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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, and has cleared its neighbouring region of planetesimals.
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.
The planets were thought by Ptolomy to orbit the Earth in deferent and epicycle motions. Copernicus suggested that the planets orbited the Sun, and this view was supported by Galileo after the development of the telescope. By careful analysis of the observation data, Johannes Kepler found their orbits to be not circular, but elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes and some share 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, through the discovery of hundreds of extrasolar planets (planets around other stars), scientists are beginning to observe similar features throughout the Milky Way Galaxy.
Planets are generally divided into two main types: large, low-density gas giants and smaller, rocky terrestrials. Under IAU definitions, there are eight planets in the Solar System; in order they are the four terrestrials: Mercury, Venus, Earth and Mars, with the four gas giants: Jupiter, Saturn, Uranus, and Neptune. Many of these planets are orbited by one or more moons, which can be larger than small planets. As of July 2008 there are 307 known extrasolar planets, ranging from the size gas giants to that of terrestrial planets. This brings the total number of identified planets to 315. The Solar System also contains at least four dwarf planets: Ceres, Pluto, Makemake and Eris. No extrasolar dwarf planets have been detected.
The idea of planets has evolved over its history, from the divine wandering stars of antiquity to the earthly objects of the scientific age. The concept has also now expanded to include worlds not only in our Solar System, but in hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy.
In ancient times, astronomers noted how certain lights moved across the sky in relation to the other stars. Ancient Greeks called these lights "πλάνητες ἀστέρες" (planetes asteres: wandering stars) or simply "πλανήτοι" (planētoi: wanderers), from which the today's word "planet" was derived.
In ancient Greece as well as in ancient China, ancient Babylon and indeed all pre-modern civilisations, it was almost universally believed that Earth was in the centre of the Universe and that all the "planets" circled the Earth. The reasons for this perception was that stars and planets appeared to revolve around the Earth each day, and the apparently common sense perception that the Earth was solid and stable and that it is not moving but at rest.
The Greek cosmological system was taken from that of the Babylonians, a contemporary Mesopotamian civilisation from whom they began to acquire astronomical learning from around 600 BC, including the constellations and the zodiac. In the 6th century BC, the Babylonians had a highly advanced level of astronomical knowledge, and had a theory of the planets centuries before the ancient Greeks. The oldest planetary astronomical text that we possess is the Babylonian Venus tablet of Ammisaduqa, a 7th century BC copy of a list of observations of the motions of the planet Venus that probably dates as early as the second millennium BC. The Babylonians also laid the foundations of what would eventually become Western astrology. The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC, comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets. Sumerians, predecessors of Babylonians which are credited as one of the first civilizations and the inventors of writing, had identified at least Venus by 1500 BC. Conversely, there is no evidence of a comparable knowledge of the planets in the earliest written Greek sources, such as the Iliad and the Odyssey.
By the first century BC, the Greeks had begun to develop their own mathematical schemes for predicting the positions of the planets. These schemes, which were based on geometry rather than the arithmetic of the Babylonians, would eventually eclipse the Babylonians' theories in complexity and comprehensiveness and account for much of the astronomical movements observed from Earth with the naked eye. These theories would reach their fullest expression in the Almagest written by Ptolemy in the 2nd century AD. So complete was the domination of Ptolemy's model that it superseded all previous works on astronomy and remained the definitive astronomical text in the Western world for 13 centuries.
To the Greeks and Romans there were seven known planets; each presumed to be circling the Earth according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.
The five naked-eye planets have been known since ancient times, and have had a significant impact on mythology, religious cosmology, and ancient astronomy. As scientific knowledge progressed, however, understanding of the term "planet" changed from something that moved across the sky (in relation to the star field); to a body that orbited the Earth (or that were believed to do so at the time); and in the 16th century to something that directly orbited the Sun when the heliocentric model of Copernicus, Galileo and Kepler gained sway.
Thus the Earth became included in the list of planets, while the Sun and Moon were excluded. At first, when the first satellites of Saturn were discovered at the end of the 17th century, the terms "planet" and "satellite" were used interchangeably – although the latter would gradually become more prevalent in the following century. Until the mid-19th century, the number of "planets" rose rapidly since any newly discovered object directly orbiting the Sun was listed as a planet by the scientific community.
