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Dwarf planet

A dwarf planet is a category of celestial bodies in the solar system Solar System

The Solar System or solar system is the stellar system [i] comprising the Sun [i] and ... 

 as introduced in a resolution 2006 redefinition of planet

The 2006 redefinition of "planet" by the International Astronomical Union [i] states that, in the solar system [i] ... 

 by the International Astronomical Union International Astronomical Union

The International Astronomical Union unites national astronomical [i] societies from around th ... 

  on August 24 2006. The definition currently applies only to the Sun Sun

|+ The Sun   |+ |- ... 

's solar system.resolution describes a dwarf planet as an object that: *Is in orbit around the Sun Sun

|+ The Sun   |+ |- ... 

*Has sufficient mass Mass

Mass is a property of a physical [i] object that quantifies the amount of matter [i] and energy [i] ... 

 for its self-gravity Gravitation

In physics [i], gravitation or gravity is the tendency of objects with mass [i] to accelerate [i] ... 

 to overcome rigid body forces so that it assumes a hydrostatic equilibrium Hydrostatic equilibrium

Hydrostatic equilibrium occurs when compression due to gravity [i] is balanced by a pressure [i] gradien ... 

  shape *Has not "cleared the neighbourhood Clearing the neighbourhood

"Clearing the neighbourhood" is an informal description of part of the process of planet formation [i]. ... 

" around its orbit

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Timeline

1999   Pluto Pluto

Pluto is the second-largest known dwarf planet [i] in the solar system [i]. ... 

, a dwarf planet with an eccentric orbit, moves further from the Sun Sun

|+ The Sun   |+ |- ... 

 than Neptune Neptune

Neptune is the eighth and outermost planet [i] in our solar system [i]. ... 

. It had been nearer than Neptune since 1979, and will become again in ''2231''.



Encyclopedia


A dwarf planet is a category of celestial bodies in the solar system Solar System

The Solar System or solar system is the stellar system [i] comprising the Sun [i] and ... 

 as introduced in a resolution 2006 redefinition of planet

The 2006 redefinition of "planet" by the International Astronomical Union [i] states that, in the solar system [i] ... 

 by the International Astronomical Union International Astronomical Union

The International Astronomical Union unites national astronomical [i] societies from around th ... 

  on August 24 2006. The definition currently applies only to the Sun Sun

|+ The Sun   |+
|-
... 

's solar system. 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 Planet

The International Astronomical Union [i] , the official scientific [i] body for astronomical [i] nomenclature [i]... 

" and also from another new category, "Small Solar System Body".

The resolution describes a dwarf planet as an object that:
  • Is in orbit around the Sun Sun

    |+ The Sun   |+

|-
... 


  • Has sufficient mass Mass

    Mass is a property of a physical [i] object that quantifies the amount of matter [i] and energy [i] ... 

     for its self-gravity Gravitation

    In physics [i], gravitation or gravity is the tendency of objects with mass [i] to accelerate [i] ... 

     to overcome rigid body forces so that it assumes a hydrostatic equilibrium Hydrostatic equilibrium

    Hydrostatic equilibrium occurs when compression due to gravity [i] is balanced by a pressure [i] gradien ... 

      shape
  • Has not "cleared the neighbourhood Clearing the neighbourhood

    "Clearing the neighbourhood" is an informal description of part of the process of planet formation [i]. ... 

    " around its orbit
  • Is not a satellite Natural satellite

    A natural satellite [i] is a non-man-made object that orbits a planet [i] or other body larger than ... 

     of a planet Planet

    The International Astronomical Union [i] , the official scientific [i] body for astronomical [i] nomenclature [i]... 

    , or other nonstellar body


It differs from the definition of "planet" in that the dwarf planet has not cleared its orbital neighbourhood. This definition reclassified Pluto Pluto

Pluto is the second-largest known dwarf planet [i] in the solar system [i]. ... 

 from a planet to a dwarf planet because it has not cleared the neighbourhood of its orbit .

List of dwarf planets

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

Dwarf planets
Name Pluto Pluto

Pluto is the second-largest known dwarf planet [i] in the solar system [i]. ... 

 
MPC number 1 134340 136199
Region of Solar system Solar System

The Solar System or solar system is the stellar system [i] comprising the Sun [i] and ... 

Asteroid belt Asteroid belt

The asteroid belt is a region of the solar system [i] falling roughly between the planet [i]s Mars [i] ... 

 
Kuiper belt Kuiper belt

The Kuiper belt is an area of the solar system [i] extending from the orbit [i] of Neptune [i] ... 

 
Scattered disc Scattered disc

The scattered disc is a distant region of our solar system [i], thinly populated by icy planetoid [i]s k ... 

Diameter 975×909 km 2306±20 km 2400±100 km
Mass in kg Kilogram

The kilogram or kilogramme, is the SI base unit [i] of mass [i]. ... 


compared to Earth
.00016 .0022
Mean equatorial radius*
in km
0.0738
471
0.180
1,148.07
0.19
~1,200
Volume*
0.00042
0.005
0.007
Density 2.08 2.0  
Equatorial gravity Gravitation

In physics [i], gravitation or gravity is the tendency of objects with mass [i] to accelerate [i] ... 

