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Encyclopedia > Definition of a planet

Although planets are the principal component of the solar system other than the Sun, a precise definition of the term is surprisingly elusive. This article details the questions that may arise when trying to formulate a strict definition of the word. A planet is generally considered to be a relatively large mass of accreted matter in orbit around a star that is not a star itself. ... Presentation of the solar system (not to scale) The solar system comprises our Sun and the retinue of celestial objects gravitationally bound to it. ... The Sun is the star at the center of our Solar system. ... A definition may be a statement of the essential properties of a certain thing, or a statement of equivalence between one expression and another, usually more complex expression that gives the meaning of the first. ...


For most astronomers the issue will be decided by the International Astronomical Union (IAU). According to a published report from Nature magazine, the discovery of 2003 UB313 (which is a Kuiper Belt object bigger than Pluto) has forced the issue. An IAU committee which had already been working on a definition is now expected to promulgate one soon. Logo of the IAU The International Astronomical Union (IAU) unites national astronomical societies from around the world. ... Nature is one of the oldest and most reputable scientific journals, first published on 4 November 1869. ... 2003 UB313 (also written 2003 UB313) is a trans-Neptunian object (TNO) which California astronomers at Mount Palomar observatory describe as definitely bigger than the planet Pluto. ... Artists rendering of the Kuiper Belt and more distant Oort cloud. ... Atmospheric characteristics Atmospheric pressure 0. ...

Contents


History and etymology

The "seven heavenly objects" of antiquity
The "seven heavenly objects" of antiquity

There has never been a single, precise definition for the word "planet." When originally coined by the ancient Greeks, a planet was any object that appeared to wander against the field of fixed stars that made up the night sky (asteres planetai "wandering stars"). This included not only the five "classical" planets, that is, Mercury, Venus, Mars, Jupiter and Saturn, but also the Sun and the Moon (the "seven heavenly objects"). Sometimes a distinction was made in terminology, so that if one said the "five planets", it would mean all of the "naked eye" ones except for the sun and moon. There was no need for as exact a classification of a planet in those days, and so the sun and moon could be included or excluded as planets in discussions. Image File history File links Download high resolution version (616x650, 169 KB)Ancient earth-centered model of the solar system and universe. ... Image File history File links Download high resolution version (616x650, 169 KB)Ancient earth-centered model of the solar system and universe. ... Ancient Greece is the term used to describe the Greek_speaking world in ancient times. ... The Pleiades star cluster A star is a massive body of plasma in outer space that is currently producing or has produced energy through nuclear fusion. ... Atmospheric characteristics Atmospheric pressure trace Potassium 31. ... (*min temperature refers to cloud tops only) Atmospheric characteristics Atmospheric pressure 9. ... Mars is the fourth planet from the Sun in the solar system, named after the Roman god of war (the counterpart of the Greek Ares), on account of its blood red color as viewed in the night sky. ... Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... Atmospheric characteristics Atmospheric pressure 140 kPa Hydrogen >93% Helium >5% Methane 0. ... The Sun is the star at the center of our Solar system. ... Crust composition Oxygen 43% Silicon 21% Aluminium 10% Calcium 9% Iron 9% Magnesium 5% Titanium 2% Nickel 0. ... Ancient Earth-centred model of the universe The Seven heavenly objects are the bright objects in the sky which can be seen with the naked eye from anywhere on the Earth: the planets Mercury, Venus, Mars, Jupiter, and Saturn, the Sun, and Moon The days of the week are named...


