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In popular culture

The depiction of comets in popular culture is firmly rooted in the long Western tradition of seeing comets as harbingers of doom and as omens of world-altering change. Halley's Comet alone has caused a slew of frightful or excited publications of all sorts at each of its reappearances. HP Compaq 6720s Adapter

It was especially noted that the birth and death of some notable persons coincided with separate appearances of the comet, such as with writers Mark Twain (who correctly speculated that he'd "go out with the comet" in 1910)[87] and Eudora Welty, to whose life Mary Chapin Carpenter dedicated the song Halley Came to Jackson.Dell Latitude D630 Adapter

In science fiction, the impact of comets has been depicted as a threat overcome by technology and heroism (Deep Impact, 1998), or as a trigger of global apocalypse (Lucifer's Hammer, 1979) or of waves of zombies (Night of the Comet, 1984). Near impacts have been depicted in Jules Verne's Off on a Comet and Tove Jansson's Comet in Moominland, HP Pavilion dm4 Battery

while a large manned space expedition visits Halley's Comet in SirArthur C. Clarke's novel 2061: Odyssey Three.

Centaurs are an unstable orbital class of minor planets that behave with characteristics of both asteroids and comets. They are named after the mythological race of beings, centaurs, which were a mixture of horse and human. Sony NP-FS11 Battery

Centaurs have transient orbits that cross or have crossed the orbits of one or more of the giant planets, and have dynamic lifetimes of a few million years. It has been estimated that there are around 44,000 centaurs in the Solar System with diameters larger than 1 km.Dell Inspiron 1520 Adapter

The first centaur-like object to be discovered was 944 Hidalgo in 1920. However, they were not recognized as a distinct population until the discovery of 2060 Chiron in 1977. The largest known centaur is 10199 Chariklo, discovered in 1997, which at 260 km in diameter is as big as a mid-sized main-belt asteroid.Toshiba PA3399U-1BRS Battery

No centaur has been photographed up close, although there is evidence that Saturn's moon Phoebe, imaged by the Cassini probe in 2004, may be a captured centaur. In addition, the Hubble Space Telescope has gleaned some information about the surface features of8405 Asbolus.Acer Aspire 5920G Adapter

As of 2008, three centaurs have been found to display cometary comas: Chiron, 60558 Echeclus, and 166P/NEAT. Chiron and Echeclus are therefore classified as both asteroids and comets. Other centaurs such as 52872 Okyrhoe are suspected of showingcometary activity. Acer Aspire 5920 Battery

Any centaur that is perturbed close enough to the Sun is expected to become a comet.

Classification

The generic definition of a centaur is a small body that orbits the Sun between Jupiter and Neptune and crosses the orbits of one or more of the giant planets.IBM Thinkpad T40 Adapter

Due to the inherent long-term instability of orbits in this region, even centaurs such as 2000 GM137 and 2001 XZ255, which do not currently cross the orbit of any planet, are in gradually changing orbits that will be perturbed until they start to cross the orbit of one or more of the giant planets.Nikon COOLPIX 4300 Battery

However, different institutions have different criteria for classifying borderline objects, based on particular values of their orbital elements:

  • The Minor Planet Center (MPC) defines centaurs as having a perihelion beyond the orbit of Jupiter and a semi-major axis less than that of Neptune.Apple MB771 Battery
  • The Jet Propulsion Laboratory (JPL) similarly defines centaurs as having a semi-major axis, a, between those of Jupiter and Neptune (5.5 AU < a < 30.1 AU).
  • In contrast, the Deep Ecliptic Survey (DES) defines centaurs using a dynamical classification scheme. Sony DSC-P5 Battery
  • These classifications are based on the simulated change in behavior of the present orbit when extended over 10 million years. The DES defines centaurs as non-resonant objects whose instantaneous (osculating) perihelia are less than the osculating semi-major axis of Neptune at any time during the simulation.Dell Inspiron 6400 Adapter
  • This definition is intended to be synonymous with planet-crossing orbits and to suggest comparatively short lifetimes in the current orbit.

The collection The Solar System Beyond Neptune (2008) uses the traditional definition of centaurs, limited to semi-major axes smaller than that of Neptune,Toshiba PA3534U-1BRS Battery

classifying the objects on unstable orbits beyond this limit as members of the scattered disk.[5] Yet, other astronomers still prefer to define centaurs as objects that are non-resonant with a perihelion inside the orbit of Neptune that can be shown to likely cross the Hill sphere of a gas giant within the next 10 million years.Sony NP-FP50 Battery

Thus centaurs can be thought of as inward scattered objects that interact more aggressively and scatter more quickly than typical scattered disc objects.