0.27 0.60  
Escape velocity 0.51 1.2  
Rotation period
0.3781 -6.38718
 
Orbital radius*
mean
mean in km
2.5-2.9
2.766
413,715,000
29.66-49.30
39.48168677
5,906,376,200
37.77-97.56
67.6681
10,210,000,000
Orbital period*
4.599 248.09 557
Mean orbital speed
17.882 4.7490 3.436
Orbital Eccentricity 0.080 0.24880766 0.44177
Orbital inclination Inclination

Inclination in general is the angle [i] between a reference plane [i] and another plane or axis [i] ... 

10.587° 17.14175° 44.187°
Inclination of the equator Equator

The equator is an imaginary circle [i] drawn around a planet [i] at a distance halfway between the pole [i] ... 

 from the orbit
119.61°  
Mean surface temperature 167 40 30
Number of natural satellite Natural satellite

A natural satellite [i] is a non-man-made object that orbits a planet [i] or other body larger than ... 

s
0 3 Pluto's natural satellites

The dwarf planet [i] Pluto [i] has three known moons [i]. ... 

 
1
Date of Discovery January 1, 1801 February 18, 1930 January 5, 2005


*Measured relative to the Earth.

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:

Possible dwarf planets
Name Category Diameter Mass
Cubewano Cubewano

In astronomy a cubewano is a Kuiper belt [i] object that orbit [i]s beyond Neptune [i] and is... 

 
1600 – 2000? km unknown
Plutino Plutino

In astronomy [i], a plutino is a trans-Neptunian object [i] in 2:3 mean motion resonance [i] ... 

 
840 - 1880 km6.2 - 7.0 × 1020 kg
Scattered-Extended object Scattered disc

The scattered disc is a distant region of our solar system [i], thinly populated by icy planetoid [i]s k ... 

 
1180–1800 km1.7-6.1 × 1021 kg
Cubewano Cubewano

In astronomy a cubewano is a Kuiper belt [i] object that orbit [i]s beyond Neptune [i] and is... 

 
~ 1500 km ~4.2 × 1021 kg
Cubewano Cubewano

In astronomy a cubewano is a Kuiper belt [i] object that orbit [i]s beyond Neptune [i] and is... 

 
989 - 1346? km 1.0-2.6 × 1021 kg
Charon
Plutino Plutino

In astronomy [i], a plutino is a trans-Neptunian object [i] in 2:3 mean motion resonance [i] ... 

 
1207 km ± 3 km ×1021 kg
Scattered disc object Scattered disc

The scattered disc is a distant region of our solar system [i], thinly populated by icy planetoid [i]s k ... 

 
= 1200 kmunknown
Cubewano Cubewano

In astronomy a cubewano is a Kuiper belt [i] object that orbit [i]s beyond Neptune [i] and is... 

 
~936 km~5.9 × 1020 kg
Cubewano Cubewano

In astronomy a cubewano is a Kuiper belt [i] object that orbit [i]s beyond Neptune [i] and is... 

 
~910 km ~7.9 × 1020 kg
Cubewano Cubewano

In astronomy a cubewano is a Kuiper belt [i] object that orbit [i]s beyond Neptune [i] and is... 

 
<900 km unknown
Plutino Plutino

In astronomy [i], a plutino is a trans-Neptunian object [i] in 2:3 mean motion resonance [i] ... 

 
<822 km unknown


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 system. The original draft resolution presented to the IAU stated that Charon could be considered a planet because:
  1. Charon independently would satisfy the size and shape criteria for planetary status
  2. Charon, on account of its large mass relative to Pluto, revolves with Pluto around a common barycentre Center of mass

    In physics [i], the center of mass of a system of particles is a specific point at which, for many purpo ... 

     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 second, third, and fourth largest asteroids could be classified as dwarf planets if it is shown that their shape is determined by hydrostatic equilibrium Hydrostatic equilibrium

Hydrostatic equilibrium occurs when compression due to gravity [i] is balanced by a pressure [i] gradien ... 

. At present this has not been demonstrated conclusively.

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.

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 kg and diameter greater than 800 km", 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.

Orbital dominance


Astronomers S. Alan Stern, Harold F. Levison, Steven Soter, and others have argued for a distinction between dwarf planets and the other eight planets Planet

The International Astronomical Union [i] , the official scientific [i] body for astronomical [i] nomenclature [i]... 

 based on their inability to "clear the neighborhood around their orbits Clearing the neighbourhood

"Clearing the neighbourhood" is an informal description of part of the process of planet formation [i]. ... 

", 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 too small in mass to significantly alter their environment in the manner of a planet.

There are several other theories that try to differentiate between planets and dwarf planets, but the current definition of what constitutes a planet uses this concept.

