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Why is Pluto not a planet any more?

Well, it’s still a planet of sorts. Since 2006 Pluto has been classified by the International Astronomical Union as a ‘dwarf planet’. In recent decades, powerful telescopes have enabled astronomers to discover several Pluto-sized objects beyond Neptune’s orbit, and there are probably lots more out there. So, either they had to expand the list of planets (which would mean you’d have to remember a lot more for your science exams) or it was time to come up with an official definition of what counts as a planet.

After some fierce debate, the international committee agreed a condition that a planet must be the biggest thing in its orbital neighbourhood. Pluto and the newly found similar objects are all in the same neighbourhood, so that rules them out. However, the defi nition of ‘planet’ is controversial, so the ‘Save Pluto’ campaigners hope it will be re-instated one day.




  • The IAU definition is not “official”; it is merely one view in an ongoing debate. There is no reason we cannot expand the list of planets. Memorization is not very important to learning. Does anyone memorize the names of Jupiter’s 63 moons? Restricting the number of planets artificially just to make them easier to memorize is hardly scientific.

    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. These reasons are why many astronomers, lay people, and educators are either ignoring the demotion entirely or working to get it overturned.

  • veterinary technician

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