Encyclopedia Search Results
|
Planet
This article is about the astronomical term. For a related but distinct class of objects, see dwarf planet. For the astrological concept, see Planets in astrology. For other uses, see Planet (disambiguation).
 A planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion in its core, and has cleared its neighbouring region of planetesimals.[1][2] After stars and stellar remnants, planets are some of the most massive objects known to humanity. They play an important part in the structure of planetary systems, and are also considered, along with large moons, the most feasible environment for life.[3] Thus planetary science is essential not only to comprehend the structure of the universe, but also to better understand the development of life, and to aid the search for extraterrestrial intelligence. Additionally, the planets visible from Earth have played a vital role in the shaping of human culture, religion and philosophy in numerous civilisations. 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. Under IAU definitions, there are eight planets in the Solar System (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and also at least three dwarf planets (Ceres, Pluto, and Eris). Many of these planets are orbited by one or more moons, which can be larger than small planets. There have also been more than two hundred planets discovered orbiting other stars.[4] Planets are generally divided into two main types: large, low-density gas giants and smaller, rocky terrestrials. Dwarf planets, a separate category, can either be terrestrials or frozen ice dwarfs. As there has been no formal scientific definition of "planet" historically, the number of objects described as such has varied through the centuries. This changed in 2006, when the IAU officially adopted a resolution defining planets within the Solar System. This definition has been both praised and criticised, and remains disputed by some scientists.
Etymology
In ancient times, astronomers noted how certain lights moved across the sky in relation to the other stars. These objects were believed to orbit the Earth, which was considered to be stationary. The lights were first called "πλανήτης" (planētēs), meaning "wanderer", by the ancient Greeks, and it is from this that the word "planet" was derived. In near-universal practice in the Western world, the planets in the Solar system are named after Graeco-Roman gods, as, in Europe, it was the Greeks who first named them. However, the practice of naming planets after gods originated in the Western world with the Sumerians, who lived in modern-day Iraq in about 3000 BCE. Successive Mesopotamian civilizations, such as the Babylonians, retained the Sumerian naming convention but adapted it to their own very different pantheons. The Greeks borrowed much of their astronomy, including constellations and the zodiac, from the Babylonians, and by 600 BCE had already begun using Babylonian concepts in their calculations.[5] The Greeks grafted the names of their own gods onto the Babylonian planet list, although there was some confusion in translation. For instance, the Babylonian Nergal was a god of war, and the Greeks, seeing this aspect of Nergal's persona, identified him with Ares, their god of war. However, Nergal, unlike Ares, was also a god of the dead and a god of pestilence.[6] Because of the influence of the Roman Empire and, later, the Catholic Church, in most countries in the West the planets are known by their Roman (or Latin) names rather than the Greek. The Romans, who, like the Greeks, were Indo-Europeans, shared with them a common pantheon under different names but lacked the rich narrative traditions that Greek poetic culture had given their gods. During the later period of the Roman Republic, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable. When the Romans studied Greek astronomy, they gave the planets their own gods' names. To the Greeks and Romans, there were five known planets; each presumed to be circling the Earth according to the complex laws laid out by Claudius Ptolemy in the 2nd century. They were, in increasing order from Earth: Mercury (called Hermes by the Greeks), Venus (Aphrodite), Mars (Ares), Jupiter (Zeus), and Saturn (Kronos). Although strictly the term "planetes" referred only to those five objects, the term was often expanded to include the Sun and the Moon.[7] When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained: Uranus (Ouranos) and Neptune (Poseidon). The Greeks still use their original names for the planets. Some Romans, following a belief imported from Mesopotamia into Hellenistic Egypt,[8] believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts began with Jupiter and worked inwards; as a result, a list of which god had charge of the first hour in each day became Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn, i.e. the usual weekday name order.[9] Sunday, Monday, and Saturday are straightforward translations of these Roman names. In English the other days were renamed after Tiw, Wóden, Thunor, and Fríge, 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. Many of the Romance languages (including French, Italian, Spanish and Portuguese), which are descended from Latin, retain the old Roman name of Terra or some variation thereof. However, the non-Romance languages use their own respective native words. Again, the Greeks retain their original name, Γή (Ge or Yi); the Germanic languages, including English, use a variation of an ancient Germanic word ertho, "ground," as can be seen in the English Earth, the German Erde, the Dutch Aarde, and the Scandinavian Jorde. The same is true for the Sun and the Moon, though they are no longer considered planets. Some non-European cultures use their own planetary naming systems. India uses a naming system based on the Navagraha, which incorporates the seven traditional planets (Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn) and the ascending and descending lunar nodes Rahu and Ketu. China, and the countries of eastern Asia subject to Chinese cultural influence, such as Japan, Korea and Vietnam, use a naming system based on the five Chinese elements.[9] History
