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Dwarf planet



Illustration of the relative sizes, albedos, and colours of the largest TNOs (an artist's depiction of some of these, compared with the Earth, is on the IAU website)
Illustration of the relative sizes, albedos, and colours of the largest TNOs (an artist's depiction of some of these, compared with the Earth, is on the IAU website[40])

Trans-Neptunian objects (or simply TNOs) are thought to have icy cores and therefore would require a diameter of perhaps 400 km (250 mi) – only about 3% of that of Earth – to relax into gravitational equilibrium.[4] Although only rough estimates of the diameters of these objects are available, as of August 2006, it was believed that another 42 bodies in the Kuiper belt (besides Pluto and Eris), plus Sedna in the Scattered disc, were likely dwarf planets.[4][41] A team is investigating another 30 such objects, and believe that the total number will eventually prove to be about 200 in the Kuiper belt, and many more beyond it.[4]

Although the status of many of these objects may be debatable, TNOs with estimated diameters approaching that of Ceres are likely to be dwarf planets regardless of measurement errors. These include cubewanos, plutinos, and scattered disc objects (SDOs):

TNOs that are larger or comparable in diameter to Ceres
Name Category Estimated diameter (km) Mass
(×1020 kg)		 Orbital
radius
(AU)
by [42] by [43] by [44]
2003 EL61 cubewano 2000 1380 1350 1200 ~42 43.31
Sedna extended-SDO 1800 1500 <1800 <1500 17–61 486.0
2005 FY9 cubewano 1600 1500 1500 1250 ~40 45.66
Quaoar cubewano 1290 1260 1260 1200 10–26 43.58
Orcus plutino 1100 909 946 1500 6.2–7.0 39.34
Ixion plutino 980 570 650 1065 ~5.8 39.65
Ceres asteroid
975
 9.5 2.77
2002 AW197 cubewano 940 793 977 890 ~5.2 47.30
Varuna cubewano 780 874 1016 900 ~5.9 42.90
2002 TC302 SDO 710 1200 1150 — 0.78 55.02

Pluto prototype

Main article: Pluto prototype

The 2006 IAU's Resolution 6a[26] recognizes Pluto as "the prototype of a new category of trans-Neptunian objects". The name and precise nature of this category are not specified, but in the debate leading up to the resolution, the members of the category were variously referred to as plutons and plutonian objects. The former name was generally deprecated[45] and was abandoned in the final draft resolution (6b);[46] eventually even the latter name failed to win majority approval on a 183–186 vote in the IAU General Assembly on August 24, 2006.[1] Nevertheless, Pluto was recognized as the prototype of a new category of TNOs and the name of this category was announced by IAU to be established at a later date.[3]

At an earlier stage in the definition process, the category (then named "pluton") defined members as planets whose rotation period around the Sun was more than 200 Julian years, and whose orbit was more highly inclined and more elliptical than a traditional planetary orbit.[47] This category of Pluto-like objects now only applies to dwarf planets that meet the conditions of being trans-Neptunian and "like Pluto" in terms of period, inclination, and eccentricity. A dwarf planet may or may not be a member of this category, but all members of the category must be dwarf planets.[3]

The membership of this class, other than Pluto itself, remains obscure. Eris and the objects listed in the table above qualify in terms of the minimum period, and most exhibit significant orbital eccentricity and inclination (although not always equal to or greater than Pluto's). However, since Quaoar has a much smaller eccentricity and inclination,[48][27] it is unclear whether or not it would qualify as a member of this category.

Contention

In the immediate aftermath of the IAU definition of dwarf planet, a number of scientists expressed their disagreement with the IAU resolution.[49] Campaigns included car bumper stickers and T-shirts.[50] Mike Brown (the discoverer of the would-have-been-the-10th-planet Eris) agrees with the reduction of the number of planets to eight.[51]

NASA has announced that it will use the new guidelines established by the IAU.[52] However, Alan Stern, the director of the NASA's mission to Pluto, 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.[53] Thus, he and his team still refer to Pluto as the ninth planet, while accepting the characterisation of dwarf planet for Ceres and Eris (dwarf planet in this case meaning just a small planet).[54]

