Venus is the second planet from the Sun. It is named after the Roman goddess of love and beauty. As the second-brightest natural object in the night sky after the Moon, Venus can cast shadows and, rarely, is visible to the naked eye in broad daylight. Venus lies within Earths orbit, and so never appears to venture far from the Sun, either setting in the west just after dusk or rising in the east a bit before dawn. Venus orbits the Sun every 224.7 Earth days. With a rotation period of 243 Earth days, it takes longer to rotate about its axis than any planet in the Solar System and does so in the opposite direction to all but Uranus. Venus does not have any moons, a distinction it shares only with Mercury among planets in the Solar System. Venus is a terrestrial planet and is sometimes called Earths "sister planet" because of their similar size, mass, proximity to the Sun, and bulk composition. It is radically different from Earth in other respects. It has the densest atmosphere of the four terrestrial planets, consisting of more than 96% carbon dioxide. The atmospheric pressure at the planets surface is 92 times that of Earth, or roughly the pressure found 900 m 3.000 ft underwater on Earth. Venus is by far the hottest planet in the Solar System, with a mean surface temperature of 735 K 462 °C; 863 °F, even though Mercury is closer to the Sun. Venus is shrouded by an opaque layer of highly reflective clouds of sulfuric acid, preventing its surface from being seen from space in visible light. It may have had water oceans in the past, but these would have vaporized as the temperature rose due to a runaway greenhouse effect. The water has probably photodissociated, and the free hydrogen has been swept into interplanetary space by the solar wind because of the lack of a planetary magnetic field. Venuss surface is a dry desertscape interspersed with slab-like rocks and is periodically resurfaced by volcanism. As one of the brightest objects in the sky, Venus has been a major fixture in human culture for as long as records have existed. It has been made sacred to gods of many cultures, and has been a prime inspiration for writers and poets as the morning star and evening star. Venus was the first planet to have its motions plotted across the sky, as early as the second millennium BC. As the planet with the closest approach to Earth, Venus has been a prime target for early interplanetary exploration. It was the first planet beyond Earth visited by a spacecraft Mariner 2 in 1962, and the first to be successfully landed on by Venera 7 in 1970. Venuss thick clouds render observation of its surface impossible in visible light, and the first detailed maps did not emerge until the arrival of the Magellan orbiter in 1991. Plans have been proposed for rovers or more complex missions, but they are hindered by Venuss hostile surface conditions. In January 2020, astronomers reported evidence that suggests that Venus is currently volcanically active.
Mercury is the smallest and innermost planet in the Solar System. Its orbit around the Sun takes 87.97 days, the shortest of all the planets in the Solar System. It is named after the Roman deity Mercury, the messenger of the gods. Like Venus, Mercury orbits the Sun within Earths orbit as an inferior planet, and its apparent distance from the Sun as viewed from Earth never exceeds 28°. This proximity to the Sun means the planet can only be seen near the western horizon after sunset or eastern horizon before sunrise, usually in twilight. At this time, it may appear as a bright star-like object, but is often far more difficult to observe than Venus. The planet telescopically displays the complete range of phases, similar to Venus and the Moon, as it moves in its inner orbit relative to Earth, which recurs over its synodic period of approximately 116 days. Mercury rotates in a way that is unique in the Solar System. It is tidally locked with the Sun in a 3:2 spin–orbit resonance, meaning that relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun. As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two Mercurian years. Mercurys axis has the smallest tilt of any of the Solar Systems planets about 1 ⁄ 30 degree. Its orbital eccentricity is the largest of all known planets in the Solar System; at perihelion, Mercurys distance from the Sun is only about two-thirds or 66% of its distance at aphelion. Mercurys surface appears heavily cratered and is similar in appearance to the Moons, indicating that it has been geologically inactive for billions of years. Having almost no atmosphere to retain heat, it has surface temperatures that vary diurnally more than on any other planet in the Solar System, ranging from 100 K −173 °C; −280 °F at night to 700 K 427 °C; 800 °F during the day across the equatorial regions. The polar regions are constantly below 180 K −93 °C; −136 °F. The planet has no known natural satellites. Two spacecraft have visited Mercury: Mariner 10 flew by in 1974 and 1975; and MESSENGER, launched in 2004, orbited Mercury over 4.000 times in four years before exhausting its fuel and crashing into the planets surface on April 30, 2015. The BepiColombo spacecraft is planned to arrive at Mercury in 2025.
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass one-thousandth that of the Sun, but two-and-a-half times that of all the other planets in the Solar System combined. Jupiter is one of the brightest objects visible to the naked eye in the night sky, and has been known to ancient civilizations since before recorded history. It is named after the Roman god Jupiter. When viewed from Earth, Jupiter can be bright enough for its reflected light to cast shadows, and is on average the third-brightest natural object in the night sky after the Moon and Venus. Jupiter is primarily composed of hydrogen with a quarter of its mass being helium, though helium comprises only about a tenth of the number of molecules. It may also have a rocky core of heavier elements, but like the other giant planets, Jupiter lacks a well-defined solid surface. Because of its rapid rotation, the planets shape is that of an oblate spheroid it has a slight but noticeable bulge around the equator. The outer atmosphere is visibly segregated into several bands at different latitudes, resulting in turbulence and storms along their interacting boundaries. A prominent result is the Great Red Spot, a giant storm that is known to have existed since at least the 17th century when it was first seen by telescope. Surrounding Jupiter is a faint planetary ring system and a powerful magnetosphere. Jupiter has 79 known moons, including the four large Galilean moons discovered by Galileo Galilei in 1610. Ganymede, the largest of these, has a diameter greater than that of the planet Mercury. Pioneer 10 was the first spacecraft to visit Jupiter, making its closest approach to the planet on December 4, 1973; Pioneer 10 identified plasma in Jupiters magnetic field and also found that Jupiters magnetic tail was nearly 800 million kilometers long, covering the entire distance to Saturn. Jupiter has been explored on a number of occasions by robotic spacecraft, beginning with the Pioneer and Voyager flyby missions from 1973 to 1979, and later by the Galileo orbiter, which arrived at Jupiter in 1995. In late February 2007, Jupiter was visited by the New Horizons probe, which used Jupiters gravity to increase its speed and bend its trajectory en route to Pluto. The latest probe to visit the planet is Juno, which entered into orbit around Jupiter on July 4, 2016. Future targets for exploration in the Jupiter system include the probable ice-covered liquid ocean of its moon Europa.
