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Astronomy Question of the Week: Why is Pluto no longer a planet?

Published by Matt on Mon Oct 5, 2009 10:29 am via: source
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It happened on 24 August 2006: instead of the nine planets it had up to that time, our Solar System suddenly had only eight – the planet Pluto was no longer a planet. What happened?

In August 2006, astronomers from all over the world gathered at the 26th General Assembly of the International Astronomical Union (IAU) in Prague. Among other things, they reorganised our planetary system and agreed on the scientific definition of a planet. A reorganisation had become necessary as an increasing number of heavenly bodies were being discovered beyond Pluto’s orbit that were about the same size as Pluto.

The eight planets of our Solar System.

The eight planets of our Solar System.

If these bodies were also granted the status of being planets, this would lead to a real flood of planets in the long term. Under the chair of the well-known female astronomer Jocelyn Bell, the astronomers thus agreed on three criteria that a heavenly body must fulfil in order to be a planet. First, the body must orbit the Sun or a star and must not be a star itself. Second, it must have sufficient mass that is has become spherical due to its own gravity. (See also the astronomic question from week 31: Why aren’t all heavenly bodies perfectly spherical?). Thirdly, since its formation, it must have cleared the area around its orbit of small bodies.

Three classes: classical planets, dwarf and planetoids

Pluto does not satisfy the third criterion – although it fulfils the first two, it was named a ‘dwarf planet’ together with Ceres (see also the astronomic question from week 31) and Eris (which orbits the Sun outside Neptune’s orbit). The updated Solar System now has three categories of planet: the eight classical planets – Mercury to Neptune, a slowly growing number of dwarf planets and the irregularly formed planetoids or Small Solar System Bodies (SSSBs).

There are already new mnemonics for remembering the order of the eight planets (moving away from the Sun), including “My Very Elegant Mother Just Served Us Nachos” – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

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. 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
Rob Goldsmith
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