Friends, our solar system has eight planets, not nine as you've been told all your life--or 12 as a committee of experts suggested last week. So ruled the International Astronomical Union (IAU) after a huge debate over the definition of "planet" at its meeting in Prague this week.
Last week, a crack IAU committee had proposed to fill a glaring hole in astronomy--the lack of an official scientific definition for "planet"--with a definition that would have raised the number of planets from nine to 12, adding the asteroid Ceres, Pluto's moon Charon, and a distant object called 2003 UB313. But the IAU's members mutinied and adopted a different definition.
Under the new definition, Pluto is not a planet, and neither are the other erstwhile candidates. So, official solar system, take 2: what is the new official definition of "planet"? And what are Pluto and the others, if not planets?
"Planets" Rule Their Space
According to the new definition, approved by the IAU, a planet in our solar system is a celestial body that meets three criteria:
- It orbits the sun.
- It's round. More technically, it "has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape."
- It "has cleared the neighborhood around its orbit." Basically, it dominates its part of space.
That last criterion revokes Pluto's planetary license--little Pluto doesn't dominate anything. The only planets are Mercury (the smallest), Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
"Dwarfs" Don't Rule Their Space
So what's Pluto? The IAU has ruled that it's a "dwarf planet." According to the IAU, a dwarf planet in our solar system is a celestial body that meets four criteria:
- It orbits the sun.
- It's round. (Same as above.)
- It's not a satellite.
- It has not "cleared the neighborhood around its orbit."
Under this definition, Ceres and 2003 UB313 are "dwarf planets," too. Charon, which was slated for promotion from "Pluto's largest moon" to "planet" will apparently have to rest content with its former title. (The rejected proposal would have revised the definition of "satellite" and made Pluto and Charon a "double planet," since they orbit each other.)
Though demoted to dwarf, Pluto did win official recognition as "the prototype of a new category of trans-Neptunian objects." The crack IAU committee had recommended that these objects be called "plutons," but geologists protested that they already use that word--it's a kind of rock formation. "Whoops," said the astronomers. For now, the new plutonic category lacks a nifty name.
Miscellaneous. Etc. Et al.
What about "trans-Neptunian objects" that aren't round like 2003 UB313, or asteroids other than Ceres, or other assorted hunks in space? The IAU has ruled that these objects--things that orbit the sun, that aren't satellites, and that don't qualify either as "planets" or "dwarf planets"--are officially (and none too poetically) named "small solar-system bodies."
For a little while at least. The definitional fight over "planet" may not be finished. Alan Stern, head of NASA's New Horizon's mission to Pluto, has already called the new definition "a farce" and claimed that "it won't stand." Others have pointed out that only a fraction of the professional astronomers gazing up at the sky today weighed in on "planets" in Prague. The IAU has spoken, and schoolbooks will be revised. But astronomers say they'll continue to explore strange new words and seek out new definitions.
Steve Sampson
August 25, 2006
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School books will not necessarily be revised because the IAU has spoken. Only four percent voted, and most are not planetary scientists. Hundreds of professional astronomers rejected the decision in a petition led by Dr. Alan Stern. This debate is most certainly far from over. Textbooks and teachers are in no way obligated to treat the IAU decision as the gospel truth.
ReplyDeleteAccording to a far better planet definition supported by many scientists, our solar system actually has 13 planets and counting. This definition states that a planet is any non-self-luminous spherical body orbiting a star. The spherical part is key because an object needs reach a certain size to become spherical. It is then in a state of hydrostatic equilibrium, where it is shaped by gravity and not chemical bonds. This is true of planets and not of asteroids and shapeless Kuiper Belt Objects.
The IAU decision makes no sense for several reasons. One, it states that dwarf planets are not planets at all. That is like saying a grizzly bear is not a bear. It is also inconsistent with the use of the term "dwarf" in astronomy, as dwarf stars are still considered stars, and dwarf galaxies are still considered galaxies.
Also, the IAU decision classifies objects solely by where they are while ignoring what they are. If Earth were in Pluto's orbit, according to this definition, it would not be a planet either. Any definition that takes the same object and makes it a planet in one location and not a planet in another is untenable and needs to be overturned.
The best way to fix this mess is to establish dwarf planets as a subclass of planets to describe objects that are spherical but do not gravitationally dominate their orbits.
Thanks for ur comment, lets c what world finally adopts as their final verdict.
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