Friday 24 November 2023

Dwarf Planets

 

A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets. The prototypical dwarf planet is Pluto. The interest of dwarf planets to planetary geologists is that they may be geologically active bodies, an expectation that was borne out in 2015 by the Dawn mission to Ceres and the New Horizons mission to Pluto.


Astronomers are in general agreement that at least the nine largest candidates are dwarf planets – in rough order of size, Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna, Ceres, and Orcus – although there is some doubt for Orcus. Of these nine plus the tenth-largest candidate Salacia, two have been visited by spacecraft (Pluto and Ceres) and seven others have at least one known moon (Eris, Haumea, Makemake, Gonggong, Quaoar, Orcus, and Salacia), which allows their masses and thus an estimate of their densities to be determined. Mass and density in turn can be fit into geophysical models in an attempt to determine the nature of these worlds. Only one, Sedna, has neither been visited nor has any known moons, making an accurate estimate of mass difficult. 


Eris (then known as 2003 UB313) was discovered in January 2005; it was thought to be slightly larger than Pluto, and some reports informally referred to it as the tenth planet


  1. Ceres ⚳ – discovered January 1, 1801 and announced January 24, 45 years before Neptune. Considered a planet for half a century before reclassification as an asteroid. Considered a dwarf planet by the IAU since the adoption of Resolution 5A on August 24, 2006.
  1. Pluto ♇ ⯓ – discovered February 18, 1930 and announced March 13. Considered a planet for 76 years. Explicitly reclassified as a dwarf planet by the IAU with Resolution 6A on August 24, 2006. Five known moons.

  3. Eris ⯰ (2003 UB313) – discovered January 5, 2005 and announced July 29. Called the "tenth planet" in media reports. Considered a dwarf planet by the IAU since the adoption of Resolution 5A on August 24, 2006, and named by the IAU dwarf-planet naming committee on September 13 of that year. One known moon.

  1. Haumea 🝻 (2003 EL61) – discovered by Brown et al. December 28, 2004 and announced by Ortiz et al. on July 27, 2005. Named by the IAU dwarf-planet naming committee on September 17, 2008. Two known moons.
  2. Makemake 🝼 (2005 FY9) – discovered March 31, 2005 and announced July 29. Named by the IAU dwarf-planet naming committee on July 11, 2008. One known moon.

  1. Quaoar 🝾 (2002 LM60) – discovered June 5, 2002 and announced October 7 of that year. One known moon.
  2. Sedna ⯲ (2003 VB12) – discovered November 14, 2003 and announced March 15, 2004.
  3. Orcus 🝿 (2004 DW) – discovered February 17, 2004 and announced two days later. One known moon.
  4. Gonggong 🝽 (2007 OR10) – discovered July 17, 2007 and announced January 2009. Recognized as a dwarf planet by JPL and NASA in May 2016. One known moon.

  1. Salacia (2004 SB60)  – discovered September 22, 2004. One known moon.


There has been some debate as to whether the Pluto–Charon system should be considered a double dwarf planet. In a draft resolution for the IAU definition of planet, both Pluto and Charon were considered planets in a binary system. The IAU currently says Charon is not considered a dwarf planet but rather a satellite of Pluto, though the idea that Charon might qualify as a dwarf planet may be considered at a later date







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