Pluto, formerly known as the ninth planet of our solar system, has been reclassified to a new category called "dwarf planet." The vote by the International Astronomical Union officially upgrades Earth's neighborhood from the traditional nine planets to eleven -- eight classical planets, and three dwarf planets. The eight classical planets of our solar system are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Pluto, Ceres, and 2003 UB313 (temporary name) are the first members of the new "dwarf planet" category.
Clyde Tombaugh at Flagstaff's Lowell Observatory discovered Pluto in 1930. Pluto's designation as a dwarf planet was caused by the new rule, which says a planet must orbit the sun and be large enough to assume a nearly round shape, and must "clear the neighborhood around its orbit." Pluto's oblong orbits overlap Neptune's.
Nevertheless NASA plans to carry on with its New Horizons spacecraft mission, which in January 2006 began a journey to the dwarf planet and its moon, Charon whether Pluto is a planet or not. The seven science instruments on the piano-sized New Horizons probe will shed light on Pluto’s surface properties, geology, interior makeup and atmospheres.
For more information about the IAU ruling, see the IAU 2006 General Assembly: Result of the IAU Resolution votes.