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List of dwarf planets The IAU has officially identified three celestial bodies that have immediately received "dwarf planet" classification:•(in sidereal days) | 0.3781 || -6.38718 (retrograde) || |- align = "left" ! 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 |- align = "left" ! Orbital period (in sidereal Years) | 4.599 || 248.09 || 557 |- align = "left" ! Mean orbital speed (in km/s) | 17.882 || 4.7490 || 3.436 |- align = "left" ! Orbital Eccentricity | 0.080 || 0.24880766 || 0.44177 |- align = "left" ! Orbital inclination |10.587° || 17.14175° || 44.187° |- align = "left" ! Inclination of the equator from the orbit (see Axial tilt) | 4° || 119.61° || |- align = "left" ! Mean surface temperature (in K) | 167 || 40 || 30 |- align = "left" ! Number of natural satellites | 0 || 3 || 1 |- align = "left" ! Date of Discovery | January 1, 1801 || February 18, 1930 || January 5, 2005 |} 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 (double planet) system. The original draft resolution (5)• presented to the IAU stated that Charon could be considered a planet because: 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.• 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 Λ, 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 Λ between the smallest terrestrial planets and the largest asteroids and KBOs: 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 (dwarf-planet in this case meaning just a "small planet"), 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 (rather than intrinsic 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.• Types of dwarf planets The IAU's Resolution 6a• See also | ||||||||||
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