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Encyclopedia > Planet
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. ...

Contents

Etymology

The gods of Olympus, after whom the Solar System's planets are named
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.
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)
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 Earth Dysnomia (136199) Eris Charon (134340) Pluto (136472) 2005 FY9 (136108) 2003 EL61 (90377) Sedna (90482) Orcus (50000) Quaoar (20000) Varuna

The largest Trans-Neptunian objects that prompted the IAU's decision.
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 terrestrial planets: Mercury, Venus, Earth, Mars (Sizes to scale)
The four gas giants against the Sun: Jupiter, Saturn, Uranus, Neptune. (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.
Planetary attributes
Name Equatorial
diameter[a]
Mass[a] Orbital
radius (AU)
Orbital period
(years)
Inclination
to Sun's equator
(°)
Orbital
eccentricity
Rotation period
(days)
Moons Rings Atmosphere
Terrestrials Mercury 0.382 0.06 0.39 0.24 3.38 0.206 58.64 no minimal
Venus 0.949 0.82 0.72 0.62 3.86 0.007 -243.02 no CO2, N2
Earth[b] 1.00 1.00 1.00 1.00 7.25 0.017 1.00 1 no N2, O2
Mars 0.532 0.11 1.52 1.88 5.65 0.093 1.03 2 no CO2, N2
Gas giants Jupiter 11.209 317.8 5.20 11.86 6.09 0.048 0.41 63 yes H2, He
Saturn 9.449 95.2 9.54 29.46 5.51 0.054 0.43 60 yes H2, He
Uranus 4.007 14.6 19.22 84.01 6.48 0.047 -0.72 27 yes H2, He
Neptune 3.883 17.2 30.06 164.8 6.43 0.009 0.67 13 yes H2, He
a  Measured relative to the Earth.
b  See Earth article for absolute values.

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. ...

Dwarf planetary attributes
Name Equatorial
diameter[c]
Mass[c] Orbital
radius (AU)
Orbital period
(years)
Inclination
to ecliptic
(°)
Orbital
eccentricity
Rotation period
(days)
Moons Rings Atmosphere
Ceres 0.08 0.0002 2.76 4.60 10.59 0.080 0.38 0 no none
Pluto 0.19 0.0022 39.48 248.09 17.14 0.249 -6.39 3 no temporary
Eris 0.19 0.0025 67.67 ~557 44.19 0.442 ~0.3 1  ? temporary
c  Measured relative to the Earth.

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 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). ... Spectral type: G[8] Absolute magnitude: 3. ... For other uses, see Pluto (disambiguation). ... The planet Pluto has three known moons. ... Absolute magnitude: −1. ... Dysnomia (officially designated (136199) Eris I Dysnomia) is a moon of the dwarf planet Eris. ... For other uses, see Asteroid (disambiguation). ... The Kuiper belt, derived from data from the Minor Planet Center. ... Eris, the largest known scattered disc object (center), and its moon Dysnomia (left of center). ... Michael (Mike) E. Brown (born c. ... A trans-Neptunian object (TNO) is any object in the solar system with all or most of its orbit beyond that of Neptune. ...


Beyond the Solar System

Extrasolar planets

Main article: Extrasolar planet

Since the 1988 discovery of Gamma Cephei Ab, a number of confirmed discoveries have been made of planets orbiting stars other than the Sun. Of the 267 extrasolar planets discovered by November 2007, most have masses which are comparable to or larger than Jupiter's.[35] Exceptions include a number of planets discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12,[36] the planets orbiting the stars Mu Arae, 55 Cancri and GJ 436 which are approximately Neptune-sized,[37] and a planetary system containing at least two planets orbiting Gliese 876.[38] An extrasolar planet, or exoplanet, is a planet beyond the Solar System. ... Gamma Cephei (γ Cep / γ Cephei) is a star in the constellation Cepheus. ... An extrasolar planet, or exoplanet, is a planet beyond the Solar System. ... It has been suggested that Radio pulsar be merged into this article or section. ... The title given to this article is incorrect due to technical limitations. ... Mu Arae (μ Ara / μ Arae) is a Sunlike yellow-orange star located around 50 light years away in the constellation Ara. ... 55 Cancri (abbreviated 55 Cnc; Bayer designation ρ1 Cancri, Rho-1 Cancri) is a nearby 6th magnitude star in the constellation Cancer. ... GJ 436, also designated Gliese 436 and HIP 57087, is a red dwarf star 30 light years from Earth in the constellation of Leo. ... Gliese 876 is a red dwarf star located approximately 15 light-years away in the constellation Aquarius. ...