Eventually, when the heliocentric model was accepted over the geocentric, Earth was placed among their number and the Sun was dropped, and after Galileo discovered his four satellites of Jupiter, the Moon was also eventually reclassified. However, the Galilean satellites of Jupiter (in 1610), Saturn's satellite Titan in 1659, and Iapetus and Rhea in 1673 were initially described as "planets", not "moons"; the word "moon" at that time only referred to Earth's Moon. In astronomy, heliocentrism is the theory that the Sun is at the center of the Universe and/or the Solar System. ... The geocentric model (in Greek: geo = earth and centron = centre) of the universe is a paradigm which places the Earth at its center. ... Earth, also known as Terra, and Tellus mostly in the 19th century, is the third-closest planet to the Sun. ... Galileo Galilei Galileo Galilei (Pisa, February 15, 1564 – Arcetri, January 8, 1642), was an Italian physicist, astronomer, and philosopher who is closely associated with the scientific revolution. ... For other uses, please see Satellite (disambiguation) A satellite is an object that orbits another object (known as its primary). ... Jupiters 4 Galilean moons, in a composite image comparing their sizes and the size of Jupiter (Great Red Spot visible). ... // Events January 7 - Galileo Galilei discovers the Galilean moons of Jupiter. ... Atmospheric characteristics Pressure 146. ... // Events May 25 - Richard Cromwell resigns as Lord Protector of England following the restoration of the Long Parliament, beginning a second brief period of the republican government called the Commonwealth. ... Iapetus (eye-ap-i-tus, Greek Ιαπετός) (British spelling: Japetus) is the third-largest moon of Saturn, discovered by Giovanni Domenico Cassini in 1671. ... Atmosphere none Rhea (ree-a, Greek ‘Ρέα) is the second largest moon of Saturn and was discovered in 1672 by Giovanni Domenico Cassini. ... Events The English Test Act was passed. ... Crust composition Oxygen 43% Silicon 21% Aluminium 10% Calcium 9% Iron 9% Magnesium 5% Titanium 2% Nickel 0. ...


Over the following centuries, the parameters of planethood have changed as new observations and theories have refined our understanding of the solar system. In the last ten years, however, a new wave of observations has made many once-clear demarcations indistinct.


Issues and controversies

While there is much disagreement between current definitions of "planet", most focus on three general criteria: that it must orbit a star, be above a certain size (usually large enough to be rounded by its own gravity), and yet not be large enough to commence nuclear fusion. Each of these criteria has been challenged by various discoveries, outlined below. In physics, an orbit is the path that an object makes, around another object, whilst under the influence of a source of centripetal force, such as gravity. ... The deuterium-tritium (D-T) fusion reaction is considered the most promising for producing fusion power. ...


Minor planets

Within our planetary system, the nine objects currently accepted as planets orbit the Sun with other objects as diverse as asteroids, meteoroids, comets, and cosmic dust. The term "minor planet" is often used to describe those objects that, while they orbit the Sun, are not seen to fulfil certain criteria common to the "true" planets. What these criteria are, or even if they should exist at all, is the subject of some debate. An artists concept of a protoplanetary disk. ... The Sun is the star at the center of our Solar system. ... An asteroid is a small, solid object in our Solar System, orbiting the Sun. ... A meteoroid is a relatively small (sand- to boulder-sized) fragment of debris in the Solar System. ... Comet Hale-Bopp A comet (denoted by ☄) is a small body in the solar system that orbits the sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail — both due primarily to the effects of solar radiation upon the comets nucleus, which itself is a... This article describes dust in the astronomical cosmic context, of which interplanetary dust and interstellar dust are particular types. ... Minor planets, or planetoids are minor bodies of the Solar system orbiting the Sun (or of other planetary systems orbiting other stars) that are larger than meteoroids (the largest of which might be taken to be around 10 meters or so across) but smaller than major planets (Mercury having a...


Orbit

One possible criterion is a relatively unique orbit. The discovery of Uranus in 1781 seemed to validate Bode's Law, a mathematical function which generates the size of the semimajor axis of planetary orbits. Astronomers had considered the Law a meaningless coincidence, but Uranus fell at very nearly the exact distance it predicted. Since Bode's Law also predicted a body between Mars and Jupiter that at that point had not been observed, astronomers turned their attention to that region in the hope that it might be vindicated again. Finally, in 1801, Ceres, a world so small it was barely visible through a telescope, was found to lie at just the correct point in space. The object was hailed as a new planet. Adjective Uranian Atmospheric characteristics Atmospheric pressure 120 kPa (at the cloud level) Hydrogen 83% Helium 15% Methane 1. ... 1781 was a common year starting on Monday (see link for calendar). ... The Titius-Bode law (or Bodes law) is the observation that orbits of planets in the solar system closely follow a simple geometric rule. ... In geometry, the semi-major axis (also semimajor axis) a applies to ellipses and hyperbolas. ... In physics, an orbit is the path that an object makes, around another object, whilst under the influence of a source of centripetal force, such as gravity. ... Adjective Uranian Atmospheric characteristics Atmospheric pressure 120 kPa (at the cloud level) Hydrogen 83% Helium 15% Methane 1. ... Mars is the fourth planet from the Sun in the solar system, named after the Roman god of war (the counterpart of the Greek Ares), on account of its blood red color as viewed in the night sky. ... Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... The Union Jack, flag of the newly formed United Kingdom of Great Britain and Ireland. ... 1 Ceres (, Latin Cerēs) was the first asteroid to be discovered. ...