The JPL Small-Body Database lists 183 centaurs.HP Compaq nx7300 Adapter

There are an additional 40 trans-Neptunian objects with a semi-major axis further than Neptune (a > 30.1 AU) and perihelion closer than the orbit ofUranus (q < 19 AU).[7] The Committee on Small Body Nomenclature of the International Astronomical Union has not formally weighed in on either side of the debate. Dell Latitude D820 Adapter

Instead, it has adopted the following naming convention for such objects: befitting their Centaur-like transitional orbits between TNOs and comets, "objects on unstable, non-resonant, giant-planet-crossing orbits with semimajor axes greater than Neptune's" are to be named for other hybrid and shape-shifting mythical creatures. Toshiba PA3399U-1BRS Battery

Thus far, only the binary objects Ceto and Phorcys and Typhon and Echidna have been named according to the new policy.

Other objects caught between these differences in classification methods include (44594) 1999 OX3, which has a semi-major axis of 32 AU but crosses the orbits of both Uranus and Neptune.Dell Studio XPS 1640 Battery

Among the inner centaurs, 2005 VD, with a perihelion distance very near Jupiter, is listed as a centaur by both JPL and DES.

Distribution

The diagram at right illustrates the orbits of all known centaurs in relation to the orbits of the planets.Dell Latitude D830 Battery

For selected objects, theeccentricity of the orbits is represented by red segments (extending from perihelion to aphelion).

Centaurs' orbits are characterised by a wide range of eccentricity, from highly eccentric (Pholus, Asbolus, Amicus, Nessus) to more circular (Chariklo and the Saturn-crossers: Thereus, Okyrhoe).Dell Inspiron E1505 Adapter

To illustrate the range of the orbits' parameters, a few objects with very unusual orbits are plotted in yellow on the diagram:

  • 1999 XS35 (Apollo asteroid) follows an extremely eccentric orbit (e=0.947), leading it from inside Earth's orbit (0.94 AU) to well beyond Neptune (>34 AU)  Asus Eee PC 1201PN Battery
  • 2007 TB434 follows a quasi-circular orbit (e<0.026)
  • 2001 XZ255 has the lowest inclination (i<3°).
  • Damocles is among a few centaurs on orbits with extreme inclination (prograde i>70°, e.g. 2007 DA61, 2004 YH32, retrogradei<120° e.g. 2005 JT50; not shown) Asus Eee PC 701 Adapter
  • 2004 YH32 follows such a highly inclined orbit (nearly 80°) that, while it crosses from the distance of the Main belt from the Sun to past the distance of Saturn, it does not even cross Jupiter relative to the plane of Jupiter's orbit.

A dozen known centaurs, including Dioretsa ("asteroid" spelled backwards), follow retrograde orbits.Dell XPS M1330 Battery

Changing orbits

Since the centaurs cross the orbits of the giant planets and are not protected by orbital resonances, their orbits are unstable within a timescale of 106 –107 years. For example, 55576 Amycus is in an unstable orbit near the 3:4 resonance of Uranus.fujifilm np95 Battery

Dynamical studies of their orbits indicate that centaurs are probably an intermediate orbital state of objects transitioning from the Kuiper Belt to theJupiter family of short-period comets. Objects may be perturbed from the Kuiper Belt, whereupon they become Neptune-crossing and interact gravitationally with that planet (see theories of origin). Asus Eee PC 900 Adapter

They then become classed as centaurs, but their orbits are chaotic, evolving relatively rapidly as the centaur makes repeated close approaches to one or more of the outer planets. Some centaurs will evolve into Jupiter-crossing orbits whereupon their periheliaSony VAIO PCG-713 Battery

may become reduced into the inner solar system and they may be reclassified as active comets in the Jupiter family if they display cometary activity. Centaurs will thus ultimately collide with the Sun or a planet or else they may be ejected into interstellar space after a close approach to one of the planets, particularly Jupiter.Dell Latitude D830 Adapter

Physical characteristics

The relatively small size of centaurs precludes surface observations, but colour indices and spectra can indicate possible surface composition and can provide insight into the origin of the bodies.Dell Studio XPS 1640 Battery

[edit]Colours

Centaurs display a puzzling diversity of colour that challenges any simple model of surface composition. In the side-diagram, the colour indices are measures ofapparent magnitude of an object through blue (B), visible (V) i.e. green-yellow and red (R) filters.Panasonic LUMIX DMC-TZ4S  Battery

The diagram illustrates these differences (in enhanced colour) for all centaurs with known colour indices. For reference, two moons: Triton and Phoebe, and planet Mars are plotted (yellow labels, size not to scale).