It is far from clear if the newly discovered large planets would resemble the gas giants in the Solar System or if they are of an entirely different type as yet unknown, like ammonia giants or carbon planets. In particular, some of the newly-discovered planets, known as hot Jupiters, orbit extremely close to their parent stars, in nearly circular orbits. They therefore receive much more stellar radiation than the gas giants in the Solar System, which makes it questionable whether they are the same type of planet at all. There is also a class of hot Jupiters that orbit so close to their star that their atmospheres are slowly blown away in a comet-like tail: the Chthonian planets.[39] Artists impression of roaster extrasolar planet HD 209458b (Osiris). ... Solar irradiance spectrum at top of atmosphere. ... A Chthonian planet (sometimes misspelled Cthonian), is a gas giant with its hydrogen and helium atmosphere stripped away due to its closeness to its star. ...


More detailed observation of extrasolar planets will require a new generation of instruments, including space telescopes. Currently the CoRoT spacecraft is searching for stellar luminosity variations due to transiting planets. Several projects have also been proposed to create an array of space telescopes to search for extrasolar planets with masses comparable to the Earth. These include the proposed NASA's Kepler Mission, Terrestrial Planet Finder, and Space Interferometry Mission programs, the ESA's Darwin, and the CNES' PEGASE.[40] The New Worlds Mission is an occulting device that may work in conjunction with the James Webb Space Telescope. However, funding for some of these projects remains uncertain. The frequency of occurrence of such terrestrial planets is one of the variables in the Drake equation which estimates the number of intelligent, communicating civilizations that exist in our galaxy.[41] A space observatory is any object in outer space which is used for observation of distant planets, galaxies, and other outer space objects. ... For a French painter, see Jean-Baptiste-Camille Corot. ... 2003 Transit of Mercury The term transit or astronomical transit has two meanings in astronomy: A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, as seen by an observer at some particular vantage point. ... A space observatory is any object in outer space which is used for observation of distant planets, galaxies, and other outer space objects. ... The Kepler Mission is a space observatory being developed by NASA that will search for extrasolar planets and will only be the second space-based telescope particularly constructed for that task, the first one being COROT. For this purpose, it will observe the brightness of about 100,000 stars over... Terrestrial Planet Finder - Infrared interferometer concept The Terrestrial Planet Finder (TPF) is a plan by NASA for a telescope system that would be capable of detecting extrasolar terrestrial planets. ... Artists concept of Space Interferometry Mission spacecraft The Space Interferometry Mission (SIM), also called SIM PlanetQuest, is a NASA instrument originally expected to be launched in December of 2011; however due to budget cuts it will now launch no sooner than between October 2014 and April 2015. ... ESA redirects here. ... Darwin is a proposed European Space Agency (ESA) mission designed to directly detect Earth-like planets orbiting nearby stars, and search for evidence of life on these planets. ... PEGASE is a proposed space mission to build a double-aperture interferometer composed of three free-flying satellites. ... This article does not cite any references or sources. ... The James Webb Space Telescope (JWST) is a planned space infrared observatory, intended to be a significant improvement on the aging Hubble Space Telescope. ... The Drake equation (rarely also called the Green Bank equation or the Sagan equation) is a famous result in the speculative fields of exobiology, astrosociobiology and the search for extraterrestrial intelligence. ... Extraterrestrial life refers to forms of life that may exist and originate outside of the planet Earth. ...


Interstellar "planets"

Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space.[42] Some scientists have argued that such objects found roaming in deep space should be classed as "planets". However, others have suggested that they could be low-mass stars.[43] The IAU's working definition on extrasolar planets takes no position on the issue. An interstellar planetary mass object is an object which has equivalent mass to a planet and is not gravitationally bound to any star, and that therefore moves through space as an independent object. ... It has been suggested that simulation software be merged into this article or section. ... This article is being considered for deletion in accordance with Wikipedias deletion policy. ... This article is about the idea of space. ... IAU redirects here. ...