Then in 1802, Heinrich Olbers discovered Pallas, a second "planet" at roughly the same distance from the Sun as Ceres. The idea that two planets could occupy the same orbit was an affront to centuries of thinking. Some years later, another world, Juno, was discovered in a similar orbit, and then another and another. William Herschel suggested that these worlds be given their own separate classification, asteroids, meaning "starlike" since they were so small they resembled stars through a telescope, though most astronomers preferred to refer to them as planets. Science textbooks in 1828, after Herschel's death, still numbered the asteroids among the planets. By 1851, the number of asteroids had increased to 15, and a new method of classifying them, by adding a number before their names, was adopted, inadvertently placing them in their own distinct category. By the 1860s, observatories in Europe and America began referring to them as "minor planets", or "small planets", though it took the four largest asteroids longer to be grouped as such. [1] --69. ... Heinrich Wilhelm Olbers. ... 2 Pallas (pal-us, Greek Παλλάς) was the first asteroid discovered after 1 Ceres. ... 3 Juno (jew-noh) was discovered on September 1, 1804 by German astronomer Karl L. Harding, using a humble 2-inch telescope. ... Sir Wilhelm Friedrich Herschel, FRS (Hanover, November 15, 1738 – August 25, 1822 Slough, then in Buckinghamshire now in Berkshire) was a German-born British astronomer and composer who became famous for discovering the planet Uranus, and made many other astronomical discoveries. ... An asteroid is a small, solid object in our Solar System, orbiting the Sun. ... 1828 was a leap year starting on Tuesday (see link for calendar). ... 1851 was a common year starting on Wednesday (see link for calendar). ... // Events and trends Technology The First Transcontinental Railroad in the United States is built in the six year period between 1863 and 1869. ... Minor planet is the official term for asteroids and trans-Neptunian objects. ...

The relative sizes of Earth (on the left) with (from top to bottom) the Moon, Pluto and its moon Charon, Sedna, Quaoar, and Ceres on the right.
The relative sizes of Earth (on the left) with (from top to bottom) the Moon, Pluto and its moon Charon, Sedna, Quaoar, and Ceres on the right.

The long road from planethood to reconsideration undergone by Ceres is mirrored in the story of Pluto, which was named a planet soon after its discovery in 1930. Pluto was an anomaly, a tiny, icy world in a region of gas giants with an orbit that often sent it flying off the plane of the ecliptic or even inside that of Neptune. However, it was, as far as anyone could tell, unique. Then, beginning in 1992, astronomers began to detect large numbers of icy bodies beyond the orbit of Neptune that were similar in composition and size to Pluto. They concluded that they had discovered the long-hypothesized Kuiper Belt (sometimes called the Edgeworth-Kuiper Belt), a band of icy debris that is the source for "short-period" comets—those, like Halley, with orbital periods of up to 200 years. Image File history File links Download high resolution version (810x500, 125 KB)This image shows the relative sizes of Earth on the left, with (from top to bottom) the Moon, Pluto and its moon Charon, (90377) Sedna, (50000) Quaoar, and (1) Ceres. ... Image File history File links Download high resolution version (810x500, 125 KB)This image shows the relative sizes of Earth on the left, with (from top to bottom) the Moon, Pluto and its moon Charon, (90377) Sedna, (50000) Quaoar, and (1) Ceres. ... Earth, also known as Terra, and Tellus mostly in the 19th century, is the third-closest planet to the Sun. ... Crust composition Oxygen 43% Silicon 21% Aluminium 10% Calcium 9% Iron 9% Magnesium 5% Titanium 2% Nickel 0. ... Atmospheric characteristics Atmospheric pressure 0. ... Charon (shar-É™n or karr-É™n, Greek Χάρων) is the largest satellite of Pluto. ... 90377 Sedna is a trans-Neptunian object, discovered by Michael Brown (Caltech), Chad Trujillo (Gemini Observatory) and David Rabinowitz (Yale University) on November 14, 2003. ... Artists impression by G. Bacon of STScI / NASA 50000 Quaoar (pronounced kwah·war, kwah·wor, or kwow·ur, Tongva ) [1] is a Trans-Neptunian object orbiting the Sun in the Edgeworth-Kuiper belt. ... 1 Ceres (, Latin CerÄ“s) was the first asteroid to be discovered. ... For more uses of the term Pluto, see Pluto (disambiguation). ... 1930 (MCMXXX) is a common year starting on Wednesday. ... A gas giant is a large planet that is not composed mostly of rock or other solid matter. ... Adjective Neptunian Atmospheric characteristics Surface pressure ≫100 MPa Hydrogen - H2 80% ±3. ... 1992 (MCMXCII) was a leap year starting on Wednesday. ... Artists rendering of the Kuiper Belt and more distant Oort cloud. ... The Kuiper belt (KYE per) is an area of the solar system extending from within the orbit of Neptune (at 30 AU) to 50 AU from the sun, at inclinations consistent with the ecliptic. ... Comet Halley as taken with the Halley Multicolor Camera on the ESA Giotto mission. ...