Centaurs appear to be grouped into two classes:  HP Compaq Business NX6320 Battery

  • very red, for example 5145 Pholus
  • blue (or blue-grey, according to some authors), for example 2060 Chiron

There are numerous theories to explain this colour difference, but they can be divided broadly into two categories:Canon POWERSHOT G2 Battery

  • The colour difference results from a difference in the origin and/or composition of the centaur (see origin below)
  • The colour difference reflects a different level of space-weathering from radiation and/or cometary activity.Dell Latitude E6500 Battery

As examples of the second category, the reddish colour of Pholus has been explained as a possible mantle of irradiated red organics, whereas Chiron has instead had its ice exposed due to its periodic cometary activity, giving it a blue/grey index. Dell LATITUDE D630 Battery

The correlation with activity and color is not certain, however, as the active centaurs span the range of colors from blue (Chiron) to red (166P/NEAT).[12] Alternatively, Pholus may have been only recently expelled from the Kuiper Belt, so that surface transformation processes have not yet taken place.Asus R33030 Adapter

A. Delsanti et al. suggest multiple competing processes: reddening by the radiation, and blushing by collisions.

Spectra

The interpretation of spectra is often ambiguous, related to particle sizes and other factors, but the spectra offer an insight into surface composition. As with the colours, the observed spectra can fit a number of models of the surface.Dell Mini 1011 Battery

Water ice signatures have been confirmed on a number of centaurs (including 2060 Chiron, 10199 Chariklo and 5145 Pholus). In addition to the water ice signature, a number of other models have been put forward:toshiba pa3332u-1brs Battery

  • Chariklo's surface has been suggested to be a mixture of tholins (like those detected on Titan and Triton) with amorphous carbon.
  • Pholus has been suggested to be covered by a mixture of Titan-like tholins, carbon black, olivine[15] and methanol ice.HP Compaq 6710b Adapter
  • The surface of 52872 Okyrhoe has been suggested to be a mixture of kerogens, olivines and small percentage of water ice.
  • 8405 Asbolus has been suggested to be a mixture of 15% Triton-like tholins, 8% Titan-like tholin, 37% amorphous carbon and 40% ice tholin.HP Compaq nx6325 Adapter

Chiron, the only centaur with known cometary activity, appears to be the most complex. The spectra observed vary depending on the period of the observation. Water ice signature was detected during a period of low activity and disappeared during high activity.Acer Aspire 5720 Adapter

Similarities to comets

Observations of Chiron in 1988 and 1989 near its perihelion found it to display a coma (a cloud of gas and dust evaporating from its surface). It is thus now officially classified as both a comet and an asteroid, although it is far larger than a typical comet and there is some lingering controversy.Toshiba Equium A110-252 Battery

Other centaurs are being monitored for comet-like activity: so far two, 60558 Echeclus, and 166P/NEAT have shown such behavior. 166P/NEAT was discovered while it exhibited a coma, and so is classified as a comet, though its orbit is that of a centaur. Acer Aspire 5052ANWXMi Battery

60558 Echeclus was discovered without a coma but recently became active,[20] and so it is now accordingly also classified as both a comet and an asteroid.

There is no clear orbital distinction between centaurs and comets. Both 29P/Schwassmann-Wachmann and 39P/Oterma have been referred to as centaurs since they have typical centaur orbits. Dell Mini 1011v Battery

The comet 39P/Oterma is currently inactive and was seen to be active only before it was perturbed into a centaur orbit by Jupiter in 1963.[21] The faint comet 38P/Stephan-Oterma would probably not show a coma if it had a perihelion distance beyond Jupiter's orbit at 5 AU. HP Compaq nx7300 Adapter

By the year 2200, comet 78P/Gehrels will probably migrate outwards into a centaur-like orbit.

Theories of origin

The study of centaur development is rich in recent developments but still hampered by limited physical data. Different models have been put forward for possible origin of centaurs.Dell Vostro 1510 Battery

Simulations indicate that the orbit of some Kuiper Belt objects can be perturbed, resulting in the object's expulsion so that it becomes a centaur. Scattered disk objects would be dynamically the best candidates for such expulsions, but their colours do not fit the bicoloured nature of the centaurs. Toshiba PA3534U-1BRS Battery

Plutinos are a class of Kuiper Belt Object that display a similar bicoloured nature, and there are suggestions that not all plutinos' orbits are as stable as initially thought, due to perturbation by Pluto.[23] Further developments are expected with more physical data on KBOs.Asus M50 Battery

A dwarf planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting the Sun[1] that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite.Asus Eee PC 901 Battery

More explicitly, it has to have sufficient mass to overcome its compressive strength and achieve hydrostatic equilibrium.