Earth is the third planet from the Sun and the only astronomical object known to harbor life. According to radiometric dating and other evidence, Earth formed over 4.5 billion years ago. Earths gravity interacts with other objects in space, especially the Sun and the Moon, which is Earths only natural satellite. Earth orbits around the Sun in 365.256 days, a period known as an Earth sidereal year. During this time, Earth rotates about its axis about 365.256 times. Earths axis of rotation is tilted with respect to its orbital plane, producing seasons on Earth. The gravitational interaction between Earth and the Moon causes tides, stabilizes Earths orientation on its axis, and gradually slows its rotation. Earth is the densest planet in the Solar System and the largest and most massive of the four rocky planets. Earths outer layer lithosphere is divided into several rigid tectonic plates that migrate across the surface over many millions of years. About 29% of Earths surface is land consisting of continents and islands. The remaining 71% is covered with water, mostly by oceans but also lakes, rivers and other fresh water, which all together constitute the hydrosphere. The majority of Earths polar regions are covered in ice, including the Antarctic ice sheet and the sea ice of the Arctic ice pack. Earths interior remains active with a solid iron inner core, a liquid outer core that generates Earths magnetic field, and a convecting mantle that drives plate tectonics. Within the first billion years of Earths history, life appeared in the oceans and began to affect Earths atmosphere and surface, leading to the proliferation of anaerobic and, later, aerobic organisms. Some geological evidence indicates that life may have arisen as early as 4.1 billion years ago. Since then, the combination of Earths distance from the Sun, physical properties and geological history have allowed life to evolve and thrive. In the history of life on Earth, biodiversity has gone through long periods of expansion, occasionally punctuated by mass extinctions. Over 99% of all species that ever lived on Earth are extinct. Estimates of the number of species on Earth today vary widely; most species have not been described. Over 7.7 billion humans live on Earth and depend on its biosphere and natural resources for their survival. Politically, the world has around 200 sovereign states.
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System after Mercury. In English, Mars carries a name of the Roman god of war and is often referred to as the Red Planet. The latter refers to the effect of the iron oxide prevalent on Mars surface, which gives it a reddish appearance distinctive among the astronomical bodies visible to the naked eye. Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the valleys, deserts, and polar ice caps of Earth. The days and seasons are likewise comparable to those of Earth, because the rotational period as well as the tilt of the rotational axis relative to the ecliptic plane are very similar. Mars is the site of Olympus Mons, the largest volcano and highest known mountain on any planet in the Solar System, and of Valles Marineris, one of the largest canyons in the Solar System. The smooth Borealis basin in the northern hemisphere covers 40% of the planet and may be a giant impact feature. Mars has two moons, Phobos and Deimos, which are small and irregularly shaped. These may be captured asteroids, similar to 5261 Eureka, a Mars trojan. Mars has been explored by numerous unmanned spacecraft. Mariner 4, launched by NASA on November 28, 1964, was the first spacecraft to visit Mars, making its closest approach to the planet on July 15, 1965. Mariner 4 detected the weak Martian radiation belt, measured at about 0.1% that of Earth’s, and captured the first images of another planet from deep space. On July 20, 1976, Viking 1 performed the first successful landing on the Martian surface. Although the Soviet Mars 3 spacecraft achieved a soft landing in December 1971, contact was lost with its lander seconds after touchdown. On July 4, 1997, the Mars Pathfinder spacecraft landed on Mars, and on July 5 released its rover, Sojourner, the first robotic rover to operate on Mars. Pathfinder was followed by the Mars Exploration Rovers, Spirit and Opportunity, which landed on Mars in January 2004 and operated until March 22, 2010 and June 10, 2018, respectively. The Mars Express orbiter, the first European Space Agency spacecraft to visit Mars, arrived in orbit on December 25, 2003. On September 24, 2014, the Indian Space Research Organization became the fourth space agency to visit Mars, when its maiden interplanetary mission, the Mars Orbiter Mission spacecraft, successfully arrived in orbit. There are ongoing investigations assessing the past habitability potential of Mars, as well as the possibility of extant life. Future astrobiology missions are planned, including the Perseverance and Rosalind Franklin rovers. Liquid water cannot exist on the surface of Mars due to low atmospheric pressure, which is less than 1% of the Earths, except at the lowest elevations for short periods. The two polar ice caps appear to be made largely of water. The volume of water ice in the south polar ice cap, if melted, would be sufficient to cover the entire planetary surface to a depth of 11 meters 36 ft. In November 2016, NASA reported finding a large amount of underground ice in the Utopia Planitia region of Mars. The volume of water detected has been estimated to be equivalent to the volume of water in Lake Superior. Mars can easily be seen from Earth with the naked eye, as can its reddish coloring. Its apparent magnitude reaches −2.94, which is surpassed only by Venus, the Moon, and the Sun. Optical ground-based telescopes are typically limited to resolving features about 300 kilometers 190 mi across when Earth and Mars are closest because of Earths atmosphere.