In the 19th century astronomers began to realize that recently discovered bodies that had been classified as planets for almost half a century (such as Ceres, Pallas, and Vesta), were very different from the traditional one. These bodies shared the same region of space between Mars and Jupiter (the Asteroid belt), and had a much smaller mass; as a result they were reclassified as "asteroids". In the absence of any formal definition, a "planet" came to be understood as any "large" body that orbited the Sun. Since 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, there was no apparent need to have a formal definition.
However, in the 20th century, Pluto was discovered. After initial observations led to the belief it was larger than Earth, the object was immediately accepted as the ninth planet. Further monitoring found the body was actually much smaller: in 1936, Raymond Lyttleton suggested that Pluto may be an escaped satellite of Neptune, and Fred Whipple suggested in 1964 that Pluto may be a comet. However, as it was still larger than all known asteroids and seemingly did not exist within a larger population, it kept its status until 2006.
In the 1990s and early 2000s, there was a flood of discoveries of similar objects in the same region of the Solar System (the Kuiper belt). 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).
In 1992, astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around a pulsar, PSR B1257+12. This discovery is generally considered to be the first definitive detection of a planetary system around another star. Then, on October 6 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star ( 51 Pegasi).
The discovery of extrasolar planets led to another ambiguity in defining a planet; the point at which a planet becomes a star. Many known extrasolar planets are many times the mass of Jupiter, approaching that of stellar objects known as " brown dwarfs". Brown dwarfs are generally considered stars due to their ability to fuse deuterium, a heavier isotope of hydrogen. While stars more massive than 75 times that of Jupiter fuse hydrogen, stars of only 13 Jupiter masses can fuse deuterium. However, deuterium is quite rare, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets.
As large Kuiper belt and scattered disc objects were discovered in the late 1990s and early years of the twenty-first century, a number including Quaoar, Sedna and Eris were heralded in the popular press as the 'tenth planet', however none of these received widespread scientific recognition as such, although Eris has now been classified as a Dwarf Planet.
The table below lists Solar System bodies formerly considered to be planets:
|Sun, Moon||Classified as planets in antiquity, in accordance with the definition then used.|
|Io, Europa, Ganymede, and Callisto||The four largest moons of Jupiter, known as the Galilean moons after their discoverer Galileo Galilei. He referred to them as the "Medicean Planets" in honour of his patron, the Medici family.|
|Titan, Iapetus, Rhea, Tethys, and Dione||Five of Saturn's larger moons, discovered by Christiaan Huygens and Giovanni Domenico Cassini.|
|Ceres, Pallas, Juno, and Vesta||The first known asteroids, from their discoveries between 1801 and 1807 until their reclassification as asteroids during the 1850s.
Ceres has subsequently been classified as a dwarf planet.
|Astrea, Hebe, Iris, Flora, Metis, Hygeia, Parthenope, Victoria, Egeria, Irene, Eunomia||More Asteroids, discovered between 1845 and 1851. The rapidly expanding list of planets prompted their reclassification as asteroids by astronomers, and this was widely accepted by 1854.|
|Pluto||Kuiper belt object beyond the orbit of Neptune. In 2006, Pluto was reclassified as a dwarf planet.|
With the discovery during the latter half of the 20th 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.
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:
- 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) 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.
- Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are " brown dwarfs", no matter how they formed or where they are located.
- 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 academic journals. Although temporary, it remains an effective, working definition until a more permanent one is formally adopted. Nevertheless, it did not address the dispute over the lower mass limit, and steered clear of the controversy regarding objects within 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:
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.
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, Makemake 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.
This definition is based in modern theories of planetary formation, in which planetary embryos initially clear their orbital neighbourhood of other smaller objects. As described by astronomer Steven Soter:
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.
In the aftermath of the IAU's 2006 vote, there has been criticism of the new definition, and some astronomers have even stated that they will not use it. 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.
