Pluto

7/11 — Mathematical calculations suggest that it is on this day that dwarf planet Pluto moved inside the orbit of Neptune for the last time before 1979.

2/18 — While studying photographs taken in January, Clyde Tombaugh discovers Pluto.

3/13 — The news of the discovery of Pluto is telegraphed to the Harvard College Observatory.

5/1 — The dwarf planet Pluto is officially named.

2/7 — Pluto moves inside Neptune's orbit for the first time since either was discovered.

4/25 — Pioneer 10 travels beyond Pluto's orbit.

2/11 — Pluto crosses Neptune's orbit, ending a nearly 20-year period (since 1979) when it was closer to the Sun than the gas giant; Pluto is not expected to interact with Neptune's orbit for another 228 years.

5/18 — A second photo from the Hubble Space Telescope confirms that Pluto has two additional moons, Nix and Hydra.

6/21 — Pluto's newly discovered moons are officially named Nix and Hydra.

8/24 — The International Astronomical Union (IAU) redefines the term "planet" such that Pluto is now considered a dwarf planet.

7/14 — NASA's New Horizons probe performs the first flyby of Pluto, and thus completes the initial survey of the Solar System.

2/4 — Clyde Tombaugh, American astronomer and academic, discovered Pluto (d. 1997)

7/11 — Venetia Burney, English educator, who named Pluto (d. 2009)

1/17 — Clyde Tombaugh, American astronomer and academic, discovered Pluto (b. 1906)