The search for planets beyond Neptune began following the discovery of the planet Neptune in 1846, amid considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century but culminated at the start of the 20th with Percival Lowell's quest for Planet X. Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the gas giants, particularly Uranus and Neptune, speculating that the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the irregularities. Clyde Tombaugh's discovery of Pluto in 1930 appeared to validate Lowell's hypothesis, and Pluto was officially considered the ninth planet until 2006. In 1978, Pluto was found to be too small for its gravity to affect the gas giants, resulting in a brief search for a tenth planet. The search was largely abandoned in the early 1990s, when a study of measurements made by the Voyager 2 spacecraft found that the irregularities observed in Uranus's orbit were due to a slight overestimation of Neptune's mass.

Read the rest of this article: http://en.wikipedia.org/wiki/Planets_beyond_Neptune

_______________________________ Today's selected anniversaries:

1006:

SN 1006 , the brightest supernova in recorded history, first appeared in the constellation Lupus, and was then seen by observers in Switzerland, Egypt, China, Japan, and possibly North America within the next day. http://en.wikipedia.org/wiki/SN_1006

1557:

Arauco War: Spanish forces of the Governor Francisco de Villagra launched a dawn surprise attack against the Mapuche headed by their toqui Lautaro in what is now Chile. http://en.wikipedia.org/wiki/Battle_of_Mataquito

1894:

A crowd of workers unemployed due to the Panic of 1893 conducted the first significant popular protest march on Washington, D.C. http://en.wikipedia.org/wiki/Coxey%27s_Army

1943:

Second World War: The Royal Navy submarine HMS Seraph deposited the body of a supposed British military intelligence officer with false documents off the coast of Spain to deceive Germany about the upcoming invasion of Sicily. http://en.wikipedia.org/wiki/Operation_Mincemeat

1975:

North Vietnamese troops captured Saigon shortly after American forces ended its helicopter evacuation of U.S. citizens, South Vietnamese civilians and third country nationals from the city, ending the Vietnam War with the unconditional surrender of South Vietnam. http://en.wikipedia.org/wiki/Fall_of_Saigon

_____________________________ Wiktionary's word of the day:

consonant (adj): Characterized by harmony or agreement http://en.wiktionary.org/wiki/consonant

___________________________ Wikiquote quote of the day:

11px

--Taisen Deshimaru http://en.wikiquote.org/wiki/Taisen_Deshimaru