In 2005, astronomers announced the discovery of Cha 110913-773444, the smallest brown dwarf found to date, at only seven times Jupiter's mass. Since it was not found in orbit around a fusing star, it is a sub-brown dwarf according to the IAU's working definition. However, some astronomers believe it should be referred to as a planet.[44]For a brief time in 2006, astronomers believed they had found a binary system of such objects, Oph 162225-240515, which the discoverers described as "planemos", or "planetary mass objects". However, recent analysis[45] of the objects has determined that their masses are each greater than 13 Jupiter-masses, making the pair brown dwarfs.[46] Cha 110913-773444 (Cha 110913 for short) is an astronomical object surrounded by what appears to be a protoplanetary disk. ... Sub-brown dwarfs or brown sub-dwarfs are cold masses smaller than the low-mass cut-off for brown dwarfs. ... Oph 162225-240515 or just Oph1622 is a pair of brown dwarfs that have been reported as orbiting each other, and not any star. ... This article is being considered for deletion in accordance with Wikipedias deletion policy. ... This brown dwarf (smaller object) orbits the star Gliese 229, which is located in the constellation Lepus about 19 light years from Earth. ...


Attributes

Although each planet has unique physical characteristics, a number of broad commonalities do exist between them. Some of these characteristics, such as rings or natural satellites, have only as yet been observed in planets in the Solar System, whilst others are also common to extrasolar planets.


Dynamic characteristics

See also: Kepler's laws of planetary motion

Johannes Keplers primary contributions to astronomy/astrophysics were his three laws of planetary motion. ...

Orbit

The orbit of the planet Neptune compared to that of Pluto. Note the elongation of Pluto's orbit in relation to Neptune's (eccentricity), as well as its large angle to the ecliptic (inclination)
The orbit of the planet Neptune compared to that of Pluto. Note the elongation of Pluto's orbit in relation to Neptune's (eccentricity), as well as its large angle to the ecliptic (inclination)

All planets revolve around stars. In the Solar System, all the planets orbit in sync with the Sun's rotation. It is not yet known whether all extrasolar planets follow this pattern. The period of one revolution of a planet's orbit is known as its sidereal period or year.[47] A planet's year depends on its distance from its star; the farther a planet is from its star, not only the longer the distance it must travel, but also the slower its speed, as it is less affected by the star's gravity. Because no planet's orbit is perfectly circular, the distance of each varies over the course of its year. Its closest distance to its is called its periastron (perihelion in the Solar System), while its farthest distance from the star is called its apastron (aphelion in the Solar System). As a planet approaches periastron, its speed increases as the pull of its star's gravity strengthens; as it reaches apastron, its speed decreases.[48] Image File history File links TheKuiperBelt_Orbits_Pluto_Ecliptic. ... Image File history File links TheKuiperBelt_Orbits_Pluto_Ecliptic. ... For other uses, see Pluto (disambiguation). ... (This page refers to eccitricity in astrodynamics. ... For the science fiction novella by William Shunn, see Inclination (novella). ... The orbital period is the time it takes a planet (or another object) to make one full orbit. ... A year (from Old English gēr) is the time between two recurrences of an event related to the orbit of the Earth around the Sun. ... This article is about several astronomical terms (apogee & perigee, aphelion & perihelion, generic equivalents based on apsis, and related but rarer terms. ... This article is about several astronomical terms (apogee & perigee, aphelion & perihelion, generic equivalents based on apsis, and related but rarer terms. ... This article is about several astronomical terms (apogee & perigee, aphelion & perihelion, generic equivalents based on apsis, and related but rarer terms. ... This article is about several astronomical terms (apogee & perigee, aphelion & perihelion, generic equivalents based on apsis, and related but rarer terms. ...


Each planet's orbit is delineated by a set of elements: The elements of an orbit are the parameters needed to specify that orbit uniquely, given a model of two ideal masses obeying the Newtonian laws of motion and the inverse-square law of gravitational attraction. ...

  • The eccentricity of an orbit describes how elongated a planet's orbit is. Planets with low eccentricities have more circular orbits, while planets with a high eccentricities have more elliptical orbits. The planets in our Solar System have very low eccentricities, and thus nearly circular orbits.[47] Comets and Kuiper belt objects (as well as several extrasolar planets) have very high eccentricities, and thus exceedingly elliptical orbits.[49][50]
an illustration of the semi-major axis
an illustration of the semi-major axis
  • The semi-major axis is the distance from a planet to the half-way point along the longest diameter of its elliptical orbit (see image). This distance is not necessarily the same as its apasteron, as no planet's orbit has its star at its exact centre.[47]
  • In our Solar System, the inclination of a planet tells how far above or below the plane of Earth's orbit (called the ecliptic) a planet's orbit lies. The eight planets of our Solar System all lie very close to the ecliptic; comets and Kuiper belt objects like Pluto are at far more extreme angles to it.[51] The points at which a planet crosses above and below the ecliptic are called its ascending and descending nodes.[47] Other orbital elements used to describe the orientation of a planet's orbit within our Solar System include the argument of periapsis and longitude of the ascending node.[47]