Pluto's orbit was found to lie right in the middle of this band and thus its planetary status was thrown into question; the precedent set by Ceres in downgrading an object from planet status because of a shared orbit has led many to conclude that Pluto must be downgraded to a minor planet as well. Mike Brown of Caltech has suggested that a "planet" should be redefined as "any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit" [2]. The eight planets over that mass limit would be referred to as "major planets". There has been outcry at the prospect of Pluto's "demotion", and the International Astronomical Union officially voted in 1999 to retain Pluto's classification as a planet.[3] [4] 1 Ceres (, Latin Cerēs) was the first asteroid to be discovered. ... Minor planets, or planetoids are minor bodies of the Solar system orbiting the Sun (or of other planetary systems orbiting other stars) that are larger than meteoroids (the largest of which might be taken to be around 10 meters or so across) but smaller than major planets (Mercury having a... Mike Brown can refer to the following people: Michael E. Brown the astronomer. ... California Institute of Technology The California Institute of Technology (commonly known as Caltech) is a private, coeducational university located in Pasadena, California, in the United States. ... Logo of the IAU The International Astronomical Union (IAU) unites national astronomical societies from around the world. ... 1999 (MCMXCIX) was a common year starting on Friday, and was designated the International Year of Older Persons by the United Nations. ...


Throughout the late 1990s, the discovery of several objects approaching the size of Pluto, such as 50000 Quaoar and 90377 Sedna, continued to erode arguments that Pluto was exceptional from the rest of the trans-Neptunian population. On July 28, 2005, astronomers announced the discovery of a trans-Neptunian object as large as, and probably larger than, Pluto, designated 2003 UB313. Although its discoverers (and many in the news media) immediately referred to it as the tenth planet, it is officially designated as a minor planet—the provisional designation 2003 UB313 referring to its official listing in the minor-planet archive as the 7827th object first identified in observations made in the second half of October 2003. The 1990s decade refers to the years from 1990 to 1999, inclusive, the last decade of the 20th Century. ... Artists impression by G. Bacon of STScI / NASA 50000 Quaoar (pronounced kwah·war, kwah·wor, or kwow·ur, Tongva ) [1] is a Trans-Neptunian object orbiting the Sun in the Edgeworth-Kuiper belt. ... 90377 Sedna is a trans-Neptunian object, discovered by Michael Brown (Caltech), Chad Trujillo (Gemini Observatory) and David Rabinowitz (Yale University) on November 14, 2003. ... July 28 is the 209th day (210th in leap years) of the year in the Gregorian Calendar, with 156 days remaining. ... 2005 (MMV) was a common year starting on Saturday of the Gregorian calendar. ... A trans-Neptunian object (TNO) is any object in the solar system which orbits the sun at a greater distance on average than Neptune. ... 2003 UB313 (also written 2003 UB313) is a trans-Neptunian object (TNO) which California astronomers at Mount Palomar observatory describe as definitely bigger than the planet Pluto. ... Ever since the discovery of Pluto, the existence of a tenth planet has been speculated by astronomers and the general public alike. ... Provisional designation of in astronomy is the naming convention applied to astronomical objects immediately following their discovery. ...


