American astronomer Clyde Tombaugh, working at the Lowell Observatory in Flagstaff, Ariz., discovers a ninth planet.

The new planet is named Pluto, after the Roman god of the underworld. The name was originally suggested by 11-year-old Venetia Burney of Oxford, England.

Astronomers discover Charon, Pluto's largest moon.

Astronomers discover the first Kuiper belt object, 1992 QB1. The Kuiper belt is an area beyond Neptune containing icy bodies similar to Pluto, raising questions about whether Pluto should be considered a member of the Kuiper belt rather than a planet.

Press reports leak that the International Astronomical Union is considering reclassifying Pluto as a "minor planet," inspiring public protest. The IAU issues a press release denying that it's considering the reclassification.

The newly redesigned Rose Center for Earth and Space Science at the American Museum of Natural History in New York opens, excluding Pluto from its solar system exhibition.

Scientists discover two more moons of Pluto: Nix and Hydra.

Caltech astronomer Mike Brown discovers 2003 UB313, an object about the size of Pluto outside Neptune's orbit. Some astronomers argue that if Pluto is a planet, then UB313 should be a planet as well.

An 18-person working group, convened by the IAU to come up with a definition of the word "planet," fails to reach a consensus.

NASA launches its New Horizons spacecraft, expected to reach Pluto in 2015.

IAU's second working group presents a new definition of "planet" that would include Pluto and Charon, UB313, and Ceres, bringing the number of planets to 12. However, the definition meets resistance from astronomers at the annual IAU meeting in Prague.

Four hundred members of the IAU vote to approve a definition of "planet" -- it must orbit the sun, be round, and clear its orbit of all neighboring objects. Pluto doesn't meet this definition, so it's officially reclassified as a "dwarf planet."