As scientific knowledge progressed, understanding of the term "planet" changed from something that moved across the sky (in relation to the starfield), to a body that orbited the Earth (or that were believed to do so at the time). When the heliocentric model gained sway in the 16th century, it became accepted that a planet was actually something that directly orbited the Sun. Thus the Earth was itself a planet,[10] while the Sun and Moon were not. At the end of the 17th century, when the first satellites of Saturn were discovered, the terms "planet" and "satellite" were at first used interchangeably, although "satellite" would gradually become more prevalent in the following century.[11] Until the mid-19th century, any newly discovered object orbiting the Sun was listed with the planets by the scientific community, and the number of "planets" swelled rapidly towards the end of that period. During the 1800s, astronomers began to realize most recent discoveries were unlike the traditional planets. They shared the same region of space, between Mars and Jupiter, and had a far smaller mass. Bodies such as Ceres, Pallas, and Vesta, which had been classed as planets for almost half a century, became classified with the new designation "asteroid." From this point, a "planet" came to be understood, in the absence of any formal definition, as any "large" body that orbited the Sun. There was no apparent need to create a set limit, as there was a dramatic size gap between the asteroids and the planets, and the spate of new discoveries seemed to have ended after the discovery of Neptune in 1846.[12] However, in the 20th century, Pluto was discovered. After initial observations led to the belief it was larger than Earth, the recently-created IAU accepted the object as a planet. Further monitoring found the body was actually much smaller, but, as it was still larger than all known asteroids and seemingly did not exist within a larger population, it kept its status for some seventy years.[13] In the 1990s and early 2000s, there was a flood of discoveries of similar objects in the same region of the Solar System. Like Ceres and the asteroids before it, Pluto was found to be just one small body in a population of thousands. A growing number of astronomers argued for it to be declassified as a planet, since many similar objects approaching its size were found. The discovery of Eris, a more massive object widely publicised as the tenth planet, brought things to a head. The IAU set about creating the definition of planet, and eventually produced one in 2006. The number of planets dropped to the eight significantly larger bodies that had cleared their orbit (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus & Neptune), and a new class of dwarf planets was created, initially containing three objects (Ceres, Pluto and Eris).[14] Former planetsIn ancient times, astronomers accepted as "planets" the seven visible objects that moved across the starfield: the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn. Since then, many objects have qualified as planets for a time:
Definition and disputesWith the discovery during the latter half of the twentieth century of more objects within the Solar System and large objects around other stars, disputes arose over what should constitute a planet. There was particular disagreement over whether an object should be considered a planet if it was part of a distinct population such as a belt, or if it was large enough to generate energy by the thermonuclear fusion of deuterium.  In 2003, The International Astronomical Union (IAU) Working Group on Extrasolar Planets made a position statement on the definition of a planet that incorporated a working definition:[2]
This definition has since been widely used[citation needed] by astronomers when publishing discoveries in journals, although it remains a temporary yet effective, working definition until a more permanent one is formally adopted. It also did not address the dispute over the lower mass limit and steered clear of the controversy regarding objects within the Solar System. This matter was finally addressed during the 2006 meeting of the IAU's General Assembly. After much debate and one failed proposal, the assembly voted to pass a resolution that defined planets within the Solar System as:[1]
Under this definition, the Solar System is considered to have eight planets. Bodies which fulfill the first two conditions but not the third (such as Pluto and Eris) are classified as dwarf planets, providing they are not also natural satellites of other planets. Originally an IAU committee had proposed a definition that would have included a much larger number of planets as it did not include (c) as a criterion. After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets. This definition is based in modern theories of planetary formation, in which planetary embryos initially clear their orbital neighborhood of other smaller objects. As described by astronomer Steven Soter:
In the aftermath of the IAU's 2006 vote, there has been criticism of the new definition,[17] and some astronomers have even stated that they will not use it.[18] 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, in a similar way to Ceres and its kin in the 1800s. More recently, the discovery of Eris was widely reported in the media as the "tenth planet". The reclassification of all three objects as dwarf planets has attracted much media and public attention.[19] FormationIt is not known with certainty how planets are formed. The prevailing theory is that they are formed during the collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, surrounded by a rotating protoplanetary disk. Through accretion—a process of sticky collision—dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planetesimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever more dense until they collapse inward under gravity to form protoplanets.[20] 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.[21] 
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.[22][23] 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.[24][25] 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.[26] (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—a astronomical term describing the abundance of isotopes with an atomic number greater than 2 (Helium)—is now believed to determine the likelihood that a star will have planets.[27] 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. Within the Solar System
According to the IAU's current definitions there are eight planets in the Solar System. In increasing distance from the Sun, they are: The larger bodies of the Solar System can be divided into categories based on their composition:
AttributesAlthough each of the planets in the Solar System has unique physical characteristics, a number of broad commonalities do exist between them. Dynamic characteristics
