptune and its moon Triton, taken by Voyager 2 as it entered the outer solar system. ... (19th century - 20th century - 21st century - more centuries) Decades: 1900s 1910s 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s As a means of recording the passage of time, the 20th century was that century which lasted from 1901&#8211;2000 in the sense of the Gregorian calendar (1900&#8211;1999... This article is about the Solar System. ... Infrared Image of a possible extrasolar planet (lower left) in the Constellation Taurus, taken by the Hubble Space Telescope. ... For other uses, see Asteroid (disambiguation). ... In physics, nuclear fusion (a thermonuclear reaction) is a process in which two nuclei join, forming a larger nucleus and releasing energy. ... Deuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of Earth of approximately one atom in 6500 of hydrogen (~154 PPM). ...

The largest Trans-Neptunian objects that prompted the IAU's decision.

In 2003, The International Astronomical Union (IAU) Working Group on Extrasolar Planets made a position statement on the definition of a planet that incorporated a working definition:^[2] 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. ... IAU redirects here. ...

1. Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 times the mass of Jupiter for objects with the same isotopic abundance as the Sun)^[20] that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass and size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.
2. Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located.
3. Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).

This definition has since been widely used by astronomers when publishing discoveries in journals,^[21] although it remains a temporary yet effective, working definition until a more permanent one is formally adopted. It also did not address the dispute over the lower mass limit and steered clear of the controversy regarding objects within the Solar System. The term true mass is synonymous with the term mass, but is used in astronomy to differentiate the measured mass of a planet from the lower limit of mass usually obtained from radial velocity techniques. ... Natural abundance refers to the prevalence of different isotopes of an element as found in nature. ... This brown dwarf (smaller object) orbits the star Gliese 229, which is located in the constellation Lepus about 19 light years from Earth. ... This article or section does not cite its references or sources. ... This article is about the journal as a written medium. ... This article is about the Solar System. ...

This matter was finally addressed during the 2006 meeting of the IAU's General Assembly. After much debate and one failed proposal, the assembly voted to pass a resolution that defined planets within the Solar System as:^[1] The final definition left the solar system with eight planets. ...

A celestial body that is (a) in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. Hydrostatic equilibrium occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. ... This article or section may be confusing or unclear for some readers, and should be edited to rectify this. ...

Under this definition, the Solar System is considered to have eight planets. Bodies which fulfill the first two conditions but not the third (such as Pluto and Eris) are classified as dwarf planets, providing they are not also natural satellites of other planets. Originally an IAU committee had proposed a definition that would have included a much larger number of planets as it did not include (c) as a criterion. After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets. Artists impression of Pluto (background) and Charon (foreground). ... A natural satellite is an object that orbits a planet or other body larger than itself and which is not man-made. ...

This definition is based in modern theories of planetary formation, in which planetary embryos initially clear their orbital neighborhood of other smaller objects. As described by astronomer Steven Soter: 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. ...

"The end product of secondary disk accretion is a small number of relatively large bodies (planets) in either non-intersecting or resonant orbits, which prevent collisions between them. Asteroids and comets, including KBOs, differ from planets in that they can collide with each other and with planets."^[22]

In the aftermath of the IAU's 2006 vote, there has been criticism of the new definition,^[23] and some astronomers have even stated that they will not use it.^[24] Part of the dispute centres around the belief that point (c) (clearing its orbit) should not have been listed, and that those objects now categorised as dwarf planets should actually be part of a broader planetary definition. The next IAU conference is not until 2009, when modifications could be made to the definition, also possibly including extrasolar planets. Medicament assisted rehabilitation conference in Oslo An academic conference is a conference for researchers (not always academics) to present and discuss their work. ...

Beyond the scientific community, Pluto has held a strong cultural significance for many in the general public considering its planetary status during most of the 20th century, in a similar way to Ceres and its kin in the 1800s. More recently, the discovery of Eris was widely reported in the media as the "tenth planet". The reclassification of all three objects as dwarf planets has attracted much media and public attention.^[25] This article does not cite its references or sources. ...

Formation

Main article: Planetary formation

It is not known with certainty how planets are formed. The prevailing theory is that they are formed during the collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, surrounded by a rotating protoplanetary disk. Through accretion—a process of sticky collision—dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planetesimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever more dense until they collapse inward under gravity to form protoplanets.^[26] After a planet reaches a diameter larger than the Earth's moon, it begins to accumulate an extended atmosphere, greatly increasing the capture rate of the planetesimals by means of atmospheric drag.^[27] Artists impression of a protoplanetary disc A protoplanetary disc (also protoplanetary disk, proplyd) is an accretion disc surrounding a T Tauri star. ... The Triangulum Emission Nebula NGC 604 The Pillars of Creation from the Eagle Nebula For other uses, see Nebula (disambiguation). ... A Protostar is an object that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. ... A protoplanetary disc (also protoplanetary disk, proplyd) is an accretion disc surrounding a T Tauri star. ... In astrophysics, the term accretion is used for at least two distinct processes. ... Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks. ... Protoplanets are moon-sized planet embryos within protoplanetary discs. ... An object falling through a gas or liquid experiences a force in direction opposite to its motion. ...

