Artist's impression of Pluto (background) and its satellite Charon (foreground). Pluto, considered a planet for 76 years, was reclassified as a dwarf planet in 2006.

A dwarf planet is a category of celestial bodies in the solar system defined in a resolution passed by the International Astronomical Union (IAU) on August 24, 2006. The definition currently applies only to the Sun's solar system.^[1] It applies only to the English language, and terminology may differ in other languages. In the usage approved by the IAU, the category "dwarf planet" is distinct from that of "planet" and also from another new category, "Small Solar-System Body". Image File history File links Pluto_artistimpression. ... Image File history File links Pluto_artistimpression. ... Adjective Plutonian Atmospheric characteristics Atmospheric pressure 0. ... Moons of solar system scaled to Earths Moon A natural satellite is a moon (not capitalized), that is, any natural object that orbits a planet. ... Media:Example. ... The currently accepted eight planets with the three newly designated dwarf planets. ... See also Lists of astronomical objects Category: ... Major features of the solar system (not to scale): The Sun, the eight planets, the asteroid belt, dwarf planet Pluto, and a comet. ... The final definition left the solar system with eight planets, pictured above (not to scale) Displays the remaining eight planets with the celestial bodies that have now been designated as dwarf planets. ... Logo of the IAU The International Astronomical Union (French: Union astronomique internationale) unites national astronomical societies from around the world. ... August 24 is the 236th day of the year in the Gregorian Calendar (237th in leap years), with 129 days remaining. ... 2006 (MMVI) is a common year starting on Sunday of the Gregorian calendar. ... The Sun is the star at the center of our solar system. ... The currently accepted eight planets with the three newly designated dwarf planets. ... A small solar system body is a term defined in 2006 by the International Astronomical Union to describe solar system objects which are not planets or dwarf planets: [1] Therefore it refers to these objects that can be further classified based on their orbit or composition: all known minor planets...

The resolution describes a dwarf planet as an object that:

• Is in orbit around the Sun
• Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
• Has not "cleared the neighbourhood" around its orbit
• Is not a satellite of a planet, or other nonstellar body

It differs from the definition of the planet in that the dwarf planet has not cleared its orbital neighbourhood. This definition reclassified Pluto from a planet to a dwarf planet because it has not cleared the neighbourhood of its orbit (the Kuiper Belt). 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 Sun is the star at the center of our solar system. ... Mass is a property of a physical object that quantifies the amount of matter and energy it is equivalent to. ... Gravity is a force of attraction that acts between bodies that have mass. ... In physics, a rigid body is an idealisation of a solid body of finite size in which deformation is neglected. ... In physics, a force is anything that causes a free body with mass to accelerate. ... Hydrostatic equilibrium occurs when compression due to gravity is balanced by outward pressure. ... This article or section is in need of attention from an expert on the subject. ... Moons of solar system scaled to Earths Moon A natural satellite is a moon (not capitalized), that is, any natural object that orbits a planet. ... The currently accepted eight planets with the three newly designated dwarf planets. ... Adjective Plutonian Atmospheric characteristics Atmospheric pressure 0. ... Artists rendering of the Kuiper Belt and hypothetical more distant Oort cloud. ...

Dwarf planets are traditionally known as minor planets along with smaller objects. Minor planet is the official term for asteroids and trans-Neptunian objects. ...

List of dwarf planets

The IAU has officially identified three celestial bodies that have immediately received "dwarf planet" classification^[2]:

