Dwarf planet candidate

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Hubble image of (225088) Gonggong with the moon Xiangliu . Gonggong is the largest known dwarf planet candidate and despite its diameter of approx. 1500 km, it is not officially classified as a dwarf planet.

A dwarf planet candidate is an object of the solar system that, according to the definition of the International Astronomical Union ( IAU ) in Prague on August 24, 2006, may be a dwarf planet , although it is not certain whether the object meets all the criteria or not.

Dwarf planet definition according to IAU 2006

According to the IAU, an object is a dwarf planet if it meets the following criteria:

  • It is in orbit around the sun
  • It has enough mass that it is in hydrostatic equilibrium and takes on an approximately round shape
  • It has not cleared the vicinity of the orbit of other objects, it is not an orbit clearer.

Planets clear their orbits from other objects. Natural satellites ( moons ) do not orbit the sun directly, but another reference body. Asteroids aren't round enough.

Problems with the application of the dwarf planet definition

Some criteria can be at least approximately clarified by observation from Earth, other criteria are very difficult to verify.

  • The path of a body can be calculated, so it can be clarified whether it is going around the sun according to criterion one and whether it has cleared its path according to criterion three or not.
  • In many cases, the mass can be approximately determined by observing the orbital disruptions; the diameter of objects can at least approximately be determined by radar measurements.
  • The assessment of roundness according to the external shape causes a problem in practice that is difficult to solve remotely. The clarification of the question becomes more difficult according to the distance. The roundness cannot be derived directly from the mass and diameter. Although there is a connection between a lot of mass, large diameter and roundness, there are also objects that are relatively massive, but not round like asteroid Vesta or the moon Proteus , which is currently considered the largest non-round body. On the other hand, there are objects that have a comparatively small diameter and little mass and are nevertheless round or almost round like the moons Mimas and the borderline case Miranda . Whether a body is in hydrostatic equilibrium (or at the time of formation) depends on the material the body is made of and on the temperature inside the object when it was formed. Objects that consist mainly of iron or rock are more difficult to achieve hydrostatic equilibrium than objects that contain liquids or plastic materials such as water ice, methane ice, carbon monoxide ice, nitrogen ice or other low-melting materials. Checking the roundness requires observation from a relatively short distance; it is made even more difficult by a very dark or high-contrast surface of the object.

Dwarf planet candidates

The dependence on the answer to the question of roundness or hydrostatic equilibrium leads in practice to the formation of a further group of objects for which roundness can only be assumed with a certain probability or at least not excluded, but also with the latest available means could not yet be proven. This group now forms the group of dwarf planet candidates, called “possible dwarf planets” in English. As a result of the discoveries made in recent years, especially in the Kuiper Belt and beyond, there is a growing number of Kuiper Belt objects that cannot be clearly classified with the previous means. Many of these objects have not yet been officially named. Depending on the definition, there are dozens or hundreds of dwarf planet candidates, but in any case they exceed the number of the five dwarf planets officially confirmed by the IAU by 2019, Ceres , Pluto , Eris , Makemake and Haumea many times over.

The criteria say nothing about objects that, according to today's knowledge, were in a hydrostatic equilibrium when they were created, later cooled down and since then have lost their spherical shape again due to impacts from other objects. They also do not define exactly how much the shape of an object can deviate from a form of hydrostatic equilibrium in order to still be round enough - in the sense of the definition.

Dwarf planet candidates, according to Mike Brown

The astronomer Mike Brown was involved in the discovery of numerous dwarf planets and dwarf planet candidates. Under the heading Free the dwarf planets, he pushes the candidate debate further than the IAU. He differentiates the dwarf planet candidates based on their diameter into five different groups with different degrees of probability. The method of counting is cumulative, i.e. the 26 objects with a diameter of over 600 km include the 10 objects with a diameter of over 800 km, etc. On October 31, 2017, this included in the outer solar system (i.e. without Ceres and the other objects in the asteroid belt ) a total of 696 objects. On September 13, 2019 there were a total of 741 objects:

  • 68 objects with a diameter over 500 km, which are likely to be dwarf planets .
  • 130 objects with a diameter of more than 400 km, which are probably dwarf planets with a certain probability .
  • 741 objects with a diameter of over 200 km, which may be dwarf planets .

See also

Web links

Individual evidence

  1. ↑ International Astronomical Union | IAU. Retrieved on April 4, 2017 : "(2) A" dwarf planet "is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape [2], (c) has not cleared the neighborhood around its orbit. "
  2. ↑ Michael E. Brown: Free the dwarf planets! Retrieved April 4, 2017 .
  3. ↑ Michael E. Brown: How many dwarf planets are there in the outer solar system? Retrieved October 31, 2017 .