Size

Neptune's large moon Proteus is clearly not spherical

A number of astronomers, such as Alan Stern, contend that a unique orbit is not necessary in defining a minor planet, and that the criterion at issue is size. Objects in orbit round the Sun range in size from Jupiter to dust particles, so obviously there would need to be a lower limit. The most oft-mooted potential limit is when an object becomes spherical under its own gravity. Many astronomers favour this definition, because it would allow Pluto to retain its status as a planet. However, sphericity is not determined solely by size; the speed of an object's rotation also affects its sphericity. Therefore Ceres is spherical, but the Kuiper belt object 2003 EL61, which is several times more massive but spins at a much faster rate, is not [5]. Jupiter and Saturn may appear spherical, but are also made oblate by their rapid rotations. Temperature also plays a role; Saturn's icy moon Mimas is spherical, but Neptune's larger moon Proteus, which is similarly composed but colder because of its greater distance from the Sun, is not. Download high resolution version (727x726, 31 KB)Photo of Netptunes satellite Proteus taken by Voyager 2 (large version). ... Download high resolution version (727x726, 31 KB)Photo of Netptunes satellite Proteus taken by Voyager 2 (large version). ... Adjective Neptunian Atmospheric characteristics Surface pressure ≫100 MPa Hydrogen - H2 80% ±3. ... Atmospheric pressure 0 kPa Proteus (proe-tee-us, Greek Πρωτέας) is one of Neptunes moons. ... Minor planets, or planetoids are minor bodies of the Solar system orbiting the Sun (or of other planetary systems orbiting other stars) that are larger than meteoroids (the largest of which might be taken to be around 10 meters or so across) but smaller than major planets (Mercury having a... 1 Ceres (, Latin Cerēs) was the first asteroid to be discovered. ... The Kuiper belt (KYE per) is an area of the solar system extending from within the orbit of Neptune (at 30 AU) to 50 AU from the sun, at inclinations consistent with the ecliptic. ... The title of this article is incorrect due to technical limitations. ... Adjective Jovian Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... Adjective Saturnian Atmospheric characteristics Atmospheric pressure 140 kPa Hydrogen >93% Helium >5% Methane 0. ... Adjective Saturnian Atmospheric characteristics Atmospheric pressure 140 kPa Hydrogen >93% Helium >5% Methane 0. ... Mimas (mye-mus) is a moon of Saturn that was discovered in 1789 by William Herschel. ... Adjective Neptunian Atmospheric characteristics Surface pressure ≫100 MPa Hydrogen - H2 80% ±3. ... Atmospheric pressure 0 kPa Proteus (proe-tee-us, Greek Πρωτέας) is one of Neptunes moons. ...


Others have suggested that the diameter limit be arbitrarily pinned at that of Pluto, thus preserving the traditional nine planets while allowing the possibility of future additions, [6] while others have suggested that it be fixed at 1000 km, just below Pluto's, which would potentially define at least three smaller KBOs as planets alongside it. [7]


Double planets

A telescope image of Pluto and Charon.
A telescope image of Pluto and Charon.

Pluto is not alone in causing planetary confusion. Its satellite Charon is the only moon in the solar system whose barycenter lies above its parent planet's surface. This means that both orbit each other like the tips of a swirling baton. Since neither can be said to be orbiting the other, it is common for astronomers to refer to Pluto/Charon as a double planet: two objects orbiting the Sun in tandem. It should be noted that Charon is not profoundly physically affected by its orbit —unlike the synchronous orbital radius, for instance, which does have profound consequences for the orbiting body (see Phobos, for example). Image File history File links Download high resolution version (950x494, 11 KB) File links The following pages link to this file: Definition of planet ... Image File history File links Download high resolution version (950x494, 11 KB) File links The following pages link to this file: Definition of planet ... 50 cm refracting telescope at Nice Observatory. ... For more uses of the term Pluto, see Pluto (disambiguation). ... Charon (shar-É™n or karr-É™n, Greek Χάρων) is the largest satellite of Pluto. ... Atmospheric characteristics Atmospheric pressure 0. ... Charon (shar-É™n or karr-É™n, Greek Χάρων) is the largest satellite of Pluto. ... Presentation of the solar system (not to scale) The solar system comprises our Sun and the retinue of celestial objects gravitationally bound to it. ... The barycenter (from the Greek βαρύκεντρον) is the center of mass of two or more bodies which are orbiting each other, and is the point around which both of them orbit. ... The term double planet has several accepted usages. ... The Sun is the star at the center of our Solar system. ... Phobos (IPA , Greek Φόβος: Fright), is the larger and innermost of Mars two moons, named after Phobos, son of Ares (Mars) from Greek Mythology. ...


