There are three known dwarf planets in our Solar System: Pluto, Eris and Ceres. Up until the year 2006 Pluto was considered to be a regular planet as it has the similar traits of a planet. It has sufficient mass, Gravity, is not a satellite (moon), and it orbits the Sun. However, the International Astronomical Union officially defines a planet as 'a body that has cleared the neighbourhood round its orbit'. These three dwarf planets do not satisfy the latter and are therefore considered to be dwarves.
Pluto is small and cold with a diameter of 2,304 km’s and a temperature of -230 degrees Celsius. It is composed of rock and ice (methane and water) and has a Nitrogen rich atmosphere (99.97%). Pluto has 3 moons; Charon, Hydra and Nix. Eris, which has a diameter of 2,400 km’s (slightly larger than Pluto), also has a moon named Dysnomia and is found in the regions of the Kuiper Belt. Ceres is a spherical Asteroid with a diameter of 960 km’s. Very little is known about these dwarf planets and the first spacecraft to visit Pluto has been launched but will only arrive there in the year 2015.
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Pluto is still a planet. Only four percent of the IAU voted on the controversial demotion, and most are not planetary scientists. Their decision was immediately opposed in a formal petition by hundreds of professional astronomers led by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto. One reason the IAU definition makes no sense is it says dwarf planets are not planets at all! That is like saying a grizzly bear is not a bear, and it is inconsistent with the use of the term “dwarf” in astronomy, where dwarf stars are still stars, and dwarf galaxies are still galaxies. Also, the IAU definition classifies objects solely by where they are while ignoring what they are. If Earth were in Pluto’s orbit, according to the IAU definition, it would not be a planet either. A definition that takes the same object and makes it a planet in one location and not a planet in another is essentially useless.
Pluto is a planet because it is spherical, meaning it is large enough to be pulled into a round shape by its own gravity--a state known as hydrostatic equilibrium and characteristic of planets, not of shapeless asteroids held together by chemical bonds. Additionally, many planetary scientists are not IAU members and had no say in this vote.
These reasons are why many astronomers, lay people, and educators are either ignoring the demotion entirely or working to get it overturned. I am a writer and amateur astronomer and proud to be one of these people. You can read more about why Pluto is a planet and worldwide efforts to overturn the demotion on my Pluto Blog at http://laurele.livejournal.com
Thank you for your response. At no stage did I say Pluto was NOT a planet- I was merely stating that it is a dwarf planet (as categorised by the IAU) and how it differs from the other planets. However, I'm very interested to know more about the IAU and will be researching their merit. I always welcome valuable information.
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