An artist's impression of protoplanetary disk.

When the protostar has grown such that it ignites to form a star, the surviving disk is removed from the inside outward by photoevaporation, the solar wind, Poynting-Robertson drag and other effects.^[28][29] Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a single larger planet or release material for other larger protoplanets or planets to absorb.^[30][31] Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Meanwhile, protoplanets that have avoided collisions may become natural satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small solar system bodies. Image File history File linksMetadata Ra4-protoplanetary-disk. ... Image File history File linksMetadata Ra4-protoplanetary-disk. ... This article is about the astronomical object. ... The plasma in the solar wind meeting the heliopause The solar wind is a stream of charged particles (i. ... The Poynting-Robertson effect, also known as Poynting-Robertson drag, named after John Henry Poynting and Howard Percy Robertson, is a process by which solar radiation causes dust particles in a solar system to slowly spiral inward. ... A natural satellite is an object that orbits a planet or other body larger than itself and which is not man-made. ... Artists impression of Pluto (background) and Charon (foreground). ... It has been suggested that minor planet be merged into this article or section. ...

The energetic impacts of the smaller planetesimals (as well as radioactive decay) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by mass, developing a denser core. Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from the mantle and from the subsequent impact of comets.^[32] (Smaller planets will lose any atmosphere they gain through various escape mechanisms.) Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. ... Comet Hale-Bopp Comet West For other uses, see Comet (disambiguation). ... There are several different processes that can lead to the escape of a planetary atmosphere. ...

With the discovery and observation of planetary systems around stars other than our own, it is becoming possible to elaborate, revise or even replace this account. The level of metallicity—a astronomical term describing the abundance of isotopes with an atomic number greater than 2 (Helium)—is now believed to determine the likelihood that a star will have planets.^[33] Hence it is thought less likely that a metal-poor, population II star will possess a more substantial planetary system than a metal-rich population I star. The globular cluster M80. ... For other uses, see Isotope (disambiguation). ... See also: List of elements by atomic number In chemistry and physics, the atomic number (also known as the proton number) is the number of protons found in the nucleus of an atom. ... Metal-poor is a term that is used to describe the chemical make up of an astronomical object. ... To meet Wikipedias quality standards, this article or section may require cleanup. ...

Within the Solar System

The terrestrial planets: Mercury, Venus, Earth, Mars (Sizes to scale)

The four gas giants against the Sun: Jupiter, Saturn, Uranus, Neptune. (Sizes to scale.)

Main article: Solar System

According to the IAU's current definitions there are eight planets in the Solar System. The largest is Jupiter, at 318 Earth masses. The smallest is Mercury, at 0.055 Earth masses. In increasing distance from the Sun, the planets are: Image File history File linksMetadata Download high resolution version (1500x653, 488 KB)Terrestrial planet size comparisons. ... Image File history File linksMetadata Download high resolution version (1500x653, 488 KB)Terrestrial planet size comparisons. ... Image File history File links Download high resolution version (1046x561, 29 KB) Summary Licensing History on English Wikipedia (Delete all revisions of this file) (cur) 01:05, 19 August 2006 . ... Image File history File links Download high resolution version (1046x561, 29 KB) Summary Licensing History on English Wikipedia (Delete all revisions of this file) (cur) 01:05, 19 August 2006 . ... This article is about the Solar System. ... IAU redirects here. ... Sol redirects here. ...

1. Mercury
2. Venus
3. Earth
4. Mars
5. Jupiter
6. Saturn
7. Uranus
8. Neptune

The larger bodies of the Solar System can be divided into categories based on their composition: Image File history File links Mercury_symbol. ... This article is about the planet. ... Image File history File links Venus_symbol. ... For other uses, see Venus (disambiguation). ... Image File history File links Earth_symbol. ... This article is about Earth as a planet. ... Image File history File links Mars_symbol. ... Adjectives: Martian Atmosphere Surface pressure: 0. ... Image File history File links Jupiter_symbol. ... For other uses, see Jupiter (disambiguation). ... Image File history File links Saturn_symbol. ... This article is about the planet. ... Image File history File links Uranus_symbol. ... For other uses, see Uranus (disambiguation). ... Image File history File links Neptune_symbol. ... For other uses, see Neptune (disambiguation). ...

• Terrestrials: Planets (and possibly dwarf planets) that are similar to Earth — with bodies largely composed of rock: Mercury, Venus, Earth and Mars.
• Gas giants: Planets with a composition largely made up of gaseous material and are significantly more massive than terrestrials: Jupiter, Saturn, Uranus, Neptune. Ice giants are a sub-class of gas giants, distinguished from gas giants by their depletion in hydrogen and helium, and a significant composition of rock and ice: Uranus and Neptune.