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 – similarly to Ceres and its kin in the 1800s. The discovery of Eris was widely reported in the media as the " tenth planet" and therefore the reclassification of all three objects as dwarf planets has attracted a lot of media and public attention.
The names for the planets in the Western world are derived from the naming practices of the Romans, which ultimately derive from those of the Greeks and the Babylonians. In ancient Greece, the two great luminaries the Sun and the Moon were called Helios and Selene; the farthest planet was called Phainon, the shiner; followed by Phaethon, "bright"; the red planet was known as Pyroeis, the "fiery"; the brightest was known as Phosphoros, the light bringer; and the fleeting final planet was called Stilbon, the gleamer. The Greeks also made each planet sacred to one of their pantheon of gods, the Olympians: Helios and Selene were the names of both planets and gods; Phainon was sacred to Kronos, the Titan who fathered the Olympians; Phaethon was sacred to Zeús, Kronos's son who deposed him as king; Pyroeis was given to Ares, son of Zeus and god of war; Phosphorus was ruled by Aphrodite, the goddess of love; and Hermes, messenger of the gods and god of learning and wit, ruled over Stilbon.
The Greek practice of grafting of their gods' names onto the planets was almost certainly borrowed from the Babylonians. The Babylonians named Phosphorus after their goddess of love, Ishtar; Pyroeis after their god of war, Nergal, Stilbon after their god of wisdom Nabu, and Phaethon after their chief god, Marduk. There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately. The translation was not perfect. For instance, the Babylonian Nergal was a god of war, and thus the Greeks identified him with Ares. However, unlike Ares, Nergal was also god of pestilence and the underworld.
Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods. While modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the Roman Empire and, later, the Catholic Church, use the Roman (or Latin) names rather than the Greek ones. 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. When the Romans studied Greek astronomy, they gave the planets their own gods' names: Mercurius (for Hermes), Venus (Aphrodite), Mars (Ares), Iuppiter (Zeus) and Saturnus (Kronos). When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained: Uranus ( Ouranos) and Neptūnus ( Poseidon).
Some Romans, following a belief possibly originating in Mesopotamia but developed in Hellenistic Egypt, believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts went Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon (from the farthest to the closest planet). Therefore, the first day was started by Saturn (1st hour), second day by Sun (25th hour), followed by Moon (49th hour), Mars, Mercury, Jupiter and Venus. Since each day was named by the god that started it, this is also the order of the days of the week in the Roman calendar – and still preserved many modern languages. 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), the Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus respectively.
Since Earth was only generally accepted as a planet in the 17th century, there is no tradition of naming it after a god (the same is true, in English at least, of the Sun and the Moon, though they are no longer considered planets). The name originates from the 8th century Anglo-Saxon word erda, which means ground or soil and was first used in writing as the name of the sphere of the Earth perhaps around 1300. It is the only planet whose name in English is not derived from greco-roman mythology. Many of the Romance languages retain the old Roman word terra (or some variation of it) that was used with the meaning of "dry land" (as opposed to "sea"). However, the non-Romance languages use their own respective native words. 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," as can be seen in the English Earth, the German Erde, the Dutch Aarde, and the Scandinavian Jorde.
Non-European cultures use other planetary naming systems. India uses a naming system based on the Navagraha, which incorporates the seven traditional planets ( Surya for the Sun, Chandra for the Moon, and Budha, Shukra, Mangala, Bṛhaspati and Shani for the traditional planets Mercury, Venus, Mars, Jupiter and Saturn) and the ascending and descending lunar nodes Rahu and Ketu. China and the countries of eastern Asia influenced by it (such as Japan, Korea and Vietnam) use a naming system based on the five Chinese elements: water (Mercury), metal (Venus), fire (Mars), wood (Jupiter) and earth (Saturn).
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 denser until they collapse inward under gravity to form protoplanets. 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.
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. 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. 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.
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. (Smaller planets will lose any atmosphere they gain through various escape mechanisms.)
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 – an astronomical term describing the abundance of chemical elements with an atomic number greater than 2 (helium) – is now believed to determine the likelihood that a star will have planets. 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.
