Pluto Demoted

Pluto has been demoted from being a planet to being a dwarf planet. On Aug. 24, 2006 , the International Astronomical Union (IAU) gave a description for dwarf planets.

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 celestial body
• has not cleared out all the objects in its own orbit.

Pluto was discovered and identified as a planet in 1930.

Pluto was demoted to a dwarf planet  August 25, 2006.

Pluto is not unique in being dismissed from the planetary ranking. The celestial bodies found between Mars and Jupiter in the 1800s were listed as planets. These include Ceres. Vesta, Juno, and Pallas which were introduced as planets.
Introductory astronomy texts of the 1820s described our solar system as having eleven planets in this order from the Sun:  Mercury, Venus, Earth, Mars, Vesta, Juno, Ceres, Pallas, Jupiter, Saturn, and Uranus.

Astronomers continued to find new celestial bodies between Mars and Jupiter.Finally, astronomers realized that this large number of similar bodies all in orbit between Mars and Jupiter represented a new class of solar-system object. They called them asteroids, the name Herschel had coined 50 years earlier. Instead of listing them by distance from the Sun, as they did the planets, astronomers categorized them by their order of discovery. Astronomers today list about 100,000 known asteroids as large as 6 miles (10 kilometers) across located between Mars and Jupiter, a region now called the asteroid belt.

At the time of Pluto’s demotion, the asteroid, CERES received a promotion to join Pluto along with a newly discovered celestial body called Eris, in the dwarf planet category. Two additional celestial bodies are also in this category,  Haumea and Makemake. I am guessing that this category will quickly grow as newer and better astronomy equipment is designed.

The long debate on the definition of a planet is a good example of how scientific concepts are not etched in stone but continue to evolve with new discoveries. Our solar system now contains eight planets in this order from the Sun:

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranius, and Neptune.

See DWARF PLANET.