(This page refers to eccitricity in astrodynamics. ... Image File history File links Semimajoraxis. ... Image File history File links Semimajoraxis. ... The semi-major axis of an ellipse In geometry, the term semi-major axis (also semimajor axis) is used to describe the dimensions of ellipses and hyperbolae. ... For the science fiction novella by William Shunn, see Inclination (novella). ... The plane of the ecliptic is well seen in this picture from the 1994 lunar prospecting Clementine spacecraft. ... 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. ... For other uses, see Pluto (disambiguation). ... The ascending node is one of the orbital nodes, a point in the orbit of an object where it crosses the plane of the ecliptic from the south celestial hemisphere to the north celestial hemisphere in the direction of motion. ... The descending node is the point in the orbit of an object where it crosses the plane of the ecliptic from the north celestial hemisphere to the south celestial hemisphere in the direction of motion. ... The argument of periapsis (ω) is the orbital element describing the angle between an orbiting bodys ascending node (the point where the body crosses the plane of reference from South to North) and its periapsis (the point of closest approach to the central body), measured in the orbital plane and... The Longitude of the ascending node (☊, also noted Ω) is one of the orbital elements used to specify the orbit of an object in space. ...

Rotation

The planets also rotate around invisible axes through their centres. A planet's rotation period is known as its day. All planets in the Solar System rotate in a counter-clockwise direction, except for Venus, which rotates clockwise[52] (Uranus is generally said to be rotating clockwise as well[53] though because of its extreme axial tilt, it can be said to be rotating either clockwise or anti-clockwise, depending on whether one states it to be inclined 82° from the ecliptic in one direction, or 98° in the opposite direction).[54] There is great variation in the length of day between the planets, with Venus taking 243 Earth days to rotate, and the gas giants only a few hours.[55] The rotational periods of extrasolar planets are not known; however their close proximity to their stars means that hot Jupiters are tidelocked; their orbits are in sync with their rotations. This means they only ever show one face to their stars, with one side in perpetual day, the other in perpetual night.[56] 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. ... Look up day in Wiktionary, the free dictionary. ... Direct motion is the motion of a planetary body in a direction similar to that of other bodies within its system, and is sometimes called prograde motion. ... Tidal locking makes one side of an astronomical body always face another, like the Moon facing the Earth. ...


Axial tilt

Planets also have varying degrees of axial tilt; they lie at an angle to the plane of the their stars' equators. This causes the amount of light received by each hemisphere to vary over the course of its year; when the northern hemisphere points away from its star, the southern hemisphere points towards it, and vice versa. Each planet therefore possesses seasons; changes to the climate over the course of its year. The point at which each hemisphere is farthest or nearest from its star is known as its solstice. Each planet has two in the course of its orbit; when one hemisphere has its summer solstice, when its day is longest, the other has its winter solstice, when its day is shortest. Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either perpetually in sunlight or perpetually in darkness around the time of its solstices.[57] Among extrasolar planets, axial tilts are not known for certain, though most hot Jupiters are believed to possess negligible to no axial tilt, as a result of their proximity to their stars.[58] In astronomy, axial tilt is the inclination angle of a planets rotational axis in relation to a perpendicular to its orbital plane. ... A reference plane, in astronomy, is an arbitrary plane chosen to measure orbital elements, which are parameters needed to specify that orbit uniquely. ... For the science fiction novella by William Shunn, see Inclination (novella). ... This article or section is in need of attention from an expert on the subject. ... “Summer solstice” redirects here. ...


Orbital clearance

The defining dynamic characteristic of a planet is that it has cleared its neighborhood. In effect, it orbits its star in isolation, as opposed to sharing its orbit with a multitude of similar-sized objects. This characteristic was mandated as part of the IAU's official definiton of a planet in August, 2006. This criterion excludes such planetary bodies as Pluto, Eris and Ceres from full-fledged planethood, making them instead dwarf planets.[1] Although to date this criterion only applies to our Solar System, a number of young extrasolar systems have been found in which evidence suggests orbital clearing is taking place within their circumstellar discs.[59] 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. ... IAU redirects here. ... The final definition left the solar system with eight planets. ... For other uses, see Pluto (disambiguation). ... Absolute magnitude: −1. ... Spectral type: G[8] Absolute magnitude: 3. ... Artists impression of Pluto (background) and Charon (foreground). ...