Stern et al introduce a paramater ?, 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. Following the authors, this value can be used to estimate the capacity of a body to clear the neighbourhood of its orbit. Stern and Levison found a gap of five orders of magnitude in ? between the smallest terrestrial planets and the largest asteroids and KBOs:

align=bottom style="text-align:left;"|
Planetary discriminants
Body Mass
?/?E**
µ***
Mercury 0.055 0.0126 9.1
Venus Venus

Venus is the second-closest planet [i] to the Sun [i], orbiting it every 224.7 Earth days. ... 

0.815 1.08 1.35
Earth Earth

Earth is the third planet [i] in the solar system [i] in terms of distance from the Sun [i], and the fi ... 

1.00 1.00 1.7
Mars Mars

Mars is the fourth planet [i] from the Sun [i] in our solar system [i] and is named after Mars [i] ... 

0.107 0.0061 1.8
0.00015 8.7 0.33
Jupiter Jupiter

Jupiter is the fifth planet [i] from the Sun [i] and the largest [i] within the solar system [i] ... 

317.7 8510 6.25
Saturn Saturn

Saturn is the sixth planet [i] from the Sun [i]. ... 

95.2 308 1.9
Uranus Uranus

Uranus is the seventh planet [i] from the Sun [i]. ... 

14.5 2.51 2.9
Neptune Neptune

Neptune is the eighth and outermost planet [i] in our solar system [i]. ... 

17.1 1.79 2.4
0.0022 1.95 0.077
0.005 3.5 0.10

*ME in Earth masses.

**?/?E = 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.

Contention


A number of scientists expressed their disagreement with the currently adopted IAU definition of "dwarf planet" by means of car bumper stickers and about 300 scientists signed a petition against the related redefinition of "planet", stating that they will not use it.

While accepting the characterization of "dwarf planet" for and , Stern 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 of objects to define them as dwarf planets. Thus, he and his team will still refer to Pluto as the ninth planet. One should also note, that it will be in pages hosted by NASA and controlled by Stern's team, that the upcoming information and the first photographs of Pluto will be unveiled to the world. However, NASA has announced that it will use the new guidelines established by the IAU.

Dwarf planet and other solar system categories


The categorization of solar system objects into the three categories of planet Planet

The International Astronomical Union [i] , the official scientific [i] body for astronomical [i] nomenclature [i]... 

, 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 Asteroid

Asteroid, minor planet, and planetoid are synonyms, and are used to indicate a diverse group of small ce... 

, Trans-Neptunian object Trans-Neptunian object

A trans-Neptunian object is any object in the solar system [i] that orbit [i]s the sun at a greater dis... 

 , and comet Comet

A comet is a small body in the solar system that orbits the Sun [i] and exhibits a coma [i] and/ ... 

 .

None of these classes 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 , 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 1 Ceres

Ceres, officially designated 1 Ceres, is a dwarf planet [i] in the asteroid belt [i]. ... 

 and TNOs and as dwarf planets and not Small Solar-System Bodies. A reasonable conclusion is that continues to be the largest asteroid and and continue to be Trans-Neptunian object Trans-Neptunian object

A trans-Neptunian object is any object in the solar system [i] that orbit [i]s the sun at a greater dis... 

s despite also being categorized as dwarf planets.

Types of dwarf planets


The IAU International Astronomical Union

The International Astronomical Union unites national astronomical [i] societies from around th ... 

's Resolution 6a 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 and was abandoned in the final draft resolution ; 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.

At an earlier stage in the definition process, the category was defined to be a planet Planet

The International Astronomical Union [i] , the official scientific [i] body for astronomical [i] nomenclature [i]... 

 whose orbit took more than 200 Julian years to complete and whose orbit was more highly inclined Inclination

Inclination in general is the angle [i] between a reference plane [i] and another plane or axis [i] ... 

 and elliptical than a traditional planetary orbit.

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. and the objects listed in the table "Possible dwarf planets" 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. , however, has a much smaller eccentricity and inclination, and so possibly does not qualify as a Pluto-like object.

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




See also

  • 2006 redefinition of planet 2006 redefinition of planet

    The 2006 redefinition of "planet" by the International Astronomical Union [i] states that, in the solar system [i] ... 

  • Classical planet
  • Mesoplanet Mesoplanet

    Mesoplanet is a term coined by Isaac Asimov [i] to refer to planetary bodies [i] with sizes small ... 

  • Small solar system body
  • Trans-Neptunian object Trans-Neptunian object

    A trans-Neptunian object is any object in the solar system [i] that orbit [i]s the sun at a greater dis... 

  • List of solar system objects by planetary discriminant List of solar system objects by planetary discriminant

    This is a list of solar system objects by planetary discriminant [i], in decr ... 

  • Clearing the neighbourhood Clearing the neighbourhood

    "Clearing the neighbourhood" is an informal description of part of the process of planet formation [i]. ... 



References


  • , 16 August 2006
  • , 18 August 2006

External links

  • NPR:
  • BBC News: Wednesday, 16 August 2006, 13:36 GMT 14:36 UK
  • Ottawa Citizen Ottawa Citizen

    The Ottawa Citizen is an English [i]-language daily newspaper owned by CanWest Global [i] ... 

    : Thursday, August 24, 2006