See also

References

  1. ^ a b "IAU 2006 General Assembly: Result of the IAU Resolution votes", International Astronomical Union. Retrieved on 2008-01-26. 
  2. ^ Dwarf Planets. NASA. Retrieved on 2008-01-22.
  3. ^ a b c d e f "Definition of a Planet in the Solar System: Resolutions 5 and 6" (PDF), IAU 2006 General Assembly, International Astronomical Union, 2006-08-24. Retrieved on 2008-01-26. 
  4. ^ a b c d e f g h i Brown, Michael E.. The Dwarf Planets. California Institute of Technology, Department of Geological Sciences. Retrieved on 2008-01-26.
  5. ^ a b c d The IAU draft definition of "planet" and "plutons". International Astronomical Union (August 16, 2006).
  6. ^ Working Group on Extrasolar Planets of the International Astronomical Union. International Astronomical Union (2001). Retrieved on 2008-01-26.
  7. ^ Mager, Brad. Pluto Revealed. discoveryofpluto.com. Retrieved on 2008-01-26.
  8. ^ Cuk, Matija; Masters, Karen (2007-09-14). Is Pluto a planet?. Cornell University, Astronomy Department. Retrieved on 2008-01-26.
  9. ^ Buie, Marc W.; William M. Grundy, Eliot F. Young, Leslie A. Young and S. Alan Stern (2006). "Orbits and Photometry of Pluto's Satellites: Charon, S/2005 P1, and S/2005 P2" (abstract page). The Astronomical Journal 132 (132): 290–98. doi:10.1086/504422. 
  10. ^ Jewitt, David; Delsanti, Audrey (2006). The Solar System Beyond The Planets in Solar System Update : Topical and Timely Reviews in Solar System Sciences (PDF). Springer. DOI:10.1007/3-540-37683-6. ISBN 978-3-540-37683-5. Retrieved on 2008-02-10. 
  11. ^ Weintraub, David A. (2006). Is Pluto a Planet? A Historical Journey through the Solar System, 1–272. ISBN 978-0-691-12348-6. 
  12. ^ Phillips, Tony; Phillips, Amelia (2006-09-04). Much Ado about Pluto. PlutoPetition.com. Retrieved on 2008-01-26.
  13. ^ Planetoids Beyond Pluto. Astrobiology Magazine (2004-12-30). Retrieved on 2008-01-26.
  14. ^ "Hubble Observes Planetoid Sedna, Mystery Deepens", NASA's Hubble Space Telescope home site, 2004-04-14. Retrieved on 2008-01-26. 
  15. ^ Brown, Michael E.. The Discovery of Eris, the Largest Known Dwarf Planet. California Institute of Technology, Department of Geological Sciences. Retrieved on 2008-01-26.
  16. ^ Brown, Michael E. (2004). What is the definition of a planet?. California Institute of Technology, Department of Geological Sciences. Retrieved on 2008-01-26.
  17. ^ Brown, Mike. "War of the Worlds", New York Times, 2006-08-16. Retrieved on 2008-02-20. 
  18. ^ "Astronomers Measure Mass of Largest Dwarf Planet", NASA's Hubble Space Telescope home site, 2007-06-14. Retrieved on 2008-01-26. 
  19. ^ Brown, Michael E.. What makes a planet?. California Institute of Technology, Department of Geological Sciences. Retrieved on 2008-01-26.
  20. ^ a b Britt, Robert Roy (2006-08-19). Details Emerge on Plan to Demote Pluto. Space.com. Retrieved on 2006-08-18.
  21. ^ Bailey, Mark E.. Comments & discussions on Resolution 5: The definition of a planet - Planets Galore. Dissertatio cum Nuncio Sidereo, Series Tertia - official newspaper of the IAU General Assembly 2006. Astronomical Institute Prague. Retrieved on 2008-02-09.
  22. ^ Dos uruguayos, Julio Fernández y Gonzalo Tancredi en la historia de la astronomía:reducen la cantidad de planetas de 9 a 8 ...&Anotaciones de Tancredi (Spanish). Science and Research Institute, Mercedes, Uruguay. Retrieved on 2008-02-11.
  23. ^ a b c Soter, Steven (2006-08-16). "What is a Planet?" (PDF). The Astronomical Journal 132: 2513–19. doi:10.1086/508861. 
  24. ^ a b c Stern, S. Alan; Levison, Harold F. (2002). "Regarding the criteria for planethood and proposed planetary classification schemes" (PDF). Highlights of Astronomy 12: 205–13, as presented at the XXIVth General Assembly of the IAU–2000 Manchester, UK, 7–18 August 2000. 
  25. ^ In theory it is possible to have bodies as massive as Jupiter be classified as dwarf planets if they "did not clear their path", but in practice it is unlikely that bodies at least as large as Mercury will be discovered in the Solar system
  26. ^ a b IAU 2006 General Assembly: Result of the IAU Resolution votes. International Astronomical Union (2006-08-26). Retrieved on 2008-01-26.
  27. ^ a b Bowell, Ted. The Asteroid Orbital Elements Database. Lowell Observatory. Retrieved on 2008-02-12.
  28. ^ Thomas, P.C; Parker J.Wm.; McFadden, L.A.; et.al. (2005). "Differentiation of the asteroid Ceres as revealed by its shape". Nature 437: 224–26. doi:10.1038/nature03938. 
  29. ^ Calculated based on the known parameters. APmag and AngSize generated with Horizons (Ephemeris: Observer Table: Quantities = 9,13,20,29)
  30. ^ Williams, D.R. (2006-09-07). Pluto Fact Sheet. NASA. Retrieved on 2007-03-24.