The term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun,Dell Inspiron 6400 Battery

brought about by an increase in discoveries oftrans-Neptunian objects that rivaled Pluto in size, and finally precipitated by the discovery of an even more massive object, Eris. This classification states that bodies large enough to have cleared the neighbourhood of their orbit are defined as planets, HP Compaq Business NC6220 Battery

while those that are not massive enough to be rounded by their own gravity are defined as small solar system bodies. Dwarf planets come in between. The definition officially adopted by the IAU in 2006 has been both praised and criticized, and has been disputed by scientists such as Alan Stern.Sony VGP-BPS9 Battery

The IAU currently recognizes five dwarf planets—Ceres, Pluto, Haumea, Makemake, and Eris.[7] However, only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they fit the definition. Eris has been accepted as a dwarf planet because it is more massive than Pluto. Sony VGP-BPS13 Battery

The IAU subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 (and hence a mathematically delimited minimum diameter of 838 km[8]) are to be named under the assumption that they are dwarf planets. HP Compaq nx7300 Adapter

The only two such objects known at the time, Makemake and Haumea, went through this naming procedure and were declared to be dwarf planets.

It is suspected that at least another forty known objects in the Solar System are dwarf planets, HP Compaq 2510p Adapter

and estimates are that up to 200 dwarf planets may be found when the entire region known as the Kuiper belt is explored, and that the number might be as high as 2,000 when objects scattered outside the Kuiper belt are considered.Dell Studio PP39L Battery

The classification of bodies in other planetary systems with the characteristics of dwarf planets has not been addressed,[10] although if they were detectable they would not be considered planets.

History of the concept

The IAU ... resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way:Nikon COOLPIX 5700  Battery

(1) A planet1 is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.Dell Studio XPS 16 Battery
(2) A “dwarf planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,2 (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.HP Compaq nx7400 Adapter
(3) All other objects,3 except satellites, orbiting the Sun shall be referred to collectively as “Small Solar System Bodies.”

The term dwarf planet has caused some concern, as a grammatical reading suggests these bodies are planets, and alternative proposals such as nanoplanetand subplanet were suggested to address this.HP Pavilion DV6500 Adapter

However, this is little different from the term minor planet, which also does not qualify as a planet. In most languages equivalent terms have been created by translating dwarf planet more-or-less literally: French planète naine, German Zwergplanet, Russian ?????????? ??????? karlikovaya planeta, Dell T117C Battery

Arabic ???? ??? kaukab qazm, Chinese ????ixíngx?ng, etc., but Japanese is an exception: In Japanese these bodies are called junwakusei ???, where wakusei ?? is 'planet' and jun- ? is a prefix corresponding to English quasi-, pene-(almost), and sub-. Thus in Japanese these are called 'subplanets' or 'almost-planets'.Acer PA-1650-02 Adapter

Although concerns were raised about the classification of planets in other solar systems,[10] the issue was not resolved; it was proposed instead to decide this only when such objects start being observed.[25]

The 2006 IAU's Resolution 6a[29] recognizes Pluto as "the prototype of a new category of trans-Neptunian objects". Acer Aspire 5920G Adapter

The name and precise nature of this category were not specified but left for the IAU to establish at a later date; in the debate leading up to the resolution, the members of the category were variously referred to as plutons and plutonian objects but neither name was carried forward.Asus S1 Adapter

On June 11, 2008, the IAU Executive Committee announced a name, plutoid, and a definition: all trans-Neptunian dwarf planets are plutoids.[30] On July 18, 2008, the Working Group for Planetary System Nomenclature reclassified the object then known as (136472) 2005 FY9 as a dwarf planet, and renamed it Makemake.HP Pavilion dv6000 Battery

Orbital dominance

Alan Stern and Harold F. Levison introduced a parameter ? (lambda), expressing the probability of an encounter resulting in a given deflection of orbit. The value of this parameter in Stern's model is proportional to the square of the mass and inversely proportional to the period. Ibm Thinkpad T30 Battery

Following the authors, this value can be used to estimate the capacity of a body to clear the neighbourhood of its orbit. A gap of five orders of magnitude in ? was found between the smallest terrestrial planets and the largest asteroids and Kuiper belt objects (third column of the planetary discriminants table to the right).Dell Studio PP39L Battery

Using this parameter, Steven Soter and other astronomers argued for a distinction between dwarf planets and the other eight planets based on their inability to "clear the neighbourhood around their orbits": planets are able to remove smaller bodies near their orbits by collision, capture, or gravitational disturbance, HP Pavilion dv5 Adapter

(or establish orbital resonances that prevent collisions), while dwarf planets lack the mass to do so.[5] Soter went on to propose a parameter he called the planetary discriminant, designated with the symbol µ (mu), that represents an experimental measure of the actual degree of cleanliness of the orbital zone (HP Compaq Business NC6400 Battery

where µ is calculated by dividing the mass of the candidate body by the total mass of the other objects that share its orbital zone).[32] There are several other schemes that try to differentiate between planets and dwarf planets,[5] but the 2006 definition uses this concept.[1]HP Compaq 490306-001 Battery

Size and mass

When an object achieves hydrostatic equilibrium, also known as gravitational relaxation, there are no gravitational imbalances in its surface. A global layer of liquid placed on this surface (assuming for argument's sake it would remain a liquid) would form a liquid surface of the same shape, apart from small-scale surface features such as craters and fissures.Compaq Presario CQ35-100 Battery