Physical characteristics

Mass

A planet's defining physical characteristic is that it is massive enough for the force of its own gravity to dominate over the electromagnetic forces binding its physical structure, leading to a state of hydrostatic equilibrium. This effectively means that all planets are spherical or spheroidal. Up to a certain mass, an object can be irregular in shape, but beyond that point, which varies depending on the chemical makeup of the object, gravity begins to pull an object towards its own centre of mass until the object collapses into a sphere.[60] Electromagnetism is the physics of the electromagnetic field: a field, encompassing all of space, composed of the electric field and the magnetic field. ... Hydrostatic equilibrium occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. ...


Mass is also the prime attribute by which planets are distinguished from stars. The upper mass limit for planethood, beyond which it achieves conditions suitable for nuclear fusion, is roughly 13 times Jupiter's mass. No objects of such mass exist in our Solar System; however a number of extrasolar planets lie at that threshold. The Extrasolar Planets Encyclopedia lists the largest planets discovered to date as HD 38529 c, at 12.7 Jupiter masses, AB Pic b, at 13.5 Jupiter masses, HD 162020 b at 13.75 Jupiter masses, and HD 13189 b, at 14 Jupiter masses. A number of objects of higher mass are also listed, but since they lie above the fusion threshold, they would be better described as brown dwarfs.[61] This article is about the astronomical object. ... The deuterium-tritium (D-T) fusion reaction is considered the most promising for producing fusion power. ... Brown dwarfs are sub-stellar objects (~13 to 75 Jupiter masses) that never fuse hydrogen into helium in their cores, as do stars on the main sequence. ...


The smallest known planet, excluding dwarf planets and satellites, is PSR B1257+12 a, one of the first extrasolar planets discovered, which was found in 1992 in orbit around a pulsar. Its mass is roughly half that of the planet Mercury.[61] PSR B1257+12 (sometimes abbreviated to PSR 1257+12) is a pulsar located 980 light years from Earth. ... It has been suggested that Radio pulsar be merged into this article or section. ...


Internal differentiation

Every planet began its existence in an entirely fluid state; in early formation, the denser, heavier materials sank to the centre, leaving the lighter materials near the surface. Each therefore has a differentiated interior consisting of a dense planetary core surrounded by a mantle which either is or was a fluid. The terrestrial planets are sealed within hard crusts,[62] but in the gas giants the mantle simply dissolves into the upper cloud layers. The terrestrial planets possess cores of magnetic elements such as iron and nickel, and mantles of silicates. Jupiter and Saturn are believed to possess cores of rock and metal surrounded by mantles of metallic hydrogen.[63] Uranus and Neptune, which are smaller, possess rocky cores surrounded by mantles of water, ammonia, methane and other ices.[64] The fluid action within these planets' cores creates a geodynamo that generates a magnetic field.[62] In cosmogony, planetary differentiation is a process by which the denser portions of a planet will sink to the center; while less dense materials rise to the surface. ... The planetary core consists of the innermost layer(s) of a planet. ... Earth cutaway from core to exosphere. ... A fluid is defined as a substance that continually deforms (flows) under an applied shear stress regardless of the magnitude of the applied stress. ... Earth cutaway from core to exosphere. ... For other uses, see Iron (disambiguation). ... For other uses, see Nickel (disambiguation). ... In chemistry, a silicate is a compound consisting of silicon and oxygen (SixOy), one or more metals, and possibly hydrogen. ... For other uses, see Jupiter (disambiguation). ... This article is about the planet. ... Metallic hydrogen results when hydrogen is sufficiently compressed and undergoes a phase change; it is an example of degenerate matter. ... For other uses, see Uranus (disambiguation). ... For other uses, see Neptune (disambiguation). ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... For other uses, see Ammonia (disambiguation). ... Methane is a chemical compound with the molecular formula . ... This article is about water ice. ... The cause of Earths magnetic field (the surface magnetic field) is not known for certain, but is possibly explained by dynamo theory. ... Magnetic field lines shown by iron filings Magnetostatics Electrodynamics Electrical Network Tensors in Relativity This box:      In physics, the magnetic field is a field that permeates space and which exerts a magnetic force on moving electric charges and magnetic dipoles. ...


