Pluto is the only planet (reclassified as a dwarf planet in 2006) discovered by a U.S. citizen, astronomer Clyde Tombaugh, in 1930. Some of his family members were on hand to witness the launch on Jan. 19, 2006. Some astronomers had disputed Pluto's right to be called a planet. It is an oddball icy dwarf, unlike the rocky planets of Mercury, Venus, Earth and Mars and the gaseous planets of Jupiter, Saturn, Uranus and Neptune. The International Astronomical Union stripped Pluto of its planetary status on Aug. 24, 2006. Pluto is the brightest body in a zone of the solar system known as the Kuiper Belt, made up of thousands of icy, rocky objects, including tiny celestial objects whose development was stunted by unknown causes. Scientists believe studying those "planetary embryos" can help them understand how planets were formed.