Neptune is the eighth and farthest known planet from the Sun in the Solar System. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. Neptune is 17 times the mass of Earth, slightly more massive than its near-twin Uranus. Neptune is denser and physically smaller than Uranus because its greater mass causes more gravitational compression of its atmosphere. Neptune orbits the Sun once every 164.8 years at an average distance of 30.1 au. It is named after the Roman god of the sea and has the astronomical symbol ♆, a stylised version of the god Neptunes trident. Neptune is not visible to the unaided eye and is the only planet in the Solar System found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to gravitational perturbation by an unknown planet. The position of Neptune was subsequently calculated from Bouvards observations, independently, by John Couch Adams and Urbain Le Verrier after his death. Neptune was subsequently observed with a telescope on 23 September 1846 by Johann Galle within a degree of the position predicted by Le Verrier. Its largest moon, Triton, was discovered shortly thereafter, though none of the planets remaining 13 known moons were located telescopically until the 20th century. The planets distance from Earth gives it a very small apparent size, making it challenging to study with Earth-based telescopes. Neptune was visited by Voyager 2, when it flew by the planet on 25 August 1989; Voyager 2 remains the only spacecraft to visit Neptune. The advent of the Hubble Space Telescope and large ground-based telescopes with adaptive optics has recently allowed for additional detailed observations from afar. Like Jupiter and Saturn, Neptunes atmosphere is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, though it contains a higher proportion of "ices" such as water, ammonia and methane. However, similar to Uranus, its interior is primarily composed of ices and rock; Uranus and Neptune are normally considered "ice giants" to emphasise this distinction. Traces of methane in the outermost regions in part account for the planets blue appearance. In contrast to the hazy, relatively featureless atmosphere of Uranus, Neptunes atmosphere has active and visible weather patterns. For example, at the time of the Voyager 2 flyby in 1989, the planets southern hemisphere had a Great Dark Spot comparable to the Great Red Spot on Jupiter. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, with recorded wind speeds as high as 2.100 km/h 580 m/s; 1.300 mph. Because of its great distance from the Sun, Neptunes outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching 55 K −218 °C; −361 °F. Temperatures at the planets centre are approximately 5.400 K 5.100 °C; 9.300 °F. Neptune has a faint and fragmented ring system labelled "arcs", which was discovered in 1984, then later confirmed by Voyager 2.
A planet is an astronomical body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.
The term planet is ancient, with ties to history, astrology, science, mythology, and religion. Five planets in the Solar System are visible to the naked eye. These were regarded by many early cultures as divine, or as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union IAU officially adopted a resolution defining planets within the Solar System. This definition is controversial because it excludes many objects of planetary mass based on where or what they orbit. Although eight of the planetary bodies discovered before 1950 remain "planets" under the current definition, some celestial bodies, such as Ceres, Pallas, Juno and Vesta each an object in the solar asteroid belt, and Pluto the first trans-Neptunian object discovered, that were once considered planets by the scientific community, are no longer viewed as planets under the current definition of planet.
Planets in astrology have a different definition.
The planets were thought by Ptolemy to orbit Earth in deferent and epicycle motions. Although the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. About the same time, by careful analysis of pre-telescopic observational data collected by Tycho Brahe, Johannes Kepler found the planets orbits were elliptical rather than circular. As observational tools improved, astronomers saw that, like Earth, each of the planets rotated around an axis tilted with respect to its orbital pole, and some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by space probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics, and even hydrology.
Planets in the Solar System are divided into two main types: large low-density giant planets, and smaller rocky terrestrials. There are eight planets in the Solar System. In order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four giant planets, Jupiter, Saturn, Uranus, and Neptune. Six of the planets are orbited by one or more natural satellites.
Several thousands of planets around other stars "extrasolar planets" or "exoplanets" have been discovered in the Milky Way. As of 1 March 2020, 4.187 known extrasolar planets in 3.105 planetary systems including 681 multiple planetary systems, ranging in size from just above the size of the Moon to gas giants about twice as large as Jupiter have been discovered, out of which more than 100 planets are the same size as Earth, nine of which are at the same relative distance from their star as Earth from the Sun, i.e. in the circumstellar habitable zone. On December 20, 2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20. A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way. Around one in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone.
The idea of planets has evolved over its history, from the divine lights of antiquity to the earthly objects of the scientific age. The concept has expanded to include worlds not only in the Solar System, but in hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy.
The five classical planets, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, and ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the "fixed stars", which maintained a constant relative position in the sky. Ancient Greeks called these lights πλάνητες ἀστέρες planētes asteres, "wandering stars" or simply πλανῆται planētai, "wanderers", from which todays word "planet" was derived. In ancient Greece, China, Babylon, and indeed all pre-modern civilizations, it was almost universally believed that Earth was the center of the Universe and that all the "planets" circled Earth. The reasons for this perception were that stars and planets appeared to revolve around Earth each day and the apparently common-sense perceptions that Earth was solid and stable and that it was not moving but at rest.