This does not mean the body is a sphere; the faster a body rotates, the more oblateor even scalene it becomes, but such forces affect a liquid surface as well. The extreme example of a non-spherical body in hydrostatic equilibrium is Haumea, which is twice as long along its major axis as it is at the poles.HP Compaq HSTNN-LB51 Battery

The upper and lower size and mass limits of dwarf planets have not been specified by the IAU. There is no defined upper limit, and an object larger or more massive than Mercury that has not "cleared the neighbourhood around its orbit" would be classified as a dwarf planet.HP Pavilion ZE4500 Battery

The lower limit is determined by the requirements of achieving a hydrostatic equilibrium shape, but the size or mass at which an object attains this shape depends on its composition and thermal history. The original draft of the 2006 IAU resolution redefined hydrostatic equilibrium shape as applying "to objects with mass above 5×1020 kg and diameter greater than 800 km",HP G62-100EB Battery

but this was not retained in the final draft.Empirical observations suggest that the lower limit may vary according to the composition of the object. For example, in the asteroid belt, Ceres, with a diameter of 975 km, is the only object presently known to be self-rounded, while 2 Pallas at approximately 600 km appears to be partially but incompletely differentiated. HP Pavilion DV6-1211AX Battery

Therefore, it has been suggested that the limit where other rocky-ice bodies like Ceres become rounded might be somewhere around 900 km.[9]The rocky body Vesta, at 530 km appears to have achieved equilibrium, only to have it disrupted by a massive impact after it solidified. HP Pavilion DV6-1220SO Battery

More-icy bodies liketrans-Neptunian objects have less rigid interiors and therefore more easily relax under their self-gravity into a rounded shape.[9] The smallest icy body known to have achieved hydrostatic equilibrium is Mimas, while the largest irregular one is Proteus; both average slightly more than 400 km (250 mi) in diameter.HP EliteBook 6930P Battery

Mike Brown (a leading researcher in this field and discoverer of Eris) suggests that the lower limit for an icy dwarf planet is therefore likely to be somewhere under 400 km.[9]

It is also not clear to what extent deviations from perfect equilibrium are to be tolerated, or whether having achieved equilibrium is sufficient for inclusion. HP Pavilion DV6-1210SA Battery

All solid bodies in the Solar System, such asIapetus with its equatorial ridge and Mars with its shield volcanoes, deviate to some extent. This may be critical in the consideration of the asteroid 4 Vesta, which may deviate from equilibrium due to a large impact that removed part of one hemisphere.Compaq Presario CQ71 Battery

Current members

As of 2011, the IAU has classified five celestial bodies as dwarf planets. Two of these, Ceres and Pluto, are known to qualify as dwarf planets through direct observation. HP Pavilion DV7-1290EO Battery

The other three, Eris, Haumea, and Makemake, are thought to be dwarf planets from mathematical modeling—or in the case of Eris, because it is larger than Pluto—and qualify for the classification under IAU naming rules based on their magnitudes. HP G6000 Battery

  1. Ceres Ceres – discovered on January 1, 1801 (45 years before Neptune), considered a planet for half a century before reclassification as an asteroid. Classified as a dwarf planet on September 13, 2006.
  2. Pluto Pluto – discovered on February 18, 1930, classified as a planet for 76 years. Reclassified as a dwarf planet on August 24, 2006.HP HDX X18-1005EA Battery
  3. Eris – discovered on January 5, 2005. Called the "tenth planet" in media reports. Accepted as a dwarf planet on September 13, 2006.
  4. Makemake – discovered on March 31, 2005. Accepted as a dwarf planet on July 11, 2008.
  5. Haumea – discovered on December 28, 2004. Accepted as a dwarf planet on September 17, 2008.HP Pavilion DV6-3013SL Battery

No space probes have visited any of the dwarf planets. This will change if NASA's Dawn and New Horizons missions reach Ceres and Pluto, respectively, as planned in 2015. Dawn is also slated to orbit and observe another potential dwarf planet, Vesta, in 2011.HP Compaq HSTNN-IB62 Battery

After Ceres, the next most massive body in the asteroid belt, Vesta, might also be classified as a dwarf planet, as its shape appears to deviate from hydrostatic equilibrium only because of a large impact that occurred after it solidified;[45] the definition of dwarf planet does not specifically address this issue. HP Pavilion DV6100 Battery

The Dawn probe scheduled to enter orbit around Vesta in 2011 may help clarify matters.

