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A dwarf planet is a celestial body that:
- orbits a sun
- has enough mass to give it a nearly round shape
- is not a satellite of another object
- has not cleared out all the objects in its own orbit
This definition of a dwarf planet was established on August 24, 2006 by the International Astronomical Union (IAU).
In 1930, Pluto was discovered and identified as a plane.
On August 25, 2006, Pluto was demoted to being a dwarf planet. Thus, on this date, our solar system was reduced from nine planets to eight planets. These planets in order from the Sun are:
Mercury, Venus, Earth, Mars, Jupiter Saturn, Uranus, Neptune
Pluto’s demotion came about because of the discovery of Eris (formerly called Xena). This celestial body is farther from the Sun than is Pluto. Like Pluto, Eris has objects in its orbit. If Eris was classified as a planet, then the asteroid Ceres as well as other asteroids would have to be called planets.
Actually, Ceres was considered a planet when it was discovered on January 1, 1801. At this time there was no reason to suspect that a new class of celestial objects had been found. It was considered to be the eighth planet . Uranus, the 7th planet had been been discovered some 20 years earlier.
When astronomers continued finding numerous other asteroids in the region the astronomical community in the early 1850s demoted Ceres and the others and coined the new term “asteroid.”
Ceres has been called a planet, demoted to an asteroid, and now its rank it up a bit since it is considered a dwarf planet.
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