Eleventh Planet - Summary

Summary

The eleventh planet in the early 19th century was Uranus by order of distance from the Sun, and Vesta in order of discovery. When Ceres (1801), Pallas (1802), Juno (1804) and Vesta (1807) were discovered in the 19th century, they were known as planets, and Neptune was not discovered until 1846, there were a total of eleven planets. At the time, even Uranus had been recognized as a planet for twenty years. However, the number of planets rapidly grew from 1850 onward, and Ceres, Pallas, Juno, and Vesta were re-classified as asteroids one, two, three, and four; the number of recognized planets dropped to eight until the discovery of Pluto in 1930.

For example, in addition to the discovery of Neptune at the end of 1846, between 1845 and the end of the decade 6 more "planets" were discovered. This included Astraea (1845), Hebe (1847), Iris (1847), Flora (1847), Metis (1848), and Hygeia (1849) (later spelled 'Hygiea'). From the 1850s onwards these were slowly classified along with the earlier "planets" as asteroids.

From 1930 to 2006, the ninth planet was Pluto, with both Eris and Haumea later laying claim to being a tenth planet. Today, if counting the "dwarf planets" of Ceres and Pluto as planets, the eleventh planet from the sun would be Eris. However in 2006 the term "planet" was redefined to exclude the new category of dwarf planets (just as some planets had earlier been recategorised as asteroids). In 2006 Pluto, Eris, Haumea, Makemake, and (in the inner Solar System) the asteroid Ceres were reclassified as dwarf planets.