Planets are defined as big balls of rock or gas that orbit around the Sun, so should Pluto still be classified as a planet? Debate over Pluto continued at the Harvard-Smithsonian Center for Astrophysics on September 18 in a debate featuring three leading experts in planetary science.
The experts each presented their case on what defines a planet. The definition of a planet has been argued many times over in public and in classrooms. We were taught in grade school that there was nine planets in our solar system, but after astronomers continued to find more Pluto-sized objects that orbit further than Neptune over the years, Pluto got booted from the list.
The International Astronomical Union (IAU) redefined a planet as a celestial body that: is in orbit around the Sun, is round or nearly round, and has "cleared the neighborhood" around its orbit in 2006. Under this definition, Pluto is considered only a "dwarf planet."
"What about all those exoplanets orbiting other stars? Are they planets?" the Harvard-Smithsonian asks. "Is a dwarf planet a small planet? Not according to the IAU. Even though a dwarf fruit tree is still a small fruit tree, and a dwarf hamster is still a small hamster."
Dr. Owen Gingerich, a science historian who chaired the IAU planet definition committee, argued that a "planet is a culturally defined word that changes over time." Gingerich defended Pluto and called it a planet, whereas Dr. Gareth Williams, associate director of the Minor Planet Center, defended the IAU definition that excludes Pluto.
Dr. Dimitar Sasselov, director of the Harvard Origins of Life Initiative presented from the viewpoint of an exoplanet scientist, defining a planet as "the smallest spherical lump of matter that formed around stars or stellar remnants."
After an audience vote on the definition, Pluto was defined a planet.
While the informal vote shows only popular opinion, debate on whether or not to include Pluto will continue. NASA's New Horizons spacecraft is scheduled to examine Pluto next year for research.