Even our own Moon, surprisingly, could be considered a partner in a double planet system, since, though it certainly orbits the Earth, the timing of its orbit round the Earth with the Earth's orbit round the Sun means that, looking down on the ecliptic, the Moon never actually loops back on itself, and in essence orbits the Sun in its own right [8]. This is true of any moon sufficiently far enough away from its parent body that its orbital speed round the planet is slower than the planet's speed round the Sun. The required distance from the planet to the moon depends on the mass of the planet, and the distance from the planet to the sun, but not the mass of the moon. If the distance from the Sun to the planet increases, or the planet's mass decreases, then the required distance between the planet and moon increases. Consequently, the same argument could be used that Jupiter and Callisto or Saturn and Iapetus form double planets. Crust composition Oxygen 43% Silicon 21% Aluminium 10% Calcium 9% Iron 9% Magnesium 5% Titanium 2% Nickel 0. ... The term double planet has several accepted usages. ... Earth, also known as Terra, and Tellus mostly in the 19th century, is the third-closest planet to the Sun. ... The plane of the ecliptic is well seen in this picture from the 1994 lunar prospecting Clementine spacecraft. ... Adjective Jovian Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... Atmospheric characteristics Atmospheric pressure trace Carbon dioxide 100% Callisto (IPA: , ka-lis-toe, Greek Καλλιστώ) is a moon of the planet Jupiter, discovered in 1610 by Galileo Galilei. ... Adjective Saturnian Atmospheric characteristics Atmospheric pressure 140 kPa Hydrogen >93% Helium >5% Methane 0. ... Iapetus (eye-ap-i-tus, Greek Ιαπετός) (British spelling: Japetus) is the third-largest moon of Saturn, discovered by Giovanni Domenico Cassini in 1671. ...


There is even some disagreement about whether all moons in the solar system, even those that do not orbit the Sun directly, should be classed as "secondary planets", since they often exhibit features in common with true planets. Jupiter's moon Ganymede and Saturn's moon Titan are both larger in terms of diameter (though not mass) than Mercury, and Titan even has a substantial atmosphere, thicker than the Earth's. Moons such as Io and Triton demonstrate obvious and ongoing geological activity, and Ganymede even has a magnetic field. Moons of solar system scaled to Earths Moon The common noun moon (not capitalized) is used to mean any natural satellite of the other planets. ... Presentation of the solar system (not to scale) The solar system comprises our Sun and the retinue of celestial objects gravitationally bound to it. ... The Sun is the star at the center of our Solar system. ... Adjective Jovian Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... Atmospheric characteristics Atmospheric pressure trace Oxygen 100% Ganymede (IPA: , gan-i-meed, Greek Γανυμήδης) is Jupiters largest moon, and indeed the largest moon in the entire solar system; it is larger in diameter than Mercury but only about half its mass. ... Atmospheric characteristics Atmospheric pressure 140 kPa Hydrogen >93% Helium >5% Methane 0. ... Atmospheric characteristics Pressure 146. ... Atmospheric characteristics Atmospheric pressure trace Potassium 31. ... Earth, also known as Terra, and Tellus mostly in the 19th century, is the third-closest planet to the Sun. ... Atmospheric characteristics Atmospheric pressure trace Sulfur dioxide 90% Io (IPA: , eye-oe, Greek Ιώ, Latin Īō) is the innermost of the four Galilean moons of Jupiter. ... Triton (trye-tun, Greek Τρίτων), also designated Neptune I, is the planet Neptunes largest moon. ... Current flowing through a wire produces a magnetic field (M) around the wire. ...


Rogue planets or sub-stars

The dividing line between "star" and "planet" has been blurred considerably since 1995, with the discovery to date of over 150 extrasolar planets: planet-sized objects in orbit around other stars. Traditionally, that dividing line has been an object's ability to fuse hydrogen in its core, though stars such as brown dwarfs have always existed at the boundary of that distinction. Too small to commence sustained hydrogen fusion, they have been granted star status on their ability to fuse deuterium. However, due to the relative rarity of that isotope, this process lasts only a tiny fraction of the star's lifetime, and hence most brown dwarfs would have ceased fusion long before their discovery. 1995 (MCMXCV) was a common year starting on Sunday of the Gregorian calendar. ... Infrared Image of a possible extrasolar planet (lower left) in the Constellation Taurus, taken by the Hubble Space Telescope. ... Brown dwarfs are sub-stellar objects (~13 to 75 Jupiter masses) that never fuse hydrogen into helium in their cores, as do stars on the main sequence. ... Deuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance of one atom in 6500 of hydrogen. ...