*Measured relative to the Earth. 1 Ceres (seer-eez (key), IPA , Latin CerÄ“s) was the first asteroid to be discovered (indicated by the 1 in its name). ... Atmospheric characteristics Atmospheric pressure 0. ... 2003 UB313 is a trans-Neptunian object (TNO) believed to be larger than the planet Pluto. ... Major features of the solar system (not to scale): The Sun, the eight planets, the asteroid belt, dwarf planet Pluto, and a comet. ... Image of the main asteroid belt between the orbits of Mars and Jupiter. ... Artists rendering of the Kuiper Belt and hypothetical more distant Oort cloud. ... The scattered disc (or scattered disk) is a distant region of our solar system, thinly populated by icy planetoids known as scattered disk objects (SDOs), a subset of the broader family of trans-Neptunian objects (TNOs). ... An asteroid is a predominantly rocky body that orbits around its star. ... In astronomy, a plutino is a trans-Neptunian object that has a 3:2 orbital resonance with Neptune. ... The international prototype, made of platinum-iridium, which is kept at the BIPM under conditions specified by the 1st CGPM in 1889. ... km redirects here. ... A cubic kilometre (symbol km³) is an SI derived unit of volume. ... Density (symbol: ρ - Greek: rho) is a measure of mass per unit of volume. ... BIC pen cap, about 1 gram. ... This article is being considered for deletion in accordance with Wikipedias deletion policy. ... Gravity is a force of attraction that acts between bodies that have mass. ... Acceleration is the time rate of change of velocity, and at any point on a v-t graph, it is given by the slope of the tangent to that point In physics or physical science, acceleration (symbol: a) is defined as the rate of change (or derivative with respect to... In physics, for a given gravitational field and a given position, the escape velocity is the minimum speed an object without propulsion, at that position, needs to have to move away indefinitely from the source of the field, as opposed to falling back or staying in an orbit within a... Speed (symbol: v) is the rate of motion, or equivalently the rate of change of position, expressed as distance d moved per unit of time t. ... Water, Rabbit, and Deer: three of the 20 day symbols in the Aztec calendar, from the Aztec Sun Stone. ... The orbital period is the time it takes a planet (or another object) to make one full orbit. ... The astronomical unit (AU or au or a. ... km redirects here. ... The orbital period is the time it takes a planet (or another object) to make one full orbit. ... A year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. ... The orbital period is the time it takes a planet (or another object) to make one full orbit. ... Speed (symbol: v) is the rate of motion, or equivalently the rate of change of position, expressed as distance d moved per unit of time t. ... In astrodynamics, under standard assumptions any orbit must be of conic section shape. ... Inclination in general is the angle between a reference plane and another plane or axis of direction. ... The equator is an imaginary circle drawn around a planet (or other astronomical object) at a distance halfway between the poles. ... 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. ... Axial tilt is an astronomical term regarding the inclination angle of a planets rotational axis in relation to its orbital plane. ... The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. ... Moons of solar system scaled to Earths Moon A natural satellite is a moon (not capitalized), that is, any natural object that orbits a planet. ... The planet Pluto has three known moons. ... S/2005 (2003 UB313) 1 is a moon around the cubewano Kuiper Belt trans-Neptunian object 2003 UB313, the presumed tenth planet of our Solar System. ...

Additionally, there are several bodies potentially qualifying as "dwarf planets,". Among these the following are known or thought to be greater than around 750 km in diameter: KM, Km, or km may stand for: Khmer language (ISO 639 alpha-2, km) Kilometre Kinemantra Meditation Knowledge management KM programming language KM Culture, Korean Movie Maker. ...

The status of Charon, currently regarded as a satellite of Pluto, remains uncertain as there is presently no clear definition of what distinguishes a satellite system from a binary (double planet) system. The original draft resolution (5)^[1] presented to the IAU stated that Charon could be considered a planet because: 2005 FY9 (also written 2005 FY9), codenamed Easterbunny by its discoverers, is a very large Kuiper belt object discovered on March 31, 2005 by the team led by Michael Brown. ... A cubewano is any substantial Kuiper belt object, orbiting beyond about 41 AU and not controlled by resonances with the outer planets. ... 90482 Orcus (originally known by the provisional designation 2004 DW) is a Kuiper Belt object (KBO) that was discovered by Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. ... In astronomy, a plutino is a trans-Neptunian object that has a 3:2 orbital resonance with Neptune. ... 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. ... A scattered disk object (or scattered disc object or SDO) is a trans-Neptunian object of the Kuiper belt with a very eccentric orbit. ... 2003 EL61 (also written 2003 EL61), nicknamed Santa (non-official designation), is a large and very unusual Kuiper belt object discovered by Mike Brown at Caltech in the United States. ... A cubewano is any substantial Kuiper belt object, orbiting beyond about 41 AU and not controlled by resonances with the outer planets. ... 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. ... A cubewano is any substantial Kuiper belt object, orbiting beyond about 41 AU and not controlled by resonances with the outer planets. ... Media:Example. ... Moons of solar system scaled to Earths Moon A natural satellite is a moon (not capitalized), that is, any natural object that orbits a planet. ... In astronomy, a plutino is a trans-Neptunian object that has a 3:2 orbital resonance with Neptune. ... (84522) 2002 TC302 is a large Scattered Disk Object (SDO), orbiting the sun at a distance of 39. ... A scattered disk object (or scattered disc object or SDO) is a trans-Neptunian object of the Kuiper belt with a very eccentric orbit. ... 20000 Varuna (VAR oo na) is a Kuiper Belt object about 1060 km in diameter, estimated from a combination of thermal and optical measurements. ... A cubewano is any substantial Kuiper belt object, orbiting beyond about 41 AU and not controlled by resonances with the outer planets. ... (55637) 2002 UX25 (also written as (55637) 2002 UX25) is a large Trans-Neptunian object discovered on October 30, 2002 by the Spacewatch program. ... A cubewano is any substantial Kuiper belt object, orbiting beyond about 41 AU and not controlled by resonances with the outer planets. ... (55636) 2002 TX300 (Also written as (55636) 2002 TX300) is a large Trans-Neptunian object discovered in October 15, 2002 by the NEAT program. ... A cubewano is any substantial Kuiper belt object, orbiting beyond about 41 AU and not controlled by resonances with the outer planets. ... (28978) Ixion (ik·sye·un) is a Kuiper belt object discovered on May 22, 2001 with a diameter of < 822 km and a semimajor axis of about 39. ... In astronomy, a plutino is a trans-Neptunian object that has a 3:2 orbital resonance with Neptune. ... (55565) 2002 AW197 (also written: (55565) 2002 AW197) is a trans-Neptunian object. ... A cubewano is any substantial Kuiper belt object, orbiting beyond about 41 AU and not controlled by resonances with the outer planets. ... Media:Example. ... The term double planet has several accepted usages. ...

