On July 24th, 2006, the International Astronomical Union (IAU) officially defined what constitutes a planet. For a celestial body in our solar system to be defined as a planet, it must:
1. Be in orbit of the Sun
2. Have sufficient mass to assume a nearly round shape (officially known as hydrostatic equilibrium)
3. “Clear the neighborhood” around its orbit
This designation meant that Pluto — first discovered in 1930 by Clyde W. Tombaugh — was no different than any of the other 70,000 icy objects that comprise the Kuiper Belt, a region that extends from the orbit of Neptune out to 55 astronomical units (55 times the distance of the Earth to the Sun).
After decades of observation, astronomers have continued to discover other large Kuiper Belt objects, such as Eris in 2005, which was determined to be larger than Pluto itself. The discovery of Eris — which has approximately 25% more mass than Pluto — posed an interesting question to the scientific community: would this object be the 10th planet in our solar system?
"If Neptune were analogized with a Chevy Impala in mass, then how big is Pluto compared to that? Pluto would be a matchbox car sitting on the curb." - Neil deGrasse Tyson
Based upon the IAU’s definition above, any object that doesn’t meet the third criteria is classified as a dwarf planet — including Pluto, Eris, and many of the other objects located in the distant reaches of the Kuiper Belt. In spite of this new designation, Pluto still holds a special spot in the hearts of scientists and astronomers, as NASA has sent their New Horizons spacecraft to observe it closely. Slated to arrive in 2015, New Horizons will capture the first close-up images of the surface.
Image Credit: PBS
1. Pluto and the Developing Landscape of Our Solar System
2. Why Pluto is No Longer a Planet