1.1. History Babylon
The first civilization known to have a functional theory of the planets were the Babylonians, who lived in Mesopotamia in the first and second millennia BC. The oldest surviving planetary astronomical text 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 MUL.APIN is a pair of cuneiform tablets dating from the 7th century BC that lays out the motions of the Sun, Moon, and planets over the course of the year. The Babylonian astrologers 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. Venus, Mercury, and the outer planets Mars, Jupiter, and Saturn were all identified by Babylonian astronomers. These would remain the only known planets until the invention of the telescope in early modern times.
1.2. History Greco-Roman astronomy
The ancient Greeks initially did not attach as much significance to the planets as the Babylonians. The Pythagoreans, in the 6th and 5th centuries BC appear to have developed their own independent planetary theory, which consisted of the Earth, Sun, Moon, and planets revolving around a "Central Fire" at the center of the Universe. Pythagoras or Parmenides is said to have been the first to identify the evening star Hesperos and morning star Phosphoros as one and the same Aphrodite, Greek corresponding to Latin Venus, though this had long been known by the Babylonians. In the 3rd century BC, Aristarchus of Samos proposed a heliocentric system, according to which Earth and the planets revolved around the Sun. The geocentric system remained dominant until the Scientific Revolution.
By the 1st century BC, during the Hellenistic period, 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 most 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 CE. So complete was the domination of Ptolemys 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 Earth according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth in Ptolemys order and using modern names: the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.
Cicero, in his De Natura Deorum, enumerated the planets known during the 1st century BCE using the names for them in use at the time:"But there is most matter for wonder in the movements of the five stars which are falsely called wandering; falsely, because nothing wanders which through all eternity preserves its forward and retrograde courses, and its other movements, constant and unaltered. For instance, the star which is farthest from the earth, which is known as the star of Saturn, and is called by the Greeks Φαίνων Phainon, accomplishes its course in about thirty years, and though in that course it does much that is wonderful, first preceding the sun, and then falling off in speed, becoming invisible at the hour of evening, and returning to view in the morning, it never through the unending ages of time makes any variation, but performs the same movements at the same times. Beneath it, and nearer to the earth, moves the planet of Jupiter, which is called in Greek Φαέθων Phaethon; it completes the same round of the twelve signs in twelve years, and performs in its course the same variations as the planet of Saturn. The circle next below it is held by Πυρόεις Pyroeis, which is called the planet of Mars, and traverses the same round as the two planets above it in four and twenty months, all but, I think, six days. Beneath this is the planet of Mercury, which is called by the Greeks Στίλβων Stilbon; it traverses the round of the zodiac in about the time of the year’s revolution, and never withdraws more than one sign’s distance from the sun, moving at one time in advance of it, and at another in its rear. The lowest of the five wandering stars, and the one nearest the earth, is the planet of Venus, which is called Φωσϕόρος Phosphoros in Greek, and Lucifer in Latin, when it is preceding the sun, but Ἕσπερος Hesperos when it is following it; it completes its course in a year, traversing the zodiac both latitudinally and longitudinally, as is also done by the planets above it, and on whichever side of the sun it is, it never departs more than two signs’ distance from it."
1.3. History India
In 499 CE, the Indian astronomer Aryabhata propounded a planetary model that explicitly incorporated Earths rotation about its axis, which he explains as the cause of what appears to be an apparent westward motion of the stars. He also believed that the orbits of planets are elliptical. Aryabhatas followers were particularly strong in South India, where his principles of the diurnal rotation of Earth, among others, were followed and a number of secondary works were based on them.
In 1500, Nilakantha Somayaji of the Kerala school of astronomy and mathematics, in his Tantrasangraha, revised Aryabhatas model. In his Aryabhatiyabhasya, a commentary on Aryabhatas Aryabhatiya, he developed a planetary model where Mercury, Venus, Mars, Jupiter and Saturn orbit the Sun, which in turn orbits Earth, similar to the Tychonic system later proposed by Tycho Brahe in the late 16th century. Most astronomers of the Kerala school who followed him accepted his planetary model.
1.4. History Medieval Muslim astronomy
In the 11th century, the transit of Venus was observed by Avicenna, who established that Venus was, at least sometimes, below the Sun. In the 12th century, Ibn Bajjah observed "two planets as black spots on the face of the Sun", which was later identified as a transit of Mercury and Venus by the Maragha astronomer Qotb al-Din Shirazi in the 13th century. Ibn Bajjah could not have observed a transit of Venus, because none occurred in his lifetime.
1.5. History European Renaissance
With the advent of the Scientific Revolution, use of the term "planet" changed from something that moved across the sky in relation to the star field; to a body that orbited Earth or that was believed to do so at the time; and by the 18th century to something that directly orbited the Sun when the heliocentric model of Copernicus, Galileo and Kepler gained sway.
Thus, Earth became included in the list of planets, whereas the Sun and Moon were excluded. At first, when the first satellites of Jupiter and Saturn were discovered in 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 because any newly discovered object directly orbiting the Sun was listed as a planet by the scientific community.
1.6. History 19th century
In the 19th century astronomers began to realize that recently discovered bodies that had been classified as planets for almost half a century were very different from the traditional ones. 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. Because 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.
1.7. History 20th century
In the 20th century, Pluto was discovered. After initial observations led to the belief that 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, Ray Lyttleton suggested that Pluto may be an escaped satellite of Neptune, and Fred Whipple suggested in 1964 that Pluto may be a comet. 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 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 Geneva Observatory 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. Although objects more massive than 75 times that of Jupiter fuse hydrogen, objects of only 13 Jupiter masses can fuse deuterium. Deuterium is quite rare, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets.