Since binary stars or other multiple-star formations are fairly common, many brown dwarfs are likely to orbit other stars, and since they would not be producing energy through fusion, they could be described as "planets". Similarly, an orbiting white dwarf, such as Sirius B, since it too has ceased fusion, could be reclassified as a planet. The recent discovery of a "planet" around a brown dwarf star has ignited a debate as to whether it should be referred to as a planet or a natural satellite. A binary star system consists of two stars both orbiting around their barycenter. ... White dwarf Sirius-B in x-rays A white dwarf is an astronomical object which is produced when a low or medium mass star dies. ... This article is about the star. ... Moons of solar system scaled to Earths Moon The common noun moon (not capitalized) is used to mean any natural satellite of the other planets. ...


But the confusion does not end with brown dwarfs. Zapatario Osorio et al. have discovered many objects in young star clusters of masses below that required to sustain fusion of any sort. [9] These have been described as "free floating planets", [10] because current theories of solar system formation suggest that planets may be ejected from solar systems altogether if their orbits become unstable; indeed the Hubble telescope may have imaged just such an ejection of a body larger than Jupiter in 1998. This would suggest that the original criterion that a planet must orbit a star should instead be amended to say that it must have originated in orbit around a star. A rogue planet is a planet that either has an extremely elongated orbit around its star so that it is not on the same orbital plane as the other planets in the system, or it is an interstellar planet, a planet that drifts freely through space and doesnt orbit...


However, it is also possible that these objects could have formed in the same manner as stars, from fragments of a collapsing nebula; thus their discoverers also term them "grey dwarfs" or "sub-brown dwarfs". If that is the case, then it raises the question of whether such an object, even one as familiar as Jupiter or Saturn, should be referred to as an orbiting low-mass star rather than a planet. After all, apart from size and relative temperature, there is little to separate Jupiter from its host star, the Sun. Both have similar overall compositions: hydrogen and helium, with trace levels of heavier elements in their atmospheres. The most oft-cited difference between them is that Jupiter formed by accretion of rocky and icy materials in orbit round the Sun, and the Sun formed from the collapsing gas of a nebula, though this distinction is not wholly certain. Adjective Jovian Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... Adjective Saturnian Atmospheric characteristics Atmospheric pressure 140 kPa Hydrogen >93% Helium >5% Methane 0. ... Adjective Jovian Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... The Sun is the star at the center of our Solar system. ...

The discovery of bodies larger than Jupiter has blurred the distinction between stars and planets.
The discovery of bodies larger than Jupiter has blurred the distinction between stars and planets.

The IAU has officially released a statement to define what constitutes an extrasolar planet and what constitutes an orbiting star: Download high resolution version (840x840, 41 KB) Original Caption Released with Image: This processed color image of Jupiter was produced in 1990 by the U.S. Geological Survey from a Voyager image captured in 1979. ... Download high resolution version (840x840, 41 KB) Original Caption Released with Image: This processed color image of Jupiter was produced in 1990 by the U.S. Geological Survey from a Voyager image captured in 1979. ... Adjective Jovian Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... The Pleiades star cluster A star is a massive body of plasma in outer space that is currently producing or has produced energy through nuclear fusion. ... A planet (from the Greek πλανήτης, planetes or wanderers) is a body of considerable mass that orbits a star and that produces very little or no energy through nuclear fusion. ... IAU is a three-letter acronym that denotes: International Astronomical Union International American University International Association of Universities International Association of Ultra Runners for ultramarathoners. ...

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

The definition, as with any definition of a planet, is uncertain and ambiguous. By ignoring the process of formation, it is essentially saying that any star too small to commence nuclear fusion in its core is a planet, thus placing it in the same company as Earth, Mars and (at this time) Pluto. The deuterium-tritium (D-T) fusion reaction is considered the most promising for producing fusion power. ... Earth, also known as Terra, and Tellus mostly in the 19th century, is the third-closest planet to the Sun. ... For the Roman god, see Mars (god). ... Atmospheric characteristics Atmospheric pressure 0. ...