All the planets revolve around the Sun in the same direction — counter-clockwise as seen from over the Sun's north pole. 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 the Sun; the farther a planet is from the Sun, not only the longer the distance it must travel, but also the slower its speed, as it is less affected by the Sun's gravity. Because no planet's orbit is perfectly circular, the distance of each varies over the course of its year. Its closest distance to the Sun is called its perihelion, while its farthest distance from the Sun is called its aphelion. As a planet approaches perihelion, its speed increases as the pull of the Sun's gravity strengthens; as it reaches aphelion, its speed decreases. Each planet's orbit is delineated by a set of elements:
Planets also have varying degrees of axial tilt; they lie at an angle to the plane of the Sun's equator. This causes the amount of sunlight received by each hemisphere to vary over the course of its year; when the northern hemisphere points away from the Sun, 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/nearest from the Sun is known as its solstice. Each planet has two in the course of its orbit; when one hemisphere has its summer solstice, when its day is longest, the other has its winter solstice, when its day is shortest. Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either perpetually in sunlight or perpetually in darkness around the time of its solstices. The planets also rotate around invisible axes through their centres. A planet's rotation period is known as its day. All the planets rotate in a counter-clockwise direction, except for Venus, which rotates clockwise (Uranus, because of its extreme axial tilt, 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. Physical characteristicsOne of a planet's defining characteristics is that it is large 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 size, 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. 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. All of the planets have atmospheres as their large masses mean gravity is strong enough to keep gaseous particles close to the surface. The larger gas giants are massive enough to keep large amounts of the light gases Hydrogen and Helium close by, although these gases mostly float into space around the smaller planets. Earth's atmosphere is greatly different to the other planets because of the various life processes that have transpired there, while the atmosphere of Mercury has mostly, although not entirely, been 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) and Earth-sized anticyclones (on Jupiter). Secondary characteristicsMany of the planets have natural satellites, often called "moons." Mercury and Venus have no moons, the Earth has one, and Mars has two, but the gas giants all have numerous moons in complex planetary systems. Many gas giant moons have similar features to the terrestrial planets and dwarf planets, and some have been studied for signs of life. The four largest 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 which fell below their parent planet's Roche limit and were torn apart by tidal forces.
Dwarf planetsBefore the August 2006 decision, several objects were proposed by astronomers, including at one stage by the IAU, as planets. However in 2006 several of these objects were reclassified as dwarf planets, objects distinct from planets. Currently three dwarf planets in the Solar System are recognized by the IAU: Ceres, Pluto and Eris. Several other objects in both the asteroid belt and the Kuiper belt are under consideration, with as many as 50 that could eventually qualify. There may be as many as 200 that could be discovered once the Kuiper Belt has been fully explored. Dwarf planets share many of the same characteristics as planets, although notable differences remain—namely that they are not dominant in their orbits. Their attributes are:
By definition, all dwarf planets are members of larger populations. Ceres is the largest body in the asteroid belt, while Pluto is a member of the Kuiper belt and Eris is a member of the scattered disc. According to Mike Brown there may soon be over forty trans-Neptunian objects that qualify as dwarf planets under the IAU's recent definition.[28] Beyond the Solar SystemExtrasolar planetsOf the 243 extrasolar planets discovered by June 2007, most have masses which are comparable to or larger than Jupiter's.[29] Exceptions include a number of planets discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12,[30] the planets orbiting the stars Mu Arae, 55 Cancri and GJ 436 which are approximately Neptune-sized,[31] and a planet orbiting Gliese 876 that is estimated to be about 6 to 8 times as massive as the Earth and is probably rocky in composition. It is far from clear if the newly discovered large planets would resemble the gas giants in the Solar System or if they are of an entirely different type as yet unknown, like ammonia giants or carbon planets. In particular, some of the newly discovered planets, known as hot Jupiters, orbit extremely close to their parent stars, in nearly circular orbits. They therefore receive much more stellar radiation than the gas giants in the Solar System, which makes it questionable whether they are the same type of planet at all. There is also a class of hot Jupiters that orbit so close to their star that their atmospheres are slowly blown away in a comet-like tail: the Chthonian planets. Several projects have been proposed to create an array of space telescopes to search for extrasolar planets with masses comparable to the Earth. The NASA Terrestrial Planet Finder was one such program, but (as of 2006-02-06) this program has been put on indefinite hold. The ESA is considering a comparable mission called Darwin. 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. Interstellar "planets"Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space. Some scientists have argued that such objects found roaming in deep space should be classed as "planets". However, many others argue that only planemos that directly orbit stars should qualify as planets, preferring to use the terms "planetary body", "planetary mass object" or "planemo" for similar free-floating objects (as well as planetary-sized moons). The IAU's working definition on extrasolar planets takes no position on the issue. The discoverers of the bodies mentioned above decided to avoid the debate over what constitutes a planet by referring to the objects as planemos. However, the original IAU proposal for the 2006 definition of planet favoured the star-orbiting criterion, although the final draft avoided the issue. 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". However, recent analysis[32] of the objects has determined that their masses are each greater than 13 Jupiter-masses, making the pair brown dwarfs.[33] See also
References
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
) 
) 
) 
) 
) 
) 
) 
)