Jupiter is the largest, at 318 Earth masses, while Mercury is smallest, at 0.055 Earth masses.
The planets of the Solar System can be divided into categories based on their composition:
- Terrestrials: 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, comprising Uranus and Neptune, are a sub-class of gas giants, distinguished from gas giants by their significantly lower mass, and by depletion in hydrogen and helium in their atmospheres together with a significantly higher proportion of rock and ice.
radius ( AU)
| Orbital period
to Sun's equator (°)
| Rotation period
|Gas giants||Jupiter||11.209||317.8||5.20||11.86||6.09||0.048||0.41||63||yes||H2, He|
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 four dwarf planets in the Solar System are recognized by the IAU: Ceres, Pluto, Makemake 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:
|Dwarf planetary attributes|
radius ( AU)
| Orbital period
to ecliptic (°)
| Rotation period
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. Scientists such as Mike Brown believe that there may soon be over forty trans-Neptunian objects that qualify as dwarf planets under the IAU's recent definition.
The first confirmed discovery of an extrasolar planet orbiting an ordinary main-sequence star occurred on 6 October 1995, when Michel Mayor and Didier Queloz of the University of Geneva announced the detection of an exoplanet around 51 Pegasi. Of the 270 extrasolar planets discovered by January 2008, most have masses which are comparable to or larger than Jupiter's, though masses ranging from just below that of Mercury to many times Jupiter's mass. The smallest extrasolar planets found to date have been discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12. There have been roughly a dozen extrasolar planets found of between 10 and 20 Earth masses, such as those orbiting the stars Mu Arae, 55 Cancri and GJ 436. These planets have been nicknamed "Neptunes" because they roughly approximate that planet's mass (17 Earths). Another new category are the so-called " super-Earths", possibly terrestrial planets far larger than Earth but smaller than Neptune or Uranus. To date, five possible super-Earths have been found: Gliese 876 d, which is roughly six times Earth's mass, OGLE-2005-BLG-390Lb and MOA-2007-BLG-192Lb, frigid icy worlds discovered through gravitational microlensing, and two planets orbiting the nearby red dwarf Gliese 581. Gliese 581 d is roughly 7.7 times Earth's mass, while Gliese 581 c is five times Earth's mass and the first terrestrial planet found within a star's habitable zone.
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 may also exist a class of hot Jupiters, called Chthonian planets, that orbit so close to their star that their atmospheres have been blown away completely by stellar radiation. While many hot Jupiters have been found in the process of losing their atmospheres, as of 2008, no genuine Chthonian planets have been discovered.
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. 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 first spectra of extrasolar planets were reported in February 2007 ( HD 209458 b and HD 189733 b). 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.
Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space. 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. The IAU's working definition on extrasolar planets takes no position on the issue.
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. 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 of the objects has determined that their masses are probably each greater than 13 Jupiter-masses, making the pair brown dwarfs.
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.
All planets revolve around stars. In the Solar System, all the planets orbit in the same direction as the Sun rotates. 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. 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. The closest approach to its star is called its periastron ( perihelion in the Solar System), while its farthest separation from the star is called its apastron ( aphelion). As a planet approaches periastron, its speed increases as the pull of its star's gravity strengthens; as it reaches apastron, its speed decreases.
Each planet's orbit is delineated by a set of elements:
- The eccentricity of an orbit describes how elongated a planet's orbit is. Planets with low eccentricities have more circular orbits, while planets with high eccentricities have more elliptical orbits. The planets in our Solar System have very low eccentricities, and thus nearly circular orbits. Comets and Kuiper belt objects (as well as several extrasolar planets) have very high eccentricities, and thus exceedingly elliptical orbits.
- 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 the same as its apasteron, as no planet's orbit has its star at its exact centre.
- The inclination of a planet tells how far above or below an established reference plane its orbit lies. In our Solar System, the reference plane is the plane of Earth's orbit, called the ecliptic. For extrasolar planets, the plane, known as the sky plane or plane of the sky, is the plane of the observer's line of sight from Earth. 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. The points at which a planet crosses above and below its reference plane are called its ascending and descending nodes. The longitude of the ascending node is the angle between the reference plane's 0 longitude and the planet's ascending node. The argument of periapsis (or perihelion in our Solar System) is the angle between a planet's ascending node and its closest approach to its star.
