Encyclopedia
A
dwarf planet is a category of celestial bodies in the
solar system as introduced in a resolution by the
International Astronomical Union on August 24 2006. The definition currently applies only to 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" 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
|-
...
It differs from the definition of "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 .
List of dwarf planets
The IAU has officially identified three celestial bodies that have immediately received "dwarf planet" classification:
Dwarf planets| Name | | Pluto | |
| MPC number | 1 | 134340 | 136199 |
|---|
| Region of Solar system | Asteroid belt | Kuiper belt | Scattered disc |
|---|
| Diameter | 975×909 km | 2306±20 km | 2400±100 km |
|---|
Mass in kg
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 | 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 | 10.587° | 17.14175° | 44.187° |
|---|
Inclination of the equator from the orbit
| 4° | 119.61° | |
|---|
| Mean surface temperature | 167 | 40 | 30 |
|---|
| Number of natural satellites | 0 | 3 | 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:
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:
- Charon independently would satisfy the size and shape criteria for planetary status
- 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 second, third, and fourth largest asteroids 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.
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 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 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 | 0.815 | 1.08 | 1.35 |
| Earth | 1.00 | 1.00 | 1.7 |
| Mars | 0.107 | 0.0061 | 1.8 |
|
| 0.00015 | 8.7 | 0.33 |
| Jupiter | 317.7 | 8510 | 6.25 |
| Saturn | 95.2 | 308 | 1.9 |
| Uranus | 14.5 | 2.51 | 2.9 |
| Neptune | 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, 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 , and
comet .
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 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 objects despite also being categorized as dwarf planets.
Types of dwarf planets
The
IAU'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 whose orbit took more than 200 Julian years to complete and whose orbit was more
highly inclined 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
References
- , 16 August 2006
- , 18 August 2006
External links
- NPR:
- BBC News: Wednesday, 16 August 2006, 13:36 GMT 14:36 UK
- Ottawa Citizen: Thursday, August 24, 2006
