Discovered in 1930, Pluto was considered the ninth planet in our solar system for decades. However, Pluto was later redefined as a dwarf planet. Learn the reasoning behind this decision.
In 2006 at the 26th General Assembly of the International Astronomical Union, a vote was taken to define a planet as an object that meets the following criteria: 1) it orbits around the sun, 2) it is (nearly) round in shape, and 3) it needs to have cleared the neighborhood of objects around its orbit. Pluto meets the first two criteria but fails the third. Earth is 1.7 million times the mass of any other object in its orbit. Pluto is hardly alone in its orbit around the sun. It’s just one of 70,000 large objects in the Kuiper Belt, and it accounts for less than 1% of the total mass of the debris in its orbit.