1.8. History 21st century
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 were particular disagreements 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.
A growing number of astronomers argued for Pluto to be declassified as a planet, because many similar objects approaching its size had been found in the same region of the Solar System the Kuiper belt during the 1990s and early 2000s. Pluto was found to be just one small body in a population of thousands.
Some of them, such as Quaoar, Sedna, and Eris, were heralded in the popular press as the tenth planet, failing to receive widespread scientific recognition. The announcement of Eris in 2005, an object then thought of as 27% more massive than Pluto, created the necessity and public desire for an official definition of a planet.
Acknowledging the problem, the IAU set about creating the definition of planet, and produced one in August 2006. The number of planets dropped to the eight significantly larger bodies that had cleared their orbit, and a new class of dwarf planets was created, initially containing three objects Ceres, Pluto and Eris.
1.9. History Extrasolar planets
There is no official definition of extrasolar planets. In 2003, the International Astronomical Union IAU Working Group on Extrasolar Planets issued a position statement, but this position statement was never proposed as an official IAU resolution and was never voted on by IAU members. The positions statement incorporates the following guidelines, mostly focused upon the boundary between planets and brown dwarfs:
- 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 the 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 working definition has since been widely used by astronomers when publishing discoveries of exoplanets in academic journals. Although temporary, it remains an effective working definition until a more permanent one is formally adopted. It does not address the dispute over the lower mass limit, and so it steered clear of the controversy regarding objects within the Solar System. This definition also makes no comment on the planetary status of objects orbiting brown dwarfs, such as 2M1207b.
One definition of a sub-brown dwarf is a planet-mass object that formed through cloud collapse rather than accretion. This formation distinction between a sub-brown dwarf and a planet is not universally agreed upon; astronomers are divided into two camps as whether to consider the formation process of a planet as part of its division in classification. One reason for the dissent is that often it may not be possible to determine the formation process. For example, a planet formed by accretion around a star may get ejected from the system to become free-floating, and likewise a sub-brown dwarf that formed on its own in a star cluster through cloud collapse may get captured into orbit around a star.
One study suggests that objects above 10 M Jup formed through gravitational instability and should not be thought of as planets.
The 13 Jupiter-mass cutoff represents an average mass rather than a precise threshold value. Large objects will fuse most of their deuterium and smaller ones will fuse only a little, and the 13 M J value is somewhere in between. In fact, calculations show that an object fuses 50% of its initial deuterium content when the total mass ranges between 12 and 14 M J. The amount of deuterium fused depends not only on mass but also on the composition of the object, on the amount of helium and deuterium present. As of 2011 the Extrasolar Planets Encyclopaedia included objects up to 25 Jupiter masses, saying, "The fact that there is no special feature around 13 M Jup in the observed mass spectrum reinforces the choice to forget this mass limit". As of 2016 this limit was increased to 60 Jupiter masses based on a study of mass–density relationships. The Exoplanet Data Explorer includes objects up to 24 Jupiter masses with the advisory: "The 13 Jupiter-mass distinction by the IAU Working Group is physically unmotivated for planets with rocky cores, and observationally problematic due to the sin i ambiguity." The NASA Exoplanet Archive includes objects with a mass or minimum mass equal to or less than 30 Jupiter masses.
Another criterion for separating planets and brown dwarfs, rather than deuterium fusion, formation process or location, is whether the core pressure is dominated by coulomb pressure or electron degeneracy pressure.
1.10. History 2006 IAU definition of planet
The matter of the lower limit was addressed during the 2006 meeting of the IAUs General Assembly. After much debate and one failed proposal, a large majority of those remaining at the meeting voted to pass a resolution. The 2006 resolution defines planets within the Solar System as follows:
A "planet", differ from planets in that they can collide with each other and with planets."
The 2006 IAU definition presents some challenges for exoplanets because the language is specific to the Solar System and because the criteria of roundness and orbital zone clearance are not presently observable. Astronomer Jean-Luc Margot proposed a mathematical criterion that determines whether an object can clear its orbit during the lifetime of its host star, based on the mass of the planet, its semimajor axis, and the mass of its host star. This formula produces a value π that is greater than 1 for planets. The eight known planets and all known exoplanets have π values above 100, while Ceres, Pluto, and Eris have π values of 0.1 or less. Objects with π values of 1 or more are also expected to be approximately spherical, so that objects that fulfill the orbital zone clearance requirement automatically fulfill the roundness requirement.
1.11. History Objects formerly considered planets
The table below lists Solar System bodies once considered to be planets.
Beyond the scientific community, Pluto still holds cultural significance for many in the general public due to its historical classification as a planet from 1930 to 2006.
2. Mythology and naming
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 Saturn was called Phainon, the shiner; followed by Phaethon Jupiter, "bright"; the red planet Mars was known as Pyroeis, the "fiery"; the brightest Venus was known as Phosphoros, the light bringer; and the fleeting final planet Mercury was called Stilbon, the gleamer. The Greeks also made each planet sacred to one among their pantheon of gods, the Olympians: Helios and Selene were the names of both planets and gods; Phainon was sacred to Cronus, the Titan who fathered the Olympians; Phaethon was sacred to Zeus, Cronuss son who deposed him as king; Pyroeis was given to Ares, son of Zeus and god of war; Phosphoros 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 their gods names onto the planets was almost certainly borrowed from the Babylonians. The Babylonians named Phosphoros 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. 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. Although 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 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 Cronus. When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained with Neptūnus Poseidon. Uranus is unique in that it is named for a Greek deity rather than his Roman counterpart.