The status of Charon (currently regarded as a satellite of Pluto) remains uncertain, as there is currently no clear definition of what distinguishes a satellite system from a binary (double planet) system.HP EliteBook 2730P Battery

The original draft resolution (5)[10] presented to the IAU stated that Charon could be considered a planet because:

  1. Charon independently would satisfy the size and shape criteria for a dwarf planet status (in the terms of the final resolution);HP Pavilion DV9000 Battery
  2. Charon revolves with Pluto around a common center of mass located between the two bodies (rather than within one of the bodies) because Charon's mass is not insignificant relative to that of Pluto.[46]

This definition was not preserved in the IAU's final resolution and it is unknown if it will be included in future debates.HP Pavilion DV7 Battery

Plutoids

Many trans-Neptunian objects (TNOs) are thought to have icy cores and therefore would require a diameter of perhaps 400 km (250 mi)—only about 3% of that of Earth—to relax into gravitational equilibrium, making them dwarf planets of the plutoid class.HP Pavilion DV7-1290EO Battery

Although only rough estimates of the diameters of these objects are available, as of August 2006, it was believed that another 42 bodies beyond Neptune (besides Pluto and Eris) were likely dwarf planets. A team is investigating another 30 such objects, and believe that the total number will eventually prove to be about 200 in the Kuiper belt, and many more beyond it.HP Compaq Business 6735S Battery

Tancredi & Favre (2008) attempt to estimate which TNOs are likely to qualify, based on both direct measurements and lightcurve data. They propose that nine of the candidates be considered dwarf planets. Six of these have been estimated by one researcher or another to be at least 900 km in diameter, HP 540 Battery

the size of the smallest known dwarf planet, Ceres, as has a tenth candidate, 2002 AW197.

A total of 19 known moons are massive enough to have relaxed into a rounded shape under their own gravity.HP Compaq Business NW9440 Battery

These bodies have no significant physical differences from the dwarf planets, but are not considered members of that class because they do not directly orbit the Sun. They are Earth's moon, the four Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto),HP Compaq Business  TC4400 Battery

seven moons of Saturn (Mimas, Enceladus, Tethys, Dione, Rhea, Titan, and Iapetus), five moons of Uranus (Miranda, Ariel, Umbriel, Titania, and Oberon), one moon of Neptune (Triton), and one moon of Pluto (Charon).HP Compaq 491278-001 Battery

Contention

In the immediate aftermath of the IAU definition of dwarf planet, a number of scientists expressed their disagreement with the IAU resolution.[5] Campaigns included car bumper stickers and T-shirts.[54]Mike Brown (the discoverer of Eris) agrees with the reduction of the number of planets to eight.HP HSTNN-LB31 Battery

NASA has announced that it will use the new guidelines established by the IAU.However, Alan Stern, the director of NASA's mission to Pluto, rejects the current IAU definition of planet, both in terms of defining dwarf planets as something other than a type of planet, and in using orbital characteristics (rather than intrinsic characteristics) of objects to define them as dwarf planets.HP Pavilion DV3-4000  Battery

Thus, as of January 2008, he and his team still referred to Pluto as the ninth planet,[58] while accepting the characterization of dwarf planet for Ceres and Eris.

A minor planet is an astronomical object in direct orbit around the Sun that is neither a dominant planet nor a comet, and thus includes the dwarf planets. HP Pavilion TX1000 Battery

The first minor planet discovered was Ceresin 1801. The orbits of more than 540,000 objects have been archived at the Minor Planet Center.[1] Most of these are in the asteroid belt.[2]

The term "minor planet" has been used since the 19th century to describe these objects.HP Pavilion DV6-1210SA  Battery

The term planetoid has also been used, especially for larger objects.[4] Historically, the terms asteroid,minor planet, and planetoidhave been more or less synonymous,[4][5] but the issue has been complicated by the discovery of numerous minor planets beyond the orbit of Jupiter and especially Neptunethat are not universally considered asteroids.HP 448007-001 Battery

Before 2006 the International Astronomical Union had officially used the term minor planet. During its 2006 meeting, the Union reclassified minor planets and comets into dwarf planets and Small Solar System bodies.HP Compaq Business  6530B  Battery

Objects are called dwarf planets if their self-gravity is sufficient to achieve hydrostatic equilibrium, that is, an ellipsoidal shape, with all other minor planets and comets called "Small Solar System bodies".[6] The IAU states: "the term 'minor planet' may still be used, but generally the term 'Small Solar System body' will be preferred."HP Pavilion DV5-1015EA  Battery

However, for purposes of numbering and naming, the traditional distinction between minor planet and comet is still followed.

The Saturnian moon Mimas is the smallest body known to be in hydrostatic equilibrium (though not eligible to be a dwarf planet since it does not directly orbit the Sun), while the asteroid Pallas may be the largest that is not. HP Pavilion DV6-1230SB Battery

The IAU has so far officially classified five objects as dwarf planets. In order both of discovery and distance from the Sun, they are Ceres, Pluto, Haumea, Makemake, and Eris.

