Pluto Reclassified: Remembering the Former Ninth Planet
WASHINGTON – In August 2006, a decision by the International Astronomical Union (IAU) dramatically altered our solar system’s planetary lineup, reclassifying Pluto as a “dwarf planet” after 76 years as the ninth planet. the move stemmed from a need to define what constitutes a planet, a discussion spurred by the discovery of other celestial bodies of similar size and composition to Pluto.
The IAU established three criteria for full planetary status: the object must orbit the Sun,possess sufficient mass for its self-gravity to achieve a nearly round shape (hydrostatic equilibrium),and have ”cleared the neighborhood” around its orbit.Pluto met the first two criteria but failed the third. Its orbit is oval-shaped and intersects with Neptune’s, disqualifying it under the new definition.
Discovered on February 18, 1930, by Clyde Tombaugh at the Lowell Observatory in Flagstaff, Arizona, Pluto was named for the Roman god of the underworld. Measuring approximately 2,377 kilometers in diameter – roughly one-fifth the size of Earth - Pluto orbits the Sun at a distance of 5,900 million kilometers, completing one orbit every 248 Earth years. Its rotation is notably slow, taking about 6.4 days for a single rotation, and, like Venus and Uranus, it exhibits a retrograde rotation.
Despite its reclassification, Pluto is not without distinction. It boasts five known satellites: Charon, Styx, Nix, Kerberos, and Hydra.
In 2015, NASA’s New Horizons spacecraft provided the first close-up images of Pluto, revealing a complex surface featuring a large, heart-shaped region called “Tombaugh Regio” – frequently enough referred to as “Pluto’s heart” - alongside mountains and impact craters.
The redefinition of “planet” and Pluto’s subsequent reclassification remain a topic of discussion, but the event underscores the evolving nature of scientific understanding. (source: nasa.gov - NASA’s New Horizons to Continue exploring Outer Solar System)
