Dwarf planet

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Dwarf planets are a class of celestial bodies in the solar system defined by the International Astronomical Union (IAU) on August 24, 2006 in Prague . Five celestial bodies have officially been considered dwarf planets since 2008, but there are several hundred other dwarf planet candidates in the solar system that could still be classified as dwarf planets.

Artist's impression of some large Trans-Neptunian objectsTransneptunisches Objekt (136199) Eris (136199) Eris Dysnomia (Mond) Dysnomia (Mond) Pluto Pluto Charon (Mond) Charon (Mond) Styx (Mond) Nix (Mond) Kerberos (Mond) Hydra (Mond) (136472) Makemake Namaka (Mond) Hiʻiaka (Mond) (136108) Haumea (90377) Sedna (225088) Gonggong (50000) Quaoar (50000) Quaoar Weywot (Mond) (90482) Orcus (90482) Orcus Vanth (Mond) Erde
Comparison of some large trans-Neptunian objects with the earth (mostly fantasy drawings. Caption as of June 2015) . To get to the corresponding article, click on the object ( large display ).



The dwarf planet category is one of the three following categories that the IAU has defined for objects in the solar system, excluding satellites (moons):

  1. A planet is an astronomical object that
  2. A dwarf planet has the same properties as a planet, except that it has not cleared its orbit .
  3. Small bodies are all other objects (most asteroids ,trans-Neptunian objects (TNOs), comets, and other small bodies in the solar system), with the exception of satellites, which are in orbit around the sun.

Plutoids are a subclass of the dwarf planets that orbitthe sunin orbits outside of Neptune's orbit and are therefore part of the trans- Neptunian objects (TNO).

Criticism of the definition

The demarcation between planets and dwarf planets is controversial because critics keep coming up who interpret the criterion of the cleaned environment differently. For example, the earth , classified as a planet, still has about ten thousand objects in its orbit. There are still many objects ( Trojans ) in Jupiter's orbit . Earth and Jupiter have cleared their orbits insofar as the mass ratio of the main body to the remaining bodies in the same orbit is so great that the mass fraction of the remaining bodies in the orbit of these planets is negligibly small; in the case of the earth, for example, it is only 1: 1,700,000 and in the case of Jupiter 1: 625,000.

The definition so far only refers to the solar system and not generally to planetary systems . That is also a point of criticism.

Concept history

The original name Plutons for a new subclass of dwarf planets, which revolve around the Sun beyond Neptune, met with opposition , especially in the community of geoscientists who already use the term elsewhere (see Pluton (geology) ), and was rejected. But even the Latinized form Plutoiden , a suggestion by members of the IAU committee for naming small celestial bodies, could not prevail for the time being, but the initially nameless subclass was created anyway. In June 2008 the IAU Executive Committee finally named this subclass Plutoids at its meeting in Oslo .

Classified dwarf planets

Currently (2016) five celestial bodies are classified as dwarf planets by the IAU. The dates of the objects are given in the list of dwarf planets of the solar system .

Dwarf planet in the asteroid belt

Ceres , image:
Dawn space probe
With a spherical and planet-like structure and an equatorial diameter of 963 km, Ceres is the only dwarf planet in the asteroid belt because it has sufficient mass for hydrostatic equilibrium. In contrast, the numerous other asteroids are just irregularly shaped boulders. Ceres was first classified as a planet after its discovery by Giuseppe Piazzi in 1801. Until the discovery of Neptune in 1846, Pallas , Juno , Vesta and Astraea were discovered, which at that time were considered full planets. So there were a total of 13 planets in 1846. Because new objects were continuously discovered between Mars and Jupiter from 1847 onwards, astronomers introduced the new category of asteroids (planetoids) in 1851. Ceres, Pallas, Juno, Vesta, and Astraea have been listed as asteroids since then, and the number of large planets returned to eight.

Transneptunian dwarf planets (Plutoids)

Pluto, image:
New Horizons space probe
It was in 1930 by Clyde Tombaugh at Lowell Observatory in Flagstaff ( Arizona discovered) and was 76 years long as the ninth planet of the solar system. It has an equatorial diameter of 2,374 km and has five moons, the largest of which, Charon , is half the diameter of Pluto.
On August 24, 2006, however, the IAU revoked the status of a full-fledged planet because it is not the dominant object in its orbit like the other large planets. Its orbit around the sun is also strongly inclined and shows great eccentricity . In addition, with Eris an object was discovered that initially appeared to be larger than Pluto, which is why both were classified in the new category of dwarf planets.
Eris and her moon Dysnomia , image: Hubble space telescope
The object Eris, announced on July 29, 2005, is minimally smaller and slightly heavier than Pluto with an equatorial diameter of 2326 km. Because both objects have similar properties, an assignment to different categories would not have made sense, which is why they were both assigned to the new category of dwarf planets.
On July 14, 2008, the Kuiper belt object 2005 FY 9 with an equatorial diameter of 1502 km was assigned the name Makemake and the status of a dwarf planet.
Makemake and his moon S / 2015 (136472) 1 (aka 'MK 2'), image: Hubble space telescope
On September 17, 2008, the Kuiper property 2003 EL 61 was given the name Haumea and the status of a dwarf planet. Because of its fast rotation, it has a strongly ellipsoidal shape with an equatorial diameter of about 2200 km and a distance between the poles of only about 1100 km.


Several hundred other objects in the trans-Neptunian area (such as Gonggong , Quaoar , Sedna , 2002 MS 4 , Orcus , Salacia or Varuna ) could also fall into the category of dwarf planets. The IAU is working on the classification of other dwarf planets. According to new findings from the European Southern Observatory in Chile , the asteroid (10) Hygiea in the main belt with a diameter of around 430 kilometers can also be classified as a dwarf planet. The dwarf planet candidates are kept on a watch list. The observations available for these objects are currently insufficient to ensure that they are in hydrostatic equilibrium .

See also


  • Silvia Protopapa: Surface characterization of Pluto, Charon and (47171) 1999 TC36 . Dissertation, Technical University of Braunschweig 2009, 143 pages ( English ). Copernicus Publishing, Katlenburg-Lindau 2009, ISBN 978-3-936586-96-1 .

Web links

Commons : Dwarf Planets  - Collection of Images

Individual evidence

  1. ^ Resolutions B5 and B6: "Definition of a Planet in the Solar System" AND "Pluto" . International Astronomical Union (IAU). August 24, 2006. Accessed March 22, 2020.
  2. ^ IAU 2006 General Assembly: Result of the IAU Resolution votes . August 24, 2006, Prague
  3. ^ IAU 2008: Plutoid chosen as name for Solar System objects like Pluto . June 11, 2008, Paris
  4. ^ IAU names fifth dwarf planet Haumea. September 17, 2008, accessed September 4, 2012 .
  5. ^ IAU 2008: Fourth dwarf planet named Makemake . July 17, 2008, Paris
  6. ^ IAU 2008: IAU names fifth dwarf planet Haumea . September 17, 2008, Paris
  7. How many dwarf planets are there in the outer solar system? (updates daily). Retrieved September 4, 2012 .