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pluto, object 134340, dwarf planet
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Pluto: The Dwarf Planet

March 26, 2010
by findingDulcinea Staff
In 2006 the International Astronomical Union defined the term “planet” for the first time. Pluto did not meet the qualifications and was downgraded to a “dwarf planet,” or object #134340.

Pluto’s Discovery

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Pluto was discovered in 1930 by Clyde Tombaugh. Tombaugh was working at the Lowell Observatory; he was trying to find a planet the Observatory's founder, Percival Lowell, thought was responsible for the strange orbits of Uranus and Neptune. Although the theory turned out to be incorrect, it was true that another body was orbiting the sun beyond Neptune.

Pluto and Its Moons

Pluto is smaller than the Earth's moon. One orbit around the sun takes 248 Earth years. Although Pluto is typically farther away from the sun than any of the eight planets, its eccentric orbit brings it closer to the sun than Neptune for about 20 of those 248 years.

In 1996, the Hubble Telescope was able to show scientists the details of the surface of Pluto. It appears that it has lots of distinctive features and may even have icy polar caps.

In 1978 it was discovered that Pluto had a moon, Charon. The Charon is the largest moon relative to its planet's size in the solar system: Charon is half the size of Pluto. Charon and Pluto are locked in their orbits together, with the same faces of each orb always facing each other. Pluto also has two smaller moons, Nix and Hydra.

Eris Creates Discord About Pluto's Status as a Planet

Scientists began observing in the 1990s that there were many distant bodies orbiting the Sun beyond Neptune. In 2003 it was one of these Trans-Neptunian Objects (TNOs) that put into question Pluto's status as a planet.

Eventually named Eris for the Greek goddess of discord, the object was first seen by astronomers in October of 2003. Eris proved to be larger than Pluto, and scientists at the time called for the naming of what they thought was the 10th planet in our solar system. Later it was discovered that the “planet” had a moon, and debate ignited about what exactly made something a planet.

What Makes Something a Planet?

The International Astronomical Union has been in charge of naming planets since the early 1900s, and so before naming the newly found planet-like object, the IAU put together the Planet Definition Committee to decide what exactly made something a planet.

On August 24, 2006 the IAU voted and agreed on the new definitions. According to NASA, “A ‘planet’ is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.”

Because Pluto exists in an area of the solar system where there are many other bodies, it did not fit the definition of a planet, and was demoted to the status of dwarf planet. Eris was also labeled a dwarf planet. In addition, Pluto has been put in a new category of objects called “plutoids.” It is thought that more dwarf planets may be discovered in the future.
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