From 1905 until his death in 1916, Percival Lowell searched for a ninth planet, which he called Planet X. Lowell had predicted the planet based on irregularities in the orbit of Uranus. In 1929, ...

February 18 marks the discovery of Pluto by astronomer Clyde W. Tombaugh. The discovery reshaped the understanding of the solar system, as Pluto was long considered the ninth planet until the ...

Pluto was downgraded from a planet to a dwarf planet in 2006 by the International Astronomical Union. The main reason for Pluto's demotion is that it's not able to "clear its orbit of debris." Other ...