The inner planets, Mercury, Venus, Earth, and Mars, their sizes to scale. ... Rock redirects here. ... This article does not cite any references or sources. ... Gas phase particles (atoms, molecules, or ions) move around freely Gas is one of the four major states of matter, consisting of freely moving atoms or molecules without a definite shape and without a definite volume. ... From top: Neptune, Uranus, Saturn, and Jupiter. ... The astronomical unit (AU or au or a. ... The orbital period is the time it takes a planet (or another object) to make one full orbit. ... For the science fiction novella by William Shunn, see Inclination (novella). ... In astrodynamics, under standard assumptions any orbit must be of conic section shape. ... In astronomy, a rotation period is the time an astronomical object takes to complete one revolution around its rotation axis relative to the background stars. ... -1... For other uses, see Atmosphere (disambiguation). ... The inner planets, Mercury, Venus, Earth, and Mars, their sizes to scale. ... This article is about the planet. ... (*min temperature refers to cloud tops only) Atmospheric characteristics Atmospheric pressure 9. ... Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. ... General Name, symbol, number nitrogen, N, 7 Chemical series nonmetals Group, period, block 15, 2, p Appearance colorless gas Standard atomic weight 14. ... Earth, also known as the Earth or Terra, is the third planet outward from the Sun. ... This article is about Earths moon. ... General Name, symbol, number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, period, block 16, 2, p Appearance colourless (gas) colourless (liquid) Standard atomic weight 15. ... Mars is the fourth planet from the Sun in the solar system, named after the Roman god of war (the counterpart of the Greek Ares), on account of its blood red color as viewed in the night sky. ... Mars has two natural moons - Phobos and Deimos. ... This article does not cite any references or sources. ... Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... Jupiters outer moons and their highly inclined orbits. ... A schema of Jupiters ring system showing the four main components The rings of Jupiter are a system of planetary rings around the planet Jupiter. ... This article is about the chemistry of hydrogen. ... General Name, symbol, number helium, He, 2 Chemical series noble gases Group, period, block 18, 1, s Appearance colorless Standard atomic weight 4. ... Atmospheric characteristics Atmospheric pressure 140 kPa Hydrogen >93% Helium >5% Methane 0. ... The Saturnian system (photographic montage) Saturn has 60 confirmed natural satellites, plus three hypothetical moons. ... The full set of rings, photographed as Saturn eclipsed the sun from the vantage of the Cassini spacecraft on September 15, 2006 (brightness has been exaggerated in this image). ... Atmospheric characteristics Atmospheric pressure 120 kPa Hydrogen 83% Helium 15% Methane 1. ... Uranus has 27 known moons. ... This is a list of the named planetary rings of Uranus. ... Atmospheric characteristics Surface pressure ≫100 MPa Hydrogen - H2 80% ±3. ... Neptune (top) and Triton (bottom), 3 days after the Voyager 2 flyby. ... This is a list of the named rings and ring arcs of Neptune. ... Earth, also known as the Earth or Terra, is the third planet outward from the Sun. ...

Dwarf planets

Main article: Dwarf planet

Before the August 2006 decision, several objects were proposed by astronomers, including at one stage by the IAU, as planets. However in 2006 several of these objects were reclassified as dwarf planets, objects distinct from planets. Currently three dwarf planets in the Solar System are recognized by the IAU: Ceres, Pluto and Eris. Several other objects in both the asteroid belt and the Kuiper belt are under consideration, with as many as 50 that could eventually qualify. There may be as many as 200 that could be discovered once the Kuiper Belt has been fully explored. Dwarf planets share many of the same characteristics as planets, although notable differences remain—namely that they are not dominant in their orbits. Their attributes are: Artists impression of Pluto (background) and Charon (foreground). ... The final definition left the solar system with eight planets, pictured above (not to scale) Displays the remaining eight planets with the celestial bodies that have now been designated as dwarf planets. ... IAU redirects here. ... Artists impression of Pluto (background) and Charon (foreground). ... This article is about the Solar System. ... Spectral type: G[8] Absolute magnitude: 3. ... For other uses, see Pluto (disambiguation). ... Absolute magnitude: −1. ... For other uses, see Asteroid (disambiguation). ... The Kuiper belt, derived from data from the Minor Planet Center. ... The word characteristic has several meanings: In mathematics, see characteristic (algebra) characteristic function characteristic subgroup Euler characteristic method of characteristics In genetics, see characteristic (genetics). ... This article or section may be confusing or unclear for some readers, and should be edited to rectify this. ...

By definition, all dwarf planets are members of larger populations. Ceres is the largest body in the asteroid belt, while Pluto is a member of the Kuiper belt and Eris is a member of the scattered disc. According to Mike Brown there may soon be over forty trans-Neptunian objects that qualify as dwarf planets under the IAU's recent definition.^[34] The astronomical unit (AU or au or a. ... The orbital period is the time it takes a planet (or another object) to make one full orbit. ... For the science fiction novella by William Shunn, see Inclination (novella). ... In astrodynamics, under s