Atmospheres

All of the planets have atmospheres as their large masses mean gravity is strong enough to keep gaseous particles close to the surface. The larger gas giants are massive enough to keep large amounts of the light gases Hydrogen and Helium close by, although these gases mostly float into space around the smaller planets.[65] Earth's atmosphere is greatly different to the other planets because the various life processes that have transpired there have introduced massive amounts of free oxygen,[66] while the atmosphere of Mercury has mostly, although not entirely, been blasted away by the solar wind.[67] Planetary atmospheres are affected by the varying degrees of energy received from either the Sun or their interiors, leading to the formation of dynamic weather systems such as hurricanes, (on Earth), planet-wide dust storms (on Mars) and Earth-sized anticyclones (on Jupiter).[57] At least one extrasolar planet, HD 189733b, has been shown to possess such a weather system, similar to the Great Red Spot on Jupiter but twice as large.[68] Hot Jupiters have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets.[69] These planets have vast differences in temperature between their day and night sides which produce supersonic windspeeds.[70] For other uses, see Atmosphere (disambiguation). ... 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. ... This article is about the idea of space. ... 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. ... The plasma in the solar wind meeting the heliopause The solar wind is a stream of charged particles (i. ... A large low-pressure system swirls off the southwestern coast of Iceland, illustrating the maxim that nature abhors a vacuum. ... This article is about weather phenomena. ... “Sandstorm” redirects here. ... A false-color image of the Great Red Spot of Jupiter from Voyager 1. ... HD 189733 (HD 189733 A) is a yellow dwarf star about 63 light-years away in the constellation Vulpecula, and host to at least one extrasolar planet. ...


Secondary characteristics

Many of the planets have natural satellites, often called "moons." Mercury and Venus have no moons, the Earth has one, and Mars has two, but the gas giants all have numerous moons in complex planetary systems. Many gas giant moons have similar features to the terrestrial planets and dwarf planets, and some have been studied for signs of life.[71][72][73] A natural satellite is an object that orbits a planet or other body larger than itself and which is not man-made. ... This article does not cite any references or sources. ...


The four largest planets in the Solar System are also orbited by planetary rings of varying size and complexity. The rings are composed primarily of dust or particulate matter, but can host tiny 'moonlets' whose gravity shapes and maintains their structure. Although the origins of planetary rings is not precisely known, they are believed to be the result of natural satellites which fell below their parent planet's Roche limit and were torn apart by tidal forces.[74][75] A planetary ring is a ring of dust and other small particles orbiting around a planet in a flat disc-shaped region. ... The tiny moonlet Dactyl (right) in orbit around the asteroid 243 Ida. ... The Roche limit, sometimes referred to as the Roche radius, is the distance within which a celestial body held together only by its own gravity will disintegrate due to a second celestial bodys tidal forces exceeding the first bodys gravitational self-attraction. ... The tidal force is a secondary effect of the force of gravity and is responsible for the tides. ...


See also

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 is being considered for deletion in accordance with Wikipedias deletion policy. ... 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. ... Artists impression of Pluto (background) and Charon (foreground). ... Planetary science, also known as planetology or planetary astronomy, is the science of planets, or planetary systems, and the solar system. ... An artists concept of a protoplanetary disc. ... Overview of the planets and dwarf planets in the Solar System. ... The sky of a world refers to the view of the heavens from its surface. ... Understanding planetary habitability is partly an extrapolation of the Earths conditions, as it is the only planet currently known to support life. ... See also: List of hypothetical planetary bodies A hypothetical planet is a planet whose existence is not known, but has been inferred from observational scientific evidence. ... This is a list of all spacecraft landings on other planets and bodies in the solar system, including both intended and unintended crash-landings. ... Planets in science fiction are fictional planets that appear in various media, especially those of the science fiction genre, as story-settings or depicted locations. ... Planets in astrology have a different meaning to the modern astronomical understanding of what a planet is. ... 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). ...