1. Charon independently would satisfy the size and shape criteria for planetary status (and in the terms of the final resolution, for the status of dwarf planet)
2. Charon, on account of its large mass relative to Pluto, revolves with Pluto around a common barycentre located in space between Pluto and Charon rather than around a point located within Pluto.

This definition, however, was not preserved in the IAU's final resolution. It is unknown if it will be taken up at a future date. If a similar definition were to be adopted, Charon would be added to the list of dwarf planets. 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 second, third, and fourth largest asteroids (Vesta, Pallas and Hygiea) could be classified as dwarf planets if it is shown that their shape is determined by hydrostatic equilibrium. At present this has not been demonstrated conclusively.^[3] 4 Vesta (ves-ta) is the second most massive asteroid in the asteroid belt, with a mean diameter of about 530 km and an estimated mass 12% the mass of the entire asteroid belt. ... 2 Pallas (pal-us, Greek Παλλάς) was the first asteroid discovered after 1 Ceres. ... 10 Hygiea (hye-jee-a or hi-jee-a) is the fourth largest Main belt asteroid with a diameter of 407 km. ... Hydrostatic equilibrium occurs when compression due to gravity is balanced by outward pressure. ...

Size and mass of dwarf planets

The upper and lower limits to the size and mass of dwarf planets are not specified in IAU resolution 5A. There is strictly no upper limit, and an object larger or more massive than Mercury that is considered not to have "cleared the neighborhood around its orbit" may still be classified as a dwarf planet. Note: This article contains special characters. ...

The lower limit is determined by the concept of hydrostatic equilibrium shape, but the size or mass at which an object attains this shape is undefined, and empirical observations suggest that it may vary according to the composition and history of the object. The original draft of IAU resolution 5 defined hydrostatic equilibrium shape as applying "to objects with mass above 5×10²⁰ kg and diameter greater than 800 km" ^[1], but this language was not retained in the final resolution 5A that was passed.

According to some astronomers, the new definition could mean the addition of up to 45 new dwarf planets.^[4][5]

Orbital dominance

Main article: Clearing the neighbourhood

Astronomers S. Alan Stern, Harold F. Levison, Steven Soter, and others have argued for a distinction between dwarf planets and the other eight planets based on their inability to "clear the neighborhood around their orbits", that is, to remove smaller bodies whose orbits bring them nearby by collision, capture, or gravitational disturbance. This concept is combined with a concept of orbital dominance measured in terms of the ratio of the mass of a planetary candidate to the combined mass of all other objects in its vicinity. Dwarf planets are considered to be too small in mass to significantly alter their environment in the manner of a planet. Stern and Levison found a gap of five orders of magnitude in Λ between the smallest terrestrial planets and the largest asteroids and KBOs: This article or section is in need of attention from an expert on the subject. ... S. Alan Stern is a planetary scientist at the Southwest Research Institute. ... 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. ... This article or section is in need of attention from an expert on the subject. ...

