New Horizons Pluto Flyby Preview And Why It Became A Dwarf Planet

Pluto is the black sheep of the solar system, and family dinners must be awkward. We all grew up thinking that there were nine planets in our solar system, but that all changed in 2006 when Pluto was reclassified as a dwarf planet. The NASA New Horizon mission will make its close approach to Pluto Tuesday and there are plenty of reasons to celebrate a mission that's been more than a decade in the making.

People are still not happy that Pluto got bumped from the official roster of planets in our solar system. Until August 2006, Pluto was a planet, but that changed following new classification guidelines from the International Astronomical Union (IAU). The "Pluto is a dwarf planet" party includes luminaries like Neil deGrasse Tyson and "Pluto Killer" Mike Brown.

Pluto was discovered by Clyde Trombaugh, from the Lowell Observatory, in 1930 and has a 248-year orbit around the Sun. The dwarf planet has five satellites -- Charon, Nix, Hydra, Styx and Kerebos. As we learned more about the solar system, our understanding of what constitutes a planet has also changed. Brown and astronomers Chad Trujillo, from the Gemini Observatory and David Rabinowitz, from Yale University, discovered an object that was larger and more massive than Pluto but was a trans-Neptunian object. Confusion about what a dwarf planet is led to quite the discussion.

The IAU debated a change in the planetary classification, which led to a new class of "dwarf planets." IAU defines one as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit." Pluto failed the planetary definition thanks to its size and other objects, such as Eris, of a similar size and mass in its neighborhood.

Just because Pluto was downgraded doesn't mean it's unworthy of study. Pluto has yet to be visited by a spacecraft, and NASA will conduct a whole slew of science projects when New Horizons flies by the dwarf planet on Tuesday. Pluto and Charon's atmosphere will be analyzed, surfaces will be mapped, and we'll learn more about how Pluto and its satellite formed.