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Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 120th day of the year (121st in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 145th day of the year (146th in leap years) in the Gregorian calendar. ... is the 303rd day of the year (304th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 304th day of the year (305th in leap years) in the Gregorian calendar. ... There are several methods for the romanization of Greek, especially depending whether the language written with Greek letters is Ancient Greek or Modern Greek and whether rather phonetic transcription or a graphematic transliteration is intended. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 204th day of the year (205th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 204th day of the year (205th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 120th day of the year (121st in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 120th day of the year (121st in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 56th day of the year in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 120th day of the year (121st in leap years) in the Gregorian calendar. ... The Royal Astronomical Society of Canada is the Canadian equivalent of the British Royal Astronomical Society, which began informally in the 1800s, but received a royal charter in 1903 from King Edward VII. The society incorporated nationally in 1968, prior to which its incorporation was limited to Ontario. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 204th day of the year (205th in leap years) in the Gregorian calendar. ... The 23rd century (Gregorian Calendar) comprises the years 2201-2300. ... is the 275th day of the year (276th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... April 8 is the 98th day of the year (99th in leap years) in the Gregorian calendar. ... is the 260th day of the year (261st in leap years) in the Gregorian calendar. ... Year 2001 (MMI) was a common year starting on Monday (link displays the 2001 Gregorian calendar). ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... April 8 is the 98th day of the year (99th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 131st day of the year (132nd in leap years) in the Gregorian calendar. ... 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. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 131st day of the year (132nd in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 204th day of the year (205th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 204th day of the year (205th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 353rd day of the year (354th in leap years) in the Gregorian calendar. ... is the 236th day of the year (237th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 236th day of the year (237th in leap years) in the Gregorian calendar. ... is the 243rd day of the year (244th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 236th day of the year (237th in leap years) in the Gregorian calendar. ... is the 291st day of the year (292nd in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 131st day of the year (132nd in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 204th day of the year (205th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 204th day of the year (205th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 133rd day of the year (134th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 133rd day of the year (134th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 206th day of the year (207th in leap years) in the Gregorian calendar. ... is the 25th day of the year in the Gregorian calendar. ... Year 2003 (MMIII) was a common year starting on Wednesday of the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 206th day of the year (207th in leap years) in the Gregorian calendar. ... is the 152nd day of the year (153rd in leap years) in the Gregorian calendar. ... Year 1998 (MCMXCVIII) was a common year starting on Thursday (link will display full 1998 Gregorian calendar). ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 204th day of the year (205th in leap years) in the Gregorian calendar. ... is the 6th day of the year in the Gregorian calendar. ... Year 2004 (MMIV) was a leap year starting on Thursday of the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 238th day of the year (239th in leap years) in the Gregorian calendar. ... is the 59th day of the year in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 305th day of the year (306th in leap years) in the Gregorian calendar. ... is the 345th day of the year (346th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 345th day of the year (346th in leap years) in the Gregorian calendar. ... is the 42nd day of the year in the Gregorian calendar. ... Year 2005 (MMV) was a common year starting on Saturday (link displays full calendar) of the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 209th day of the year (210th in leap years) in the Gregorian calendar. ... is the 237th day of the year (238th in leap years) in the Gregorian calendar. ... Year 2004 (MMIV) was a leap year starting on Thursday of the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 204th day of the year (205th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 253rd day of the year (254th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 253rd day of the year (254th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 204th day of the year (205th in leap years) in the Gregorian calendar. ... is the 272nd day of the year (273rd in leap years) in the Gregorian calendar. ... Year 2003 (MMIII) was a common year starting on Wednesday of the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 204th day of the year (205th in leap years) in the Gregorian calendar. ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 145th day of the year (146th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 145th day of the year (146th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 120th day of the year (121st in leap years) in the Gregorian calendar. ... is the 253rd day of the year (254th in leap years) in the Gregorian calendar. ... Year 2004 (MMIV) was a leap year starting on Thursday of the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 72nd day of the year (73rd in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 251st day of the year (252nd in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 251st day of the year (252nd in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 251st day of the year (252nd in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 252nd day of the year (253rd in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 252nd day of the year (253rd in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 252nd day of the year (253rd in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 251st day of the year (252nd in leap years) in the Gregorian calendar. ... is the 121st day of the year (122nd in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 252nd day of the year (253rd in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 251st day of the year (252nd in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 258th day of the year (259th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 252nd day of the year (253rd in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 336th day of the year (337th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 253rd day of the year (254th in leap years) in the Gregorian calendar. ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 360th day of the year (361st in leap years) in the Gregorian calendar. ... is the 129th day of the year (130th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 227th day of the year (228th in leap years) in the Gregorian calendar. ... is the 31st day of the year in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 227th day of the year (228th in leap years) in the Gregorian calendar. ... is the 285th day of the year (286th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 227th day of the year (228th in leap years) in the Gregorian calendar. ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... is the 345th day of the year (346th in leap years) in the Gregorian calendar. ... Year 2001 (MMI) was a common year starting on Monday (link displays the 2001 Gregorian calendar). ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 253rd day of the year (254th in leap years) in the Gregorian calendar. ...

