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The Question

(Submitted January 20, 1999)

I'm a High school Senior and have a fair amount of knowledge of the outer planets. At one point in my career planning, I wanted to be an field expert for You (NASA). As of this morning I heard on GOOD MORNING AMERICA that Pluto is no longer considered a planet, just a big ball of ice. Is this true, and if so what is Pluto's current status in regards to its classification?

The Answer

There have been a lot of reports, such as http://news.bbc.co.uk/hi/english/sci/tech/newsid_259000/259767.stm about this subject, with varying degrees of accuracy.

The group who will decide the official status of Pluto for the professional astronomers world-wide (as they do all official questions related to objects in the Universe) is called the International Astronomical Union; in this particular case, IAU Division III (Planetary Systems Sciences) is taking the lead.

Pluto has been known as the ninth planet of our solar system since it was discovered by Clyde Tombaugh at Lowell Observatory in 1930. On the other hand, it has been clear for decades that Pluto does not fit in with the pattern of the other planets. Over the last few years, the accumulated information on Pluto and the discovery of an increasing number of other objects in the outer solar system with orbital characteristics very similar to those of Pluto have been discussed within the community of astronomers called "minor-planet researchers". The question of the official status of Pluto has recently come to the forefront because the orbits of some of these other objects are now sufficiently well determined that it is reasonable to begin including them in the catalog of orbits of what are now generically known as "Trans-Neptunian Objects" (TNOs).

IAU Division III has already recommended that Pluto be included as number 1 in a catalog of TNOs.

Does this mean that Pluto has been demoted? The answer is no. Pluto will have dual classification as a planet and a TNO, at least for the time being.

Currently, the definition of a planet (as opposed to an asteriod or a TNO) is rather arbitrary. If astronomers reach a consensus on what the defintion of a planet should be, then IAU may reclassify some Solar System objects. However, in the absense of such a consensus, the definition is historical and arbitrary; moreover, many people outside the professional astronomy community have an interest in this issue, as the media attention attests. "Until there is a consensus that one of the physical definitions is clearly the most useful approach in thinking about the solar system, the IAU will not 'demote' Pluto or 'promote' Ceres," says the IAU.

Brian Marsden, head of the IAU's Minor Planet Center, has also addes his voice, as quoted in a press release.

"There is no plan to 'downgrade' or 'demote' Pluto. It will stay as a planet."

Allie Hajian, John Cannizzo, Laura Whitlock
and the Ask an Astrophysicist team


Note added in January 2007: the above answer correctly describes the situation in 1999. However, the discovery in 2003 of the object Eris, which is almost certainly bigger than Pluto, prompted a new round of discussion among astronomers. In August 2006, the International Astronomical Union decided to re-classify Pluto as a "dwarf planet," a category separate from the 8 planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune). Not everybody is happy with this decision, however, so this may not be the end of the story.

You can also read about Eris, Pluto, and the definition of planets on the web pages of Eris's discoverer, Dr. Michael Brown:
http://www.gps.caltech.edu/~mbrown/

Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See http://imagine.gsfc.nasa.gov/docs/ask_astro/ask_an_astronomer.html for help on other astronomy Q&A services.

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