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. Because 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 after the Nundinal cycle was rejected – and still preserved in many modern languages. In English, Saturday, Sunday, and Monday are straightforward translations of these Roman names. The other days were renamed after Tiw Tuesday, Woden Wednesday, Thunor Thursday, and Frige Friday, the Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus, respectively.
Earth is the only planet whose name in English is not derived from Greco-Roman mythology. Because it was only generally accepted as a planet in the 17th century, there is no tradition of naming it after a god. 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 Earth perhaps around 1300. As with its equivalents in the other Germanic languages, it derives ultimately from the Proto-Germanic word ertho, "ground", as can be seen in the English earth, the German Erde, the Dutch aarde, and the Scandinavian jord. 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". The non-Romance languages use their own native words. The Greeks retain their original name, Γή Ge.
Non-European cultures use other planetary-naming systems. India uses a system based on the Navagraha, which incorporates the seven traditional planets and the ascending and descending lunar nodes Rahu and Ketu.
China and the countries of eastern Asia historically subject to Chinese cultural influence 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.
In traditional Hebrew astronomy, the seven traditional planets have for the most part descriptive names – the Sun is חמה Hammah or "the hot one," the Moon is לבנה Levanah or "the white one," Venus is כוכב נוגה Kokhav Nogah or "the bright planet," Mercury is כוכב Kokhav or "the planet" given its lack of distinguishing features, Mars is מאדים Maadim or "the red one," and Saturn is שבתאי Shabbatai or "the resting one" in reference to its slow movement compared to the other visible planets. The odd one out is Jupiter, called צדק Tzedeq or "justice". Steiglitz suggests that this may be a euphemism for the original name of כוכב בעל Kokhav Baal or "Baals planet", seen as idolatrous and euphemized in a similar manner to Ishbosheth from II Samuel.
In Arabic, Mercury is عُطَارِد ʿUtārid, cognate with Ishtar / Astarte, Venus is الزهرة, Earth is الأرض al-ʾArd, from the same root as eretz, Mars is اَلْمِرِّيخ al-Mirrīkh, meaning "featherless arrow" due to its retrograde motion, Jupiter is المشتري and Saturn is زُحَل Zuhal, "withdrawer".
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 mass somewhat larger than Mars mass, it begins to accumulate an extended atmosphere, greatly increasing the capture rate of the planetesimals by means of atmospheric drag. Depending on the accretion history of solids and gas, a giant planet, an ice giant, or a terrestrial planet may result.
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. 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 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 the Sun, 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 thought to determine the likelihood that a star will have planets. Hence, it is thought that a metal-rich population I star will likely have a more substantial planetary system than a metal-poor, population II star.
4. Solar System
There are eight planets in the Solar System, which are in increasing distance from the Sun:
- ♄ Saturn
- ♃ Jupiter
- ⊕ Earth
- ♆ Neptune
- ☿ Mercury
- ♅ Uranus
- ♀ Venus
- ♂ Mars
Jupiter is the largest, at 318 Earth masses, whereas Mercury is the smallest, at 0.055 Earth masses.
The planets of the Solar System can be divided into categories based on their composition:
- Ice giants, Uranus and Neptune, are primarily composed of low-boiling-point materials such as water, methane, and ammonia, with thick atmospheres of hydrogen and helium. They have a significantly lower mass than the gas giants only 14 and 17 Earth masses.
- Giant planets Jovians: Massive planets significantly more massive than the terrestrials: Jupiter, Saturn, Uranus, Neptune.
- Terrestrials: Planets that are similar to Earth, with bodies largely composed of rock: Mercury, Venus, Earth and Mars. At 0.055 Earth masses, Mercury is the smallest terrestrial planet and smallest planet in the Solar System. Earth is the largest terrestrial planet.
- Gas giants, Jupiter and Saturn, are giant planets primarily composed of hydrogen and helium and are the most massive planets in the Solar System. Jupiter, at 318 Earth masses, is the largest planet in the Solar System, and Saturn is one third as massive, at 95 Earth masses.
An exoplanet extrasolar planet is a planet outside the Solar System. As of 1 March 2020, there are 4.187 confirmed exoplanets in 3.105 systems, with 681 systems having more than one planet.
In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12. This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of giant planets that survived the supernova and then decayed into their current orbits.
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. From then until the Kepler mission most known extrasolar planets were gas giants comparable in mass to Jupiter or larger as they were more easily detected. The catalog of Kepler candidate planets consists mostly of planets the size of Neptune and smaller, down to smaller than Mercury.
There are types of planets that do not exist in the Solar System: super-Earths and mini-Neptunes, which could be rocky like Earth or a mixture of volatiles and gas like Neptune - a radius of 1.75 times that of Earth is a possible dividing line between the two types of planet. There are hot Jupiters that orbit very close to their star and may evaporate to become chthonian planets, which are the leftover cores. Another possible type of planet is carbon planets, which form in systems with a higher proportion of carbon than in the Solar System.
A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way.
On December 20, 2011, the Kepler Space Telescope team reported the discovery of the first Earth-size exoplanets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20.
Around 1 in 5 Sun-like stars have an "Earth-sized" planet in the habitable zone, so the nearest would be expected to be within 12 light-years distance from Earth. 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 the Milky Way.
