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with plutoids)|
(IAU = International Astronomical Union) According to this new definition we now have eight planets, and a slowly growing number of dwarf planets. The new definition for planets is the following:
The first condition is like what people since long think what is a planet. The second condition means, that a planet needs a certain minimum mass to possess sufficient gravity to get "round" by itself. That does not equal to a sphere - by the definition of the "hydrostatic equilibrium" the body may have adopted the shape of an ellipsoid as consequence of the gravitational and rotational forces effecting it. The third condition means, that a celestial body can be called a planet only in case if it has collected practically all material in the "neighbourhood around its orbit" that could eventually collide with it. Smaller bodies that are gravitationally dominated by the planet, like moons and other satellites like trojans or horseshoe-satellites, do not count. With "neighbourhood around its orbit" all celestial bodies in similar distance from the sun are summarized. Because Pluto sometimes is found in the same distance from the sun as the much bigger Neptune, Pluto is no further a planet, and we have again 8 planets as in the 19th Century.
A celestial body that fulfils the first two conditions, but not the third, and also is no moon, will henceforth be called a dwarf planet, according to the definition of the IAU. The first three official dwarf planets are already decided, one is Ceres, with about 950 km diameter the largest member of the asteroid belt, another one is Pluto that does not have any more planetary status, and the third is Eris, the largest of the trans-neptunian objects known so far.
According to the IAU most asteroids, TNOs and comets will be dubbed collectively SSSB, however it is still unclear, which are the ones to be excluded by the wording "except satellites orbiting the Sun". If you take this wording seriously, then all of them should be excluded ?? I suppose that the correct wording should have been "all other objects orbiting the Sun except satellites" ... ;-) | |||
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The new definition for plutoids is the following:
So every transneptunian dwarf planet is a plutoid at the same time, this is valid also for Pluto and Eris. Additionally it was decided that every newly discovered object brighter than H=1.0 (absolute magnitude) will be classified immediately as plutoid and consequently also as dwarf planet and will be given an official name by the IAU WGPSN and the IAU CSBN groups. If further investigations should reveal that this object does not qualify as a dwarf planet, it will keep its name, but will be reclassified. | |||
| All current planets and
dwarf planets: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, 2003 EL61, Makemake, Eris |
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