Populations

Hundreds of thousands of minor planets have been discovered within the Solar System, with the 2009 rate of discovery currently running at over 3,000 per month.HP Pavilion ZD8181EA  Battery

Of the more than 535,000 registered minor planets, 251,651 have orbits known well enough to be assigned permanent official numbers. Of these, 16,154 have official names[1]. As of September 2010, the lowest-numbered unnamed minor planet is (3708) 1974 FV1;[9] but there are also some named minor planets above number 240,000.HP Pavilion DV9500  Battery

There are various broad minor-planet populations:

  • Asteroids; traditionally, most have been bodies in the inner solar system.[5]
    • Main-belt asteroids, those following roughly circular orbits between Mars and Jupiter. These are the original and best-known group of asteroids or minor planets.HP Pavilion DV4-1125BR  Battery
    • Near-Earth asteroids, those whose orbits take them inside the orbit of Mars. Further subclassification of these, based on orbital distance, is traditional:
      • Aten asteroids, those that have semi-major axes of less than one Earth orbit. Those Aten asteroids that have their aphelion within Earth's orbit are known as Apohele asteroids;HP Pavilion DV5-1111EA Battery
      • Amor asteroids are those near-Earth asteroids that approach the orbit of the Earth from beyond, but do not cross it. Amor asteroids are further subdivided into four subgroups, depending on where their semimajor axis falls between Earth's orbit and the asteroid belt;HP Pavilion DV6-1234TX Battery
      • Apollo asteroids are those asteroids with a semimajor axis greater than Earth's, while having a perihelion distance of 1.017 AU or less. Like Aten asteroids, Apollo asteroids are Earth-crossers.
    • Mars trojans, asteroids sharing Mars's orbit and gravitationally locked to it. HP Pavilion DV3-2008TX  Battery
    • Only four such asteroids are known.
    • Jupiter trojans, asteroids sharing Jupiter's orbit and gravitationally locked to it. Numerically they are estimated to equal the main-belt asteroids.
  • Distant minor planets; an umbrella term for minor planets in the outer solar system.HP Pavilion DV8  Battery
    • Centaurs, bodies in the outer solar system between Jupiter and Neptune. They have unstable orbits due to the gravitational influence of the giant planets, and therefore must have come from elsewhere, probably outside Neptune.[11]HP Pavilion DV6-1215SA  Battery
    • Neptune trojans, bodies sharing Neptune's orbit and gravitationally locked to it. Although only a handful are known, there is evidence that Neptune trojans are more numerous than either the asteroids in the asteroid belt or the Jupiter trojans.[12]HP Compaq 490306-001  Battery
    • Trans-Neptunian objects, bodies at or beyond the orbit of Neptune, the outermost planet.
      • The Kuiper belt, objects inside an apparent population drop-off approximately 55 AU from the Sun.
        • Classical Kuiper belt objects, also known as cubewanos, are in primordial, relatively circular orbits that are not in resonance with Neptune.Compaq Presario CQ50  Battery
        • Plutinos, bodies like Pluto that are in a 2:3 resonance with Neptune, and other resonant trans-Neptunian objects. This is a rather large group.
      • Scattered disc objects, with aphelia outside the Kuiper belt. These are thought to have been scattered by Neptune.HP Pavilion DV6-1223EO  Battery
      • Detached objects such as Sedna, with both aphelia and perihelia outside the Kuiper belt.
      • The Oort Cloud, a hypothetical population thought to be the source of long period comets that may extend out to 50,000 AU from the Sun.HP Compaq 490306-001  Battery

Naming

A newly discovered minor planet is given a provisional designation (such as 2002 AT4) consisting of the year of discovery and an alphanumeric code indicating the half-month of discovery and the sequence within that half-month. Compaq Presario CQ35-100 Battery

Once an asteroid's orbit has been confirmed, it is given a number, and later may also be given a name (e.g. 433 Eros). The formal naming convention uses parentheses around the number, but dropping the parentheses is quite common. Informally, it is common to drop the number altogether, or to drop it after the first mention when a name is repeated in running text.HP Compaq HSTNN-LB51  Battery

Minor planets that have been given a number but not a name keep their provisional designation, e.g. (29075) 1950 DA. As modern discovery techniques are finding vast numbers of new asteroids, they are increasingly being left unnamed. The earliest discovered to be left unnamed was for a long time (3360) 1981 VA, now 3360 Syrinx;HP Pavilion ZE4500  Battery

as of September 2008, this distinction is held by (3708) 1974 FV1. On rare occasions, a small object's provisional designation may become used as a name in itself: the still unnamed (15760) 1992 QB1 gave its "name" to a group of Kuiper belt objects which became known as cubewanos.HP G62-100EB  Battery

A few objects are cross-listed as both comets and asteroids, such as 4015 Wilson–Harrington, which is also listed as 107P/Wilson–Harrington.