There are exoplanets that are much closer to their parent star than any planet in the Solar System is to the Sun, and there are also exoplanets that are much farther from their star. Mercury, the closest planet to the Sun at 0.4 AU, takes 88 days for an orbit, but the shortest known orbits for exoplanets take only a few hours, see Ultra-short period planet. The Kepler-11 system has five of its planets in shorter orbits than Mercurys, all of them much more massive than Mercury. Neptune is 30 AU from the Sun and takes 165 years to orbit, but there are exoplanets that are hundreds of AU from their star and take more than a thousand years to orbit, e.g. 1RXS1609 b.
6. Planetary-mass objects
A planetary-mass object PMO, planemo, or planetary body is a celestial object with a mass that falls within the range of the definition of a planet: massive enough to achieve hydrostatic equilibrium to be rounded under its own gravity, but not enough to sustain core fusion like a star. By definition, all planets are planetary-mass objects, but the purpose of this term is to refer to objects that do not conform to typical expectations for a planet. These include dwarf planets, which are rounded by their own gravity but not massive enough to clear their own orbit, the larger moons, and free-floating planemos, which may have been ejected from a system rogue planets or formed through cloud-collapse rather than accretion sometimes called sub-brown dwarfs.
6.1. Planetary-mass objects Dwarf planets
A dwarf planet is a planetary-mass object that is neither a true planet nor a natural satellite; it is in direct orbit of a star, and is massive enough for its gravity to compress it into a hydrostatically equilibrious shape usually a spheroid, but has not cleared the neighborhood of other material around its orbit. Alan Stern, who proposed the term dwarf planet, has argued that location should not matter and that only geophysical attributes should be taken into account geophysical planet definition, and that dwarf planets are thus a subtype of planet. However, the IAU classifies dwarf planets as a separate category. The number of dwarf planets in the Solar System is unknown. IAU has recognized three Ceres, Pluto and Eris and assigned the naming of two additional candidates, Haumea and Makemake, to the IAU dwarf-planet naming committee.
6.2. Planetary-mass objects Rogue planets
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", although others have suggested that they should be called low-mass brown dwarfs.
6.3. Planetary-mass objects Sub-brown dwarfs
Stars form via the gravitational collapse of gas clouds, but smaller objects can also form via cloud-collapse. Planetary-mass objects formed this way are sometimes called sub-brown dwarfs. Sub-brown dwarfs may be free-floating such as Cha 110913-773444 and OTS 44, or orbiting a larger object such as 2MASS J04414489+2301513.
Binary systems of sub-brown dwarfs are theoretically possible; Oph 162225-240515 was initially thought to be a binary system of a brown dwarf of 14 Jupiter masses and a sub-brown dwarf of 7 Jupiter masses, but further observations revised the estimated masses upwards to greater than 13 Jupiter masses, making them brown dwarfs according to the IAU working definitions.
6.4. Planetary-mass objects Former stars
In close binary star systems one of the stars can lose mass to a heavier companion. Accretion-powered pulsars may drive mass loss. The shrinking star can then become a planetary-mass object. An example is a Jupiter-mass object orbiting the pulsar PSR J1719-1438. These shrunken white dwarfs may become a helium planet or carbon planet.
6.5. Planetary-mass objects Satellite planets
Some large satellites moons are of similar size or larger than the planet Mercury, e.g. Jupiters Galilean moons and Titan. Alan Stern has argued that location should not matter and that only geophysical attributes should be taken into account in the definition of a planet, and proposes the term satellite planet for a planet-sized satellite.
6.6. Planetary-mass objects Captured planets
Rogue planets in stellar clusters have similar velocities to the stars and so can be recaptured. They are typically captured into wide orbits between 100 and 10 5 AU. The capture efficiency decreases with increasing cluster volume, and for a given cluster size it increases with the host/primary mass. It is almost independent of the planetary mass. Single and multiple planets could be captured into arbitrary unaligned orbits, non-coplanar with each other or with the stellar host spin, or pre-existing planetary system.
Although each planet has unique physical characteristics, a number of broad commonalities do exist among them. Some of these characteristics, such as rings or natural satellites, have only as yet been observed in planets in the Solar System, whereas others are also commonly observed in extrasolar planets.
7.1. Attributes Orbit
According to current definitions, all planets must revolve around stars; thus, any potential "rogue planets" are excluded. In the Solar System, all the planets orbit the Sun in the same direction as the Sun rotates counter-clockwise as seen from above the Suns north pole. At least one extrasolar planet, WASP-17b, has been found to orbit in the opposite direction to its stars rotation. The period of one revolution of a planets orbit is known as its sidereal period or year. A planets 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, because it is less affected by its stars gravity. No planets orbit is perfectly circular, and hence 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, whereas its farthest separation from the star is called its apastron aphelion. As a planet approaches periastron, its speed increases as it trades gravitational potential energy for kinetic energy, just as a falling object on Earth accelerates as it falls; as the planet reaches apastron, its speed decreases, just as an object thrown upwards on Earth slows down as it reaches the apex of its trajectory.
Each planets orbit is delineated by a set of elements:
- The inclination of a planet tells how far above or below an established reference plane its orbit lies. In the Solar System, the reference plane is the plane of Earths orbit, called the ecliptic. For extrasolar planets, the plane, known as the sky plane or plane of the sky, is the plane perpendicular to the observers line of sight from Earth. The eight planets of the 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 planes 0 longitude and the planets ascending node. The argument of periapsis or perihelion in the Solar System is the angle between a planets ascending node and its closest approach to its star.