Planets also have varying degrees of axial tilt; they lie at an angle to the plane of 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. The varying amount of light and heat received by each hemisphere creates annual changes in weather patterns for each half of the planet. 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. 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.
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 (Uranus is generally said to be rotating clockwise as well 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). 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. The rotational periods of extrasolar planets are not known; however their proximity to their stars means that hot Jupiters are tidaly locked (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.
The defining dynamic characteristic of a planet is that it has cleared its neighborhood. A planet that has cleared its neighbourhood has accumulated enough mass to gather up or sweep away all the planetesimals in its orbit. 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 definition 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. 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.
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.
Mass is also the prime attribute by which planets are distinguished from stars. The upper mass limit for planethood is roughly 13 times Jupiter's mass, beyond which it achieves conditions suitable for nuclear fusion. Other than the Sun, 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 several planets that are close to this limit: HD 38529c, AB Pictorisb, HD 162020b, and HD 13189b. 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.
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.
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, 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. Uranus and Neptune, which are smaller, possess rocky cores surrounded by mantles of water, ammonia, methane and other ices. The fluid action within these planets' cores creates a geodynamo that generates a magnetic field.
All of the Solar System 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, while the smaller planets lose these gases into space. The composition of the Earth's atmosphere is different from the other planets because the various life processes that have transpired on the planet have introduced free molecular oxygen. The only solar planet without a true atmosphere is Mercury which had it mostly, although not entirely, blasted away by the solar wind.
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), an Earth-sized anticyclone on Jupiter (called the Great Red Spot), and holes in the atmosphere (on Neptune). At least one extrasolar planet, HD 189733 b, has been claimed to possess such a weather system, similar to the Great Red Spot but twice as large.
Hot Jupiters have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets. These planets may have vast differences in temperature between their day and night sides which produce supersonic winds, although the day and night sides of HD 189733b appear to have very similar temperatures, indicating that that planet's atmosphere effectively redistributes the star's energy around the planet.
One important characteristic of the planets is their intrinsic magnetic moments which in turn give rise to magnetospheres. The presence of a magnetic field indicates that the planet is still geologically alive. In other words, magnetized planets have flows of electrically conducting material in their interiors, which generate their magnetic fields. These fields significantly change the interaction of the planet and solar wind. A magnetized planet creates a cavity around itself called magnetosphere, which the solar wind can not penetrate. The size of the magnetosphere can be much larger than that of the planet itself. In contrast, non-magnetized planets have only small magnetospheres induced by interaction of the ionosphere with the solar wind, which can not effectively protect the planet.
Of the eight planets in our Solar System, only Venus and Mars lack such a magnetic field. In addition, the moon of Jupiter Ganymede also has one. Of the magnetized planets Mercury has the weakest magnetic field, and is barely enough to deflect the solar wind. Ganymede's magnetic field is several times larger, and Jupiter's is the strongest in the Solar System. The magnetic fields of other giant planets are roughly similar in strength of that of Earth but their magnetic moments are significantly larger than the Earth's magnetic moment. The magnetic fields of Uranus and Neptune are strongly tilted relative the rotational axis and displaced from the centre of the planet.
In 2004, a team of astronomers in Hawaii observed an extrasolar planet around the star HD 179949, which appeared to be creating a sunspot on the surface of its parent star. The team hypothesised that the planet's magnetosphere was transferring energy onto the star's surface increasing its already high 14,000 degree surface temperature by an additional 750 degrees.
Planets in our Solar System possess orbital resonances in their own right. All except Mercury and Venus have natural satellites, often called "moons." Earth has one, and Mars has two, and the gas giants 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 (especially Europa).
The four gas giants 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 that fell below their parent planet's Roche limit and were torn apart by tidal forces.
No secondary characteristics have been observed around extrasolar planets. However the sub-brown dwarf Cha 110913-773444, which has been described as a rogue planet, is believed to be orbited by a tiny protoplanetary disc.