*M_E in Earth masses.
**Λ/ΛE = M²/P, in Earth masses squared per year.
***µ = M/m, where M is the mass of the body, and m is the aggregate mass of all the other bodies that share its orbital zone.
Note: This article contains special characters. ... (*min temperature refers to cloud tops only) Atmospheric characteristics Atmospheric pressure 9. ... Earth, also known as the Earth or Terra, is the third planet outward from the Sun. ... 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. ... Atmospheric characteristics Atmospheric pressure 120 kPa Hydrogen 83% Helium 15% Methane 1. ... Atmospheric characteristics Surface pressure ≫100 MPa Hydrogen - H2 80% ±3. ... 1 Ceres (seer-eez (key), IPA , Latin Cerēs) was the first asteroid to be discovered (indicated by the 1 in its name). ... Adjective Plutonian Atmospheric characteristics Atmospheric pressure 0. ... 2003 UB313 is a trans-Neptunian object (TNO) believed to be larger than the planet Pluto. ...

Dwarf planet and other solar system categories

The categorization of solar system objects into the three categories of planet, dwarf planet, and Small Solar-System Body established by IAU Resolution 5A does not supersede previous classifications based on other criteria, such as a body's location in the Solar System, its composition, or its history. The Resolution itself makes reference to the classes asteroid, Trans-Neptunian object (TNO), and comet (footnote 3)^[2]. The currently accepted eight planets with the three newly designated dwarf planets. ... A small solar system body is a term defined in 2006 by the International Astronomical Union to describe solar system objects which are not planets or dwarf planets: [1] Therefore it refers to these objects that can be further classified based on their orbit or composition: all known minor planets... An asteroid is a predominantly rocky body that orbits around its star. ... A trans-Neptunian object (TNO) is any object in the solar system that orbits the sun at a greater distance on average than Neptune. ... Comet Hale-Bopp For other uses, see Comet (disambiguation). ...

None of these classes (asteroid, TNO, comet) is coterminous with any of the categories of planet, dwarf planet, and Small Solar-System Body. They may or may not be subsets of the latter categories. The language in footnote 3 that Small Solar-System Bodies "currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies," although technically ambiguous, implies that "comets and other small bodies" are subsets of the category "Small Solar-System Body". The use of the word "most", however, indicates that asteroids and TNOs are sets that only partially overlap the category; this is consistent with the naming of asteroid 1 Ceres and TNOs Pluto and 2003 UB₃₁₃ as dwarf planets and not Small Solar-System Bodies. A reasonable conclusion is that Ceres continues to be the largest asteroid and Pluto and 2003 UB₃₁₃ continue to be Trans-Neptunian objects despite also being categorized as dwarf planets. 1 Ceres (seer-eez (key), IPA , Latin CerÄ“s) was the first asteroid to be discovered (indicated by the 1 in its name). ... Adjective Plutonian Atmospheric characteristics Atmospheric pressure 0. ... 2003 UB313 is a trans-Neptunian object (TNO) believed to be larger than the planet Pluto. ... 1 Ceres (seer-eez (key), IPA , Latin CerÄ“s) was the first asteroid to be discovered (indicated by the 1 in its name). ... Adjective Plutonian Atmospheric characteristics Atmospheric pressure 0. ... 2003 UB313 is a trans-Neptunian object (TNO) believed to be larger than the planet Pluto. ... A trans-Neptunian object (TNO) is any object in the solar system that orbits the sun at a greater distance on average than Neptune. ...

Types of dwarf planets

Illustration of the size estimates of the largest trans-Neptunian objects including Pluto and 2003 UB₃₁₃. Other objects may meet the definition of a dwarf planet.

The IAU's Resolution 6a^[2] recognizes Pluto as "the prototype of a new category of trans-Neptunian objects". The name and precise nature of this category are not specified, but in the debate leading up to the resolution, the members of the category were variously referred to as "Plutons" and "Plutonian objects". The former name was generally deprecated^[6] and was abandoned in the final draft resolution (6b)^[7]; the latter name failed to win majority approval on a 180-186 vote in the IAU General Assembly on August 24, 2006. The category, while established, remains nameless. Image File history File links TheTransneptunians_Size_Albedo_Color. ... Image File history File links TheTransneptunians_Size_Albedo_Color. ... A trans-Neptunian object (TNO) is any object in the solar system that orbits the sun at a greater distance on average than Neptune. ... Logo of the IAU The International Astronomical Union (French: Union astronomique internationale) unites national astronomical societies from around the world. ... August 24 is the 236th day of the year in the Gregorian Calendar (237th in leap years), with 129 days remaining. ... 2006 (MMVI) is a common year starting on Sunday of the Gregorian calendar. ... On August 24, 2006, the International Astronomical Union (IAU) decided to reclassify Pluto as a dwarf planet, requiring that a planet must clear the neighbourhood around its orbit. ...

