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What Defines a Planet?
What constitutes a planet? The International Astronomical Union (IAU) developed some definitions in 2001, modified them again in 2003, and as of August 24, 2006, the IAU has come up with another definition. The IAU said in a statement that the definition for a planet is now officially known as "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."
A "dwarf 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, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.
All other objects except satellites orbiting the sun shall be referred to collectively as "Small Solar-System Bodies". According to the IAU, more dwarf planets are expected to be announced in the coming months and years. Currently, a dozen candidate dwarf planets are listed on IAU's dwarf planet watchlist, which keeps changing as new objects are found and the physics of the existing candidates becomes better-known.
According to Paul Hertz, Chief Scientist for the Science Mission Directorate at NASA Headquarters, NASA will use the new guidelines established by the International Astronomical Union, and continue pursuing exploration of the most scientifically interesting objects in the solar system, regardless of how they are categorized.
Dwarf PlanetsA dwarf planet is a category of celestial bodies defined in a resolution passed by the International Astronomical Union (IAU) on August 24, 2006.
Currently, there are three celestial bodies that have been redefined by the IAU as dwarf planets:
UB313 or ErisIn July 2005, Astronomer Mike Brown of CalTech and his team announced the discovery of yet another Kuiper Belt Object - this one larger than Pluto. This object, provisionally named UB313, or Xena, has officially been named Eris by the IAU.
The new dwarf planet, as it has been defined by the IAU on August 24, 2006, has a diameter of 3,000 km (1,850 miles) which is 700 km (435 miles) larger than Pluto. These new observations were made using a sensitive sensor on the IRAM 30-m telescope that measured the heat emitted by the new object, and found it had a similar reflectivity to Pluto. This allowed them to calculate its size.
UB313 is significant because it is now known as the largest dwarf planet in the solar system. It is the largest object found in orbit around the sun since the discovery of Neptune and its moon Triton in 1846.
The new dwarf planet is the most distant object ever seen in orbit around the sun, even more distant than Sedna, the Kuiper Belt discovered in 2003. It is almost 10 billion miles from the sun and more than 3 times more distant than the next closest planet, Pluto and takes more than twice as long to orbit the sun as Pluto.
PlutoOnce known as the smallest, coldest, and most distant planet from the Sun, Pluto has a dual identity, not to mention being enshrouded in controversy since its discovery in 1930. Pluto is also a member of a group of objects that orbit in a disc-like zone beyond the orbit of Neptune called the Kuiper Belt. This distant region consists of thousands of miniature icy worlds with diameters of at least 1,000 km and is also believed to be the source of some comets.
Pluto has three known moons, Hydra and Nix, besides its companion moon, Charon. At about 1,186 km (737 miles), Charon's diameter is a little more than half of Pluto's.
The duo's gravity has locked them into a mutually synchronous orbit, which keeps each one facing the other with the same side. Many moons - including our own - keep the same hemisphere facing their planet. But this is the only case in which the planet always presents the same hemisphere to its moon. If you stood on one and watched the other, it would appear to hover in place, never moving across the sky.
Charon was discovered in 1978, while two additional moons Hydra and Nix, were discovered in 2005.
In Greek mythology, Charon was the boatman who carried the souls of the dead to the underworld - a kingdom that in Roman mythology was ruled by the god, Pluto. The U.S. Naval Observatory's James Christy suggested the name after he found the moon in 1978.
Seven years later, Charon and Pluto began a five-year period of eclipsing each other from Earth's point of view. That was lucky for us, because it enabled scientists to measure the diameters and masses of both objects as each passed in front of the other.
Charon appears to be covered by water ice, which differs from Pluto's surface of frozen nitrogen, methane, and carbon dioxide. One theory is that the materials that formed Charon were blasted out of Pluto in a collision. That's very similar to the way in which our own moon is thought to have been created.
NASA launched its New Horizons spacecraft to Pluto and Charon in January 2006, and it should arrive in 2015, becoming the first spacecraft to visit them. In preparation, the New Horizons project is organizing a search for additional moons of Pluto, using ground-based telescopes and possibly the Hubble Space Telescope.
CeresCeres has been classified as a dwarf planet that might also be classified as an asteroid. Its name is derived from the Roman goddess Ceres. Discovered on January 1, 1801, by Giuseppe Piazzi, Ceres has a diameter of about 950 kilometers and is by far the largest and most massive known body in the asteroid belt, as it contains approximately a third of the belt's total mass.
The classification of Ceres has changed several times. Even though it was classified as a planet when it was first discovered, because it resembled similar bodies in the asteroid belt it was reclassified as an asteroid for over 150 years.
As the first such body to be discovered, its name was prefixed by the number 1, under the modern system of asteroid numbering. After the discovery of the trans-Neptunian object 2003 UB313 (Eris), a proposition was made by the International Astronomical Union to reinstate Ceres to the status of planet along with Pluto's moon, Charon, and Eris.
Instead, on August 24, 2006, an alternate proposal came into effect labeling Ceres a 'dwarf planet'. It is not yet clear whether dwarf planet status is, like planet status, a sole defining category, or whether dwarf planets also retain their previous minor body classifications such as "asteroid."