  31. ^ Buie, M. W.; Grundy, W.M.; Young, E.F.; Young, L.A.; Stern, S.A. (2006). "Orbits and photometry of Pluto's satellites: Charon, S/2005 P1, and S/2005 P2". Astronomical Journal 132: 290. doi:10.1086/504422. arXiv:astro-ph/0512491. 
  32. ^ Stansberry, John; Grundy, Will; Brown, Mike; Spencer, John; Trilling, David; Cruikshank, Dale; Margot, Jean-Luc (2007). Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope (PDF). University of Arizona, Lowell Observatory, California Institute of Technology, NASA Ames Research Center, Southwest Research Institute, Cornell University. Retrieved on 2007-05-18.
  33. ^ Brown, Michael E.; Schaller, Emily L. (2007). "The Mass of Dwarf Planet Eris". Science 316 (5831): 1585. doi:10.1126/science.1139415. 
  34. ^ a b Russel, C.T.; Capaccioni, F.; Coradini, A.; et.al. (2006). "Dawn Discovery mission to Vesta and Ceres: Present status". Advances in Space Research 38: 2043–48. doi:10.1016/j.asr.2004.12.041. 
  35. ^ Britt, Robert Roy (2003). Pluto Mission a Go! Initial Funding Secured. Space.com. Retrieved on 2007-04-13.
  36. ^ Rincon, Paul. "Planets plan boosts tally to 12", BBC News, 2006-08-16. Retrieved on 2007-03-17. 
  37. ^ Battersby, Stephen (2006-08-16). Three new planets may join solar system. New Scientist.com. Retrieved on 2008-01-26.
  38. ^ Thomas, Peter C.; Binzelb, Richard P.; Gaffeyc, Michael J.; Zellnerd, Benjamin H.; Storrse, Alex D.; Wells, Eddie (1997). "Vesta: Spin Pole, Size, and Shape from HST Images". Icarus 128 (1): 88–94. doi:10.1006/icar.1997.5736. 
  39. ^ the footnote in the original text reads: For two or more objects comprising a multiple object system.... A secondary object satisfying these conditions i.e. that of mass, shape is also designated a planet if the system barycentre resides outside the primary. Secondary objects not satisfying these criteria are "satellites". Under this definition, Pluto's companion Charon is a planet, making Pluto-Charon a double planet.
  40. ^ The IAU draft definition of "planet" and "plutons". International Astronomical Union (2006-08-16). Retrieved on 2008-01-26. “Planet candidates in the Solar System artist's impression”
  41. ^ Britt, Robert Roy (2006-08-16). Nine Planets Become 12 with Controversial New Definition. Space.com. Retrieved on 2008-01-26.
  42. ^ Johnston, Robert (2007-11-24). List of Known Trans-Neptunian Objects. Johnston's Archive.net. Retrieved on 2008-01-26.
  43. ^ Barucci, M.A.; Stansberry, John; Grundy, Will; Brown, Mike; Cruikshank, Dale; Spencer, John; Trilling, David; Margot, Jean-Luc (2007). "Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope". The Solar System beyond Neptune. University of Arizona Press. 
  44. ^ David Jewitt. Kluipert Belt: The 1000 km Scale KBOs. University of Hawaii, Institute for Astronomy. Retrieved on 2008-02-10.
  45. ^ "Astronomers divided over "planet" definition", Deutsche Presse Agentur, Rawstory.com, 2006-08-22. Retrieved on 2008-01-26. 
  46. ^ The Final IAU Resolution on the definition of "planet" ready for voting. International Astronomical Union (2006-08-24). Retrieved on 2008-01-26.
  47. ^ Draft definition, IAU press release. International Astronomical Union (2006-08-16). Retrieved on 2008-01-26.
  48. ^ Compared with Mercury, Quaoar has a similar inclination and a much smaller eccentricity; the eccentricity is in fact only slightly larger than that of Earth
  49. ^ Rincon, Paul (2006-08-25). Pluto vote 'hijacked' in revolt. BBC News. Retrieved on 2008-01-26.
  50. ^ Chang, Alicia. "Online merchants see green in Pluto news", Associated Press, USA Today, 2006-08-25. Retrieved on 2008-01-25. 
  51. ^ Brown, Michael E.. The Eight Planets. California Institute of Technology, Department of Geological Sciences. Retrieved on 2008-01-26.
  52. ^ Hotly-Debated Solar System Object Gets a Name. NASA press release (2006-09-14). Retrieved on 2008-01-26.
  53. ^ Stern, Alan (2006-09-06). Unabashedly Onward to the Ninth Planet. New Horizons Web Site. Retrieved on 2008-01-26.
  54. ^ Stern, Alan (2008-01-17). Happy Birthday New Horizons! Two Years on the Road to the Ninth Planet. New Horizons Web Site. Retrieved on 2008-01-26.

External links

Look up Dwarf planet in
Wiktionary, the free dictionary.
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The Solar System
The Sun Mercury Venus The Moon Earth Phobos and Deimos Mars Ceres The asteroid belt Jupiter Moons of Jupiter Saturn Moons of Saturn Uranus Moons of Uranus Moons of Neptune Neptune Moons of Pluto Pluto The Kuiper Belt Dysnomia Eris The Scattered Disc The Oort Cloud
See also astronomical objects, the Solar System's list of objects, sorted by radius or mass, and the Solar System Portal



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