External links

A small orrery showing earth and the inner planets An orrery is a mechanical device that illustrates the relative positions and motions of the planets and moons in the solar system in heliocentric model. ...

Definition and reclassification debate

 v  d  e The Solar System
The Sun Mercury Venus The Moon Earth Phobos and Deimos Mars Ceres The asteroid belt Jupiter Moons of Jupiter Saturn Moons of Saturn Uranus Moons of Uranus Moons of Neptune Neptune Moons of Pluto Pluto The Kuiper Belt Dysnomia Eris The Scattered Disc The Oort Cloud
The Sun · Mercury · Venus · Earth · Mars · Ceres · Jupiter · Saturn · Uranus · Neptune · Pluto · Eris
Planets · Dwarf planets · Moons: Terrestrial · Martian · Jovian · Saturnian · Uranian · Neptunian · Plutonian · Eridian
Small bodies:   Meteoroids · Asteroids/Asteroid moons (Asteroid belt) · Centaurs · TNOs (Kuiper belt/Scattered disc) · Comets (Oort cloud)
See also astronomical objects, the solar system's list of objects, sorted by radius or mass, and the Solar System Portal

IAU redirects here. ... 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. ... is the 81st day of the year (82nd in leap years) in the Gregorian calendar. ... Year 2004 (MMIV) was a leap year starting on Thursday of the Gregorian calendar. ... IAU redirects here. ... This article is about the Solar System. ... Sol redirects here. ... 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. ... Spectral type: G[8] Absolute magnitude: 3. ... For other uses, see Jupiter (disambiguation). ... This article is about the planet. ... For other uses, see Uranus (disambiguation). ... For other uses, see Neptune (disambiguation). ... For other uses, see Pluto (disambiguation). ... Absolute magnitude: −1. ... 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 article is about Earths moon. ... The relative sizes of and distance between Mars, Phobos, and Deimos, to scale : Phobos (top) and Deimos (bottom). ... Jupiters 4 Galilean moons, in a composite image comparing their sizes and the size of Jupiter (Great Red Spot visible). ... The Saturnian system (photographic montage) Moons of Saturn (photographic montage) Saturn has 60 confirmed natural satellites, plus three hypothetical moons. ... Uranus has twenty-seven known moons. ... Neptune (top) and Triton (bottom), 3 days after the Voyager 2 flyby. ... Hubble image of the Plutonian system Pluto has three known moons. ... Dysnomia (officially designated (136199) Eris I Dysnomia) is a moon of the dwarf planet Eris. ... 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... “Meteor” redirects here. ... For other uses, see Asteroid (disambiguation). ... 243 Ida and its moon Dactyl An asteroid moon is an asteroid that orbits another asteroid. ... For other uses, see Asteroid (disambiguation). ... The centaurs are a class of icy planetoids that orbit the Sun between Jupiter and Neptune, named after the mythical race of centaurs. ... A trans-Neptunian object (TNO) is any object in the solar system that orbits the sun at a greater distance on average than Neptune. ... The Kuiper belt, derived from data from the Minor Planet Center. ... Eris, the largest known scattered disc object (center), and its moon Dysnomia (left of center). ... Comet Hale-Bopp Comet West For other uses, see Comet (disambiguation). ... Artists rendering of the Oort cloud and the Kuiper Belt. ... Astronomical objects are significant physical entities, associations or structures which current science has confirmed to exist in space. ... Below is a list of solar system objects with diameter >500km: The Sun, a spectral class G2 star Mercury Venus Earth Moon Mars Jupiter Io Europa Ganymede Callisto complete list of Jupiters natural satellites Saturn Tethys Dione Rhea Titan Iapetus complete list of Saturns natural satellites Uranus Ariel... It has been suggested that Planetary-size comparison be merged into this article or section. ... This is a list of solar system objects by mass, in decreasing order. ...


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Planet Feed Reader (171 words)
Planet is an awesome 'river of news' feed reader.
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Panel showing positions of planets and a selected asteroid or comet, both geocentric and from the observer's location.
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