In 2006, researchers and scientists meeting in Prague took a vote and then announced they would no longer consider Pluto to be a proper planet, due to its size. Pluto is now to be considered a "dwarf planet." "There are finally, officially eight planets in our solar system," astronomer Mark Brown told "National Geographic."
Pluto is significantly smaller than the other eight planets in the solar system. It has a diameter of only about 2,306 kilometers (1,432 miles), and is about 18 percent of Earth's size. In fact, Pluto is only twice the size of its largest moon, Charon, and is not even as big as Earth's moon.
Pluto was discovered in 1930 by 24-year-old Kansas lab assistant Clyde Tombaugh. The Lowell Observatory -- which had sponsored Tombaugh's research -- had the right to name the new planet. The name was suggested by 11-year-old schoolgirl Venetia Burney.
Pluto is a whopping 5.9 billion kilometers from the sun; this is equal to 3.67 billion miles. To put that in perspective, Earth is only 150 million kilometers -- or 93 million miles -- from the center of our solar system.
Pluto is a hard, rocky little planet comprised primarily of water and solid ice. The planet's small core is posited by scientists to be made of an iron-nickel alloy and rock. Scientists have hypothesized that there may be a liquid ocean beneath Pluto's outer ice shell. Scientists hoped to get more by launching NASA's New Horizons spacecraft, scheduled to take images of Pluto in 2015.
Scientists originally thought Pluto had no moons. However, in recent years, scientists have discovered three satellites of Pluto: Charon, Nix and Hydra. Charon -- the largest of the three, by far -- was found in 1978 by James Christy.
Many astronomers now feel that Pluto is just another object in the Kuiper Belt. The belt -- which floats out in space past Neptune -- is a collection of rock and metal objects that was discovered in 1992. In addition to Pluto, the belt is home to dwarf planets Haumea and Makemake, amongst others.
Because of its distance and size, it is difficult to get a view of Pluto, even using a powerful telescope. In the 1980s, advances in home telescope design gave amateur astronomers the ability to see Pluto from their backyards. However, Pluto appears as a mere dot in the lens.
Because of its distance from the sun, Pluto has the longest orbit of any of the -- formerly -- nine planets; it takes Pluto 248 years to circle the sun. Pluto has a strange, seemingly irregular orbit that interferes with Neptune's orbit. This is another reason scientists have declassified Pluto as a planet.
Because of its smaller mass, Pluto's gravity is not nearly as strong as Earth's. However, because of its size, a person standing on the surface of Pluto is closer to the center of gravity's pull. According to scientists' estimates, Pluto's gravitational pull is only between 4 and 7 percent of Earth's.