- The eccentricity of an orbit describes how elongated a planets orbit is. Planets with low eccentricities have more circular orbits, whereas planets with high eccentricities have more elliptical orbits. The planets in the 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 apastron, because no planets orbit has its star at its exact centre.
7.2. Attributes Axial tilt
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 has seasons, changes to the climate over the course of its year. The time at which each hemisphere points 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. Jupiters 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 have negligible to no axial tilt as a result of their proximity to their stars.
7.3. Attributes Rotation
The planets rotate around invisible axes through their centres. A planets rotation period is known as a stellar day. Most of the planets in the Solar System rotate in the same direction as they orbit the Sun, which is counter-clockwise as seen from above the Suns north pole, the exceptions being Venus and Uranus, which rotate clockwise, though Uranuss extreme axial tilt means there are differing conventions on which of its poles is "north", and therefore whether it is rotating clockwise or anti-clockwise. Regardless of which convention is used, Uranus has a retrograde rotation relative to its orbit.
The rotation of a planet can be induced by several factors during formation. A net angular momentum can be induced by the individual angular momentum contributions of accreted objects. The accretion of gas by the giant planets can also contribute to the angular momentum. Finally, during the last stages of planet building, a stochastic process of protoplanetary accretion can randomly alter the spin axis of the planet. There is great variation in the length of day between the planets, with Venus taking 243 days to rotate, and the giant planets only a few hours. The rotational periods of extrasolar planets are not known. However, for "hot" Jupiters, their proximity to their stars means that they are tidally locked i.e., their orbits are in sync with their rotations. This means, they always show one face to their stars, with one side in perpetual day, the other in perpetual night.
7.4. Attributes Orbital clearing
The defining dynamic characteristic of a planet is that it has cleared its neighborhood. A planet that has cleared its neighborhood 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 IAUs 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 the Solar System, a number of young extrasolar systems have been found in which evidence suggests orbital clearing is taking place within their circumstellar discs.
7.5. Attributes Mass
A planets 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 Jupiters mass for objects with solar-type isotopic abundance, beyond which it achieves conditions suitable for nuclear fusion. Other than the Sun, no objects of such mass exist in the Solar System; but there are exoplanets of this size. The 13-Jupiter-mass limit is not universally agreed upon and the Extrasolar Planets Encyclopaedia includes objects up to 60 Jupiter masses, and the Exoplanet Data Explorer up to 24 Jupiter masses.
The smallest known planet is PSR B1257+12A, 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. The smallest known planet orbiting a main-sequence star other than the Sun is Kepler-37b, with a mass and radius slightly higher than that of the Moon.
7.6. Attributes 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 that either is or was a fluid. The terrestrial planets are sealed within hard crusts, but in the giant planets the mantle simply blends into the upper cloud layers. The terrestrial planets have cores of elements such as iron and nickel, and mantles of silicates. Jupiter and Saturn are believed to have cores of rock and metal surrounded by mantles of metallic hydrogen. Uranus and Neptune, which are smaller, have 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.
7.7. Attributes Atmosphere
All of the Solar System planets except Mercury have substantial atmospheres because their gravity is strong enough to keep gases close to the surface. The larger giant planets are massive enough to keep large amounts of the light gases hydrogen and helium, whereas the smaller planets lose these gases into space. The composition of Earths atmosphere is different from the other planets because the various life processes that have transpired on the planet have introduced free molecular oxygen.
Planetary atmospheres are affected by the varying insolation or internal energy, leading to the formation of dynamic weather systems such as hurricanes, on Earth, planet-wide dust storms on Mars, a greater-than-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 have such a weather system, similar to the Great Red Spot but twice as large.
Hot Jupiters, due to their extreme proximities to their host stars, 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 that produce supersonic winds, although the day and night sides of HD 189733 b appear to have very similar temperatures, indicating that planets atmosphere effectively redistributes the stars energy around the planet.
7.8. Attributes Magnetosphere
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 in the solar wind around itself called the magnetosphere, which the wind cannot penetrate. The magnetosphere can be much larger than the planet itself. In contrast, non-magnetized planets have only small magnetospheres induced by interaction of the ionosphere with the solar wind, which cannot effectively protect the planet.
Of the eight planets in the 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 the magnetic field of Mercury is the weakest, and is barely able to deflect the solar wind. Ganymedes magnetic field is several times larger, and Jupiters is the strongest in the Solar System so strong in fact that it poses a serious health risk to future manned missions to its moons. The magnetic fields of the other giant planets are roughly similar in strength to that of Earth, but their magnetic moments are significantly larger. 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 hypothesized that the planets magnetosphere was transferring energy onto the stars surface, increasing its already high 7.760 °C temperature by an additional 400 °C.
7.9. Attributes Secondary characteristics
Several planets or dwarf planets in the Solar System such as Neptune and Pluto have orbital periods that are in resonance with each other or with smaller bodies this is also common in satellite systems. All except Mercury and Venus have natural satellites, often called "moons". Earth has one, Mars has two, and the giant planets have numerous moons in complex planetary-type systems. Many moons of the giant planets have features similar to those on the terrestrial planets and dwarf planets, and some have been studied as possible abodes of life especially Europa.
The four giant planets 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 planets Roche limit and were torn apart by tidal forces.
No secondary characteristics have been observed around extrasolar planets. 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 and the sub-brown dwarf OTS 44 was shown to be surrounded by a substantial protoplanetary disk of at least 10 Earth masses.
Vulcan (hypothetical planet)
Phaeton (hypothetical planet)
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