The boundary continues to blur; in December, 2005, the Spitzer Space Telescope observed the smallest brown dwarf yet found, only eight times Jupiter's mass, and ironically with what appears to be the beginnings of its own solar system. Were this object found in orbit round another star, it would have been termed a planet.[11] The Spitzer Space Telescope Facility launches from Cape Canaveral Air Force Station in Florida on Monday, Aug. ... Brown dwarfs are sub-stellar objects (~5 to 90 Jupiter masses) that do not fuse hydrogen into helium in their cores, as do stars on the main sequence, but have fully convective surfaces and interiors, with no chemical differentiation by depth. ... Presentation of the solar system (not to scale) The solar system comprises our Sun and the retinue of celestial objects gravitationally bound to it. ... The Pleiades star cluster A star is a massive body of plasma in outer space that is currently producing or has produced energy through nuclear fusion. ... A planet is generally considered to be a relatively large mass of accreted matter in orbit around a star that is not a star itself. ...


History vs. science

Ultimately, a strict scientific definition of what a planet is may prove different from the determination provided by convention and history. Mike Brown, since his discovery of 2003 UB313, has shifted his position on what constitutes a planet. "Scientists have not yet realized that the term planet no longer belongs to them. But, quite clearly, it does not... The word 'planet' has been around much longer than modern science." [12] It may very well be that children will continue to learn of the nine planets in school, while scientists work in a solar system of eight, or hundreds, or even abandon the term "planet" altogether. For now, "planet", like "continent", is a word caught between the scientific and cultural worlds without a clear meaning. Dr. Michael (Mike) E. Brown has been an associate professor of planetary astronomy at the California Institute of Technology (Caltech) since 2002. ... 2003 UB313 (also written 2003 UB313) is a trans-Neptunian object (TNO) which California astronomers at Mount Palomar observatory describe as definitely bigger than the planet Pluto. ...


See also

Mesoplanet is a term coined by Isaac Asimov to refer to planetary bodies with sizes smaller than Mercury but larger than 1 Ceres. ... This article is being considered for deletion in accordance with Wikipedias deletion policy. ...

References

  • Mike Brown's Sedna article
  • Mike Brown's webpage on 2003 EL61
  • Mike Brown's webpage: What is a planet?
  • David Jewitt's Kuiper Belt page- Pluto
  • Letter to Science Magazine- interstellar planets
  • Responses
  • What is a Planet? An essay by Gibor Basri of the University of Berkeley
  • Defining "Planet": A second essay by Gibor Basri
  • What is a Planet? Debate Forces New Definition
  • Astronomers to decide what makes a planet from the Nature.com web site (non-free subscription required)
  • IAU Statement on 2003 UB313, which also refers to efforts to formally define what a planet is.
  • IAU statement on extrasolar planet
  • The Flap Over Pluto
  • David Darling. The Universal Book of Astronomy, from the Andromeda Galaxy to the Zone of Avoidance. 2003. John Wiley & Sons Canada (ISBN 0471265691), p. 394
  • Collins Dictionary of Astronomy, 2nd ed. 2000. HarperCollins Publishers (ISBN 0-00-710297-6), p. 312-4.

Nature is one of the oldest and most reputable scientific journals, first published on 4 November 1869. ...

External links

Further Reading

  • Ken Croswell, Planet Quest. Oxford University Press, 1999. (ISBN 0192880837)

  Results from FactBites:
 
PLANET - Definition (277 words)
Note: The term planet was first used to distinguish those stars which have an apparent motion through the constellations from the fixed stars, which retain their relative places unchanged.
The inferior planets are Mercury and Venus, which are nearer to the sun than is the earth; the superior planets are Mars, the asteroids, Jupiter, Saturn, Uranus, and Neptune, which are farther from the sun than is the earth.
Primary planets are those which revolve about the sun; secondary planets, or moons, are those which revolve around the primary planets as satellites, and at the same time revolve with them about the sun.
New planet definition sparks furore - space - 25 August 2006 - New Scientist Space (1004 words)
The new planet definition that relegates Pluto to "dwarf planet" status is drawing intense criticism from astronomers.
That is because the definition stipulates that to be a planet, an object must have cleared the neighbourhood around its orbit.
A better definition would say a planet is an object that orbits a star and is large enough to be spherical, but is not large enough to be a brown dwarf – a "failed" star with between about 13 and 75 times the mass of Jupiter – or a star, he says.
  More results at FactBites »

 
 

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