Debate has been raging about whether Pluto should continue to have the status of a Planet! The International Astronomical Union (IAU) General Assembly, on 24 August 2006, has decided to make a new category - the Dwarf Planet and put Pluto into this new category.

International Astronomical Union (IAU) - which has the naming rights to space objects  - has a working group considering the definition of a "planet". But some would say that the vote to demote Pluto, on the last day of the General Assembly when most Astronomers had departed, meant that "was not truly representative" of the IAU.

Under the new ruling, the IAU defines a planet as "a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a nearly round shape, and has cleared the neighbourhood around its orbit".

Because Pluto does not meet the last criterion (its orbit cuts into that of Neptune), the IAU demoted it to 'dwarf planet' status. The decision leaves the solar system with only eight planets - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Alan Stern, the lead scientist on NASA's robotic New Horizons mission, now on its way to Pluto, and several astronomers of the Johns Hopkins University in the US, were among those who do not approve the new definition.

Johns Hopkins said in a statement that several of its astronomers described the IAU's decision as a "muddled" ruling "that is unlikely to settle ongoing debates over how to define a planet and whether the term should apply to Pluto".

According to BBC News, NASA's Stern slammed Pluto's demotion saying "it is impossible and contrived to put a dividing line between dwarf planets and planets". (Source:
Indo-Asian News Service, 28th August 2006)


Gibor Basri, an astronomer at the University of California - wants to increase the number of planets to 12! Professor Basri's definition: a planet must orbit a star, not another planet, and it must be round. That means it must be 700K in diameter, when gravity moulds it into a sphere, or bigger. Smaller objects are potato-shaped. All the accepted planets are round.

Professor Basri says that a planet can't be bigger than 13 times the mass of Jupiter. Because such objects generate heat and light, they should be called stars. But Geoff Marcy, a member of the Anglo-Australian planet search team at Siding Springs telescope says other factors need to be taken into account, including how the planet was formed.

Professor Ross Taylor, at the Australian National University, has removed Pluto from planet status in their textbooks (reported back in 2003!)

Pluto is now classified as a Dwarf Planet - Is this the best decision?



Author: Frances Moore, Managing Director, WebQuest Direct


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Last updated September 2006