At an earlier stage in the definition process, the category (then described as "pluton") was defined to be a planet whose orbit took more than 200 Julian years to complete and whose orbit was more highly inclined and elliptical than a traditional planetary orbit.^[8] The currently accepted eight planets with the three newly designated dwarf planets. ... In astronomy, a Julian year is a unit of time defined as exactly 365. ... Inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit and is the angular distance of the orbital plane from the plane of the reference (usually planets equator or the ecliptic), stated in degrees. ...

This category of Pluto-like objects only applies to dwarf planets which meet the conditions of being trans-Neptunian and "like Pluto" in terms of period, inclination and eccentricity. A dwarf planet may or may not be a member of this category, but all members of the category must be dwarf planets.

The membership of this class, other than Pluto itself, remains obscure. Pluto's largest satellite, Charon would qualify if it were to be classed as a dwarf planet in its own right. 2003 UB₃₁₃ and the objects listed in the table "Possible dwarf planets" (above) also qualify in terms of the minimum period, and most exhibit orbital eccentricity and inclination that are significant, though not always equal to or greater than Pluto's. Quaoar, however, has a much smaller eccentricity and inclination, and so possibly does not qualify as a Pluto-like object. 2003 UB313 is a trans-Neptunian object (TNO) believed to be larger than the planet Pluto. ... This article is about the trans-Neptunian object. ...

The diagrams below illustrate the changes between the original draft and the final outcome of the vote.

Illustration of the outcome of the vote.

Illustration of the draft proposal (not passed).

Image File history File links ThePlanetDefinition3. ... Image File history File links ThePlanetDefinition3. ... Image File history File links ThePlanetDefinition. ... Image File history File links ThePlanetDefinition. ...

See also

• Classical planet
• Mesoplanet
• Small solar system body
• Trans-Neptunian object
• List of solar system objects by planetary discriminant
• Clearing the neighbourhood

The final definition left the solar system with eight planets, pictured above (not to scale). ... Mesoplanet is a term coined by Isaac Asimov to refer to planetary bodies with sizes smaller than Mercury but larger than 1 Ceres. ... A small solar system body is a term defined in 2006 by the International Astronomical Union to describe solar system objects which are not planets or dwarf planets: [1] Therefore it refers to these objects that can be further classified based on their orbit or composition: all known minor planets... A trans-Neptunian object (TNO) is any object in the solar system that orbits the sun at a greater distance on average than Neptune. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... This article or section is in need of attention from an expert on the subject. ...

References

1. ^ ^a b c Draft Resolution 5 for GA-XXVI: Definition of a Planet.
2. ^ ^a b c IAU 2006 General Assembly: Result of the IAU Resolution votes.
3. ^ Three new planets may join solar system. New Scientist. Retrieved on 2006-08-16.
4. ^ Nine Planets Become 12 with Controversial New Definition. Space.com. Retrieved on 2006-08-16.
5. ^ What makes a planet?. Michael E. Brown. Retrieved on 2006-08-16.
6. ^ Astronomers divided over "planet" definition.
7. ^ The Final IAU Resolution on the definition of "planet" ready for voting.
8. ^ Draft definition, IAU press release.

• The IAU press release and supporting web site, 16 August 2006
• The Scientific American Blog on Plutons, 18 August 2006

2006 (MMVI) is a common year starting on Sunday of the Gregorian calendar. ... August 16 is the 228th day of the year (229th in leap years) in the Gregorian Calendar. ... Space. ... 2006 (MMVI) is a common year starting on Sunday of the Gregorian calendar. ... August 16 is the 228th day of the year (229th in leap years) in the Gregorian Calendar. ... Dr. Michael (Mike) E. Brown has been an associate professor of planetary astronomy at the California Institute of Technology (Caltech) since 2002. ... 2006 (MMVI) is a common year starting on Sunday of the Gregorian calendar. ... August 16 is the 228th day of the year (229th in leap years) in the Gregorian Calendar. ...

External links

• NPR: Dwarf Planets May Finally Get Respect (David Kestenbaum)
• SPACE.com: Q&A: The IAU's Proposed Planet Definition 16 August 2006 2:00 am ET
• BBC News: Q&A New planets proposal Wednesday, 16 August 2006, 13:36 GMT 14:36 UK

The Solar System  v·d·e
Star: The Sun
Planets: Mercury - Venus - Earth - Mars - Jupiter - Saturn - Uranus - Neptune
Dwarf planets: Ceres - Pluto - 2003 UB313
Other: Moon - Asteroid belt - Comets - Kuiper belt - Scattered disc - Oort cloud
See also astronomical objects and the solar system's list of objects, sorted by radius or mass.