Editing Definition of planet (section)

=== Minor planets ===
[[File:AnimatedOrbitOf1Ceres.gif|thumb|Orbit of Ceres, which fits with the description of a seemingly "missing" planet between Mars and Jupiter, as predicted by [[Bode's law]].]]
One of the unexpected results of [[William Herschel]]'s discovery of Uranus was that it appeared to validate [[Bode's law]], a mathematical function which generates the size of the [[semimajor axis]] of planetary [[orbit]]s. Astronomers had considered the "law" a meaningless coincidence, but Uranus fell at very nearly the exact distance it predicted. Since Bode's law also predicted a body between Mars and Jupiter that at that point had not been observed, astronomers turned their attention to that region in the hope that it might be vindicated again. Finally, in 1801, astronomer [[Giuseppe Piazzi]] found a miniature new world, [[Ceres (dwarf planet)|Ceres]], lying at just the correct point in space. The object was hailed as a new planet.<ref name=Hilton>{{cite web | author=Hilton, James L. | title=When did asteroids become minor planets? | work=U.S. Naval Observatory | url=http://sd-www.jhuapl.edu/weaver_projects/GPD/Contributed_Talks/hilton_gpd_poster.pdf | access-date=May 25, 2006}}</ref>

Then in 1802, [[Heinrich Wilhelm Matthäus Olbers|Heinrich Olbers]] discovered [[2 Pallas|Pallas]], a second "planet" at roughly the same distance from the Sun as Ceres. The fact that two planets could occupy the same orbit was an affront to centuries of thinking.<ref>{{cite book|title=King Henry the Fourth Part One in The Globe Illustrated Shakespeare: The Complete Works Annotated|author=William Shakespeare|publisher=Granercy Books|year= 1979|page= 559}}</ref> In 1804, another world, [[3 Juno|Juno]], was discovered in a similar orbit.<ref name=Hilton/> In 1807, Olbers discovered a fourth object, [[4 Vesta|Vesta]], at a similar orbital distance.

Herschel suggested that these four worlds be given their own separate classification, [[asteroid]]s (meaning "starlike" since they were too small for their disks to resolve and thus resembled [[star]]s), though most astronomers preferred to refer to them as planets.<ref name=Hilton/> This conception was entrenched by the fact that, due to the difficulty of distinguishing asteroids from yet-uncharted stars, those four remained the only asteroids known until 1845.<ref name=18planets>{{cite web
  |title=The Planet Hygea
  |year=1849
  |work=spaceweather.com
  |url=http://spaceweather.com/swpod2006/13sep06/Pollock1.jpg
  |access-date=June 24, 2008 }}</ref><ref name=police>{{cite journal|title=Call the Police! The story behind the discovery of the asteroids|journal=Astronomy Now|first=Keith|last= Cooper|pages=60–61|volume=21|issue=6|date=June 2007}}</ref> Science textbooks in 1828, after Herschel's death, still numbered the asteroids among the planets.<ref name=Hilton/> With the arrival of more refined star charts, the search for asteroids resumed, and a fifth and sixth were discovered by [[Karl Ludwig Hencke]] in 1845 and 1847.<ref name=police /> By 1851 the number of asteroids had increased to 15, and a new method of classifying them, by affixing a number before their names in order of discovery, was adopted, inadvertently placing them in their own distinct category. Ceres became "(1) Ceres", Pallas became "(2) Pallas", and so on. By the 1860s, the number of known asteroids had increased to over a hundred, and observatories in Europe and the United States began referring to them collectively as "[[minor planet]]s", or "small planets", though it took the first four asteroids longer to be grouped as such.<ref name=Hilton/> To this day, "minor planet" remains the official designation for all small bodies in orbit around the Sun, and each new discovery is numbered accordingly in the IAU's [[Minor Planet Catalogue]].<ref>{{cite web|title=The MPC Orbit (MPCORB) Database|url=http://www.minorplanetcenter.org/iau/MPCORB.html|access-date=October 15, 2007}}</ref>