Numbering

Minor planets are awarded an official number once their orbits are confirmed. HP Pavilion DV6-1211AX  Battery

With the increasing rapidity of discovery, these are now six-figure numbers. The switch from five figures to six figures arrived with the publication of the Minor Planet Circular (MPC) of October 19, 2005, which saw the highest numbered minor planet jump from 99947 to 118161.HP EliteBook 6930P Battery

Sources for names

The first few asteroids were named after figures from Greek and Roman mythology, but as such names started to dwindle the names of famous people, literary characters, discoverer's wives, children, and even television characters were used.HP Pavilion DV9500  Battery

The first asteroid to be given a non-mythological name was 20 Massalia, named after the Greek name for the city of Marseilles. The first to be given an entirely non-Classical name was 45 Eugenia, named after Empress Eugénie de Montijo, the wife of Napoleon III. For some time only female (or feminized) names were used;Compaq Evo N610C  Battery

Alexander von Humboldt was the first man to have an asteroid named after him, but his name was feminized to 54 Alexandra. This unspoken tradition lasted until 334 Chicago was named; even then, oddly feminised names show up in the list for years after.HP Pavilion DM3-1050EO Battery

As the number of asteroids began to run into the hundreds, and eventually the thousands, discoverers began to give them increasingly frivolous names. The first hints of this were 482 Petrina and 483 Seppina, named after the discoverer's pet dogs.Compaq Presario 1245  Battery

However, there was little controversy about this until 1971, upon the naming of 2309 Mr. Spock (the name of the discoverer's cat). Although the IAUsubsequently banned pet names as sources, eccentric asteroid names are still being proposed and accepted, such as 4321 Zero, 6042 Cheshirecat, 9007 James Bond, 13579 Allodd and 24680 Alleven, and 26858 Misterrogers.HP G6000  Battery

A well-established rule is that, unlike comets, minor planets may not be named after their discoverer(s). One way to circumvent this rule has been for astronomers to exchange the courtesy of naming their discoveries after each other.HP Compaq 451086-661  Battery

An exception to this rule is 96747 Crespodasilva, which was named after its discoverer, Lucy d'Escoffier Crespo da Silva, because she died shortly after the discovery, at age 22.

Names were adapted to various languages from the beginning. 1 Ceres, Ceres being its Anglo-Latin name, was actually named Cerere, the Italian form of the name. HP Pavilion DV2166EA  Battery

German, French, Arabic and Hindi use forms similar to the English, whereas Russian uses a form, Tserera, similar to the Italian. In Greek the name was translated to Demeter, the Greek equivalent of the Roman goddess Ceres. In the early years, before it started causing conflicts, asteroids named after Roman figures were generally translated in Greek; HP Compaq 464119-143  Battery

other examples are Hera for 3 Juno, Hestia for 4 Vesta, Chloris for 8 Flora, and Pistê for 37 Fides. In Chinese, the names are not given the Chinese forms of the deities they are named after, but rather typically have a syllable or two for the charact

er of the deity or person, Compaq Presario CQ71  Battery

followed by ? 'god(dess)' or ? 'woman' if just one syllable, plus ? 'star/planet', so that most asteroid names are written with three Chinese characters. Thus Ceres is ??? 'grain goddess planet',[15] Pallas is ??? 'wisdom goddess planet', etc.HP Pavilion DV9565EA  Battery

Special naming rules

Minor-planet naming is not always a free-for-all: there are some populations for which rules have developed about the sources of names. For instance, centaurs (orbiting between Saturn and Neptune) are all named after mythological centaurs; HP Pavilion DV6203TX Battery

Jupiter trojans after heroes from the Trojan War; resonant trans-Neptunian objects after underworld spirits; and non-resonant TNOs after creation deities.

Physical properties of comets and minor planets

Commission 15[16] of the International Astronomical Union is dedicated to the Physical Study of Comets & Minor Planets.HP Pavilion DV7-1290EO  Battery

Archival data on the physical properties of comets and minor planets are found in the PDS Asteroid/Dust Archive. This includes standard asteroid physical characteristics such as the properties of binary systems, occultation timings and diameters, masses,HP HSTNN-OB51  Battery

densities, rotation periods, surface temperatures, albedoes, spin vectors, taxonomy, and absolute magnitudes and slopes. In addition, European Asteroid Research Node (E.A.R.N.), an association of asteroid research groups, maintains a Data Base of Physical and Dynamical Properties of Near Earth Asteroids.HP Pavilion DV7-2185DX  Battery

Most detailed information is available from Category: Asteroids visited by spacecraft and Category: Comets visited by spacecraft.HP Compaq Business  6830S  Battery,HP Pavilion DM4 Battery,Dell Latitude E6400 Battery,HP Pavilion DV6-1020ED Battery,HP Pavilion DV7-1290EO Battery,Apple A1281 Battery,Apple A1280 Battery,Dell Inspiron 6400 AC Adapter

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