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Thursday, May 21, 2009


Laurel Kornfeld

Pluto still IS a planet and has always been one. Unlike most objects in the Kuiper Belt, it has attained hydrostatic equilibrium, meaning it has enough self-gravity to have pulled itself into a round shape. When an object is large enough for this to happen, it develops the same geological processes as the larger planets, processes that inert asteroids and most KBOs do not have. Spherical objects are shaped by gravity; smaller asteroids are shaped by chemical bonds.

Not distinguishing between shapeless asteroids and objects whose composition clearly makes them planets is a disservice and is sloppy science.

As of now, there are three other KBOs that meet this criterion and therefore should be classified as planets—Haumea, Makemake, and Eris. Only one KBO has been found to be larger than Pluto, and that is Eris.

The IAU definition makes no linguistic sense, as it states that dwarf planets are not planets at all. That’s like saying a grizzly bear is not a bear. And it goes against the use of the term "dwarf" in astronomy, where dwarf stars are still stars, and dwarf galaxies are still galaxies.

Second, it defines objects solely by where they are while ignoring what they are. If Earth were placed in Pluto’s orbit, by the IAU definition, it would not be a planet. That is because the further away an object is from its parent star, the more difficulty it will have in clearing its orbit. A definition that takes the same object and makes it a planet in one location and not a planet in another is essentially useless.

Significantly, this definition was adopted by only four percent of the IAU, most of whom are not planetary scientists. No absentee voting was allowed. It was done so in a highly controversial process that violated the IAU’s own bylaws, and it was immediately opposed by a petition of 300 professional astronomers saying they will not use the new definition, which they described accurately as “sloppy.” Also significant is the fact that many planetary scientists are not IAU members and therefore had no say in this matter at all.

Many believe we should keep the term planet broad to encompass any non-self-luminous spheroidal object orbiting a star.

We can distinguish different types of planets with subcategories such as terrestrial planets, gas giants, ice giants, dwarf planets, super Earths, hot Jupiters, etc.

We should be broadening, not narrowing our concept of planet as more objects are being discovered in this and other solar systems.

In a 2000 paper, Dr. Alan Stern and Dr. Hal Levison distinguish two types of planets—the gravitationally dominant ones and the smaller ones that are not gravitationally dominant. However, they never say that objects in the latter category are not planets.

I attended the Great Planet Debate, which actually took place in August 2008, and there was a strong consensus there that a broader, more encompassing planet definition is needed. I encourage anyone interested to listen to and view the conference proceedings at You can also read more about this issue on my blog at


Pluto is smaller than some moons, and is not even the largest of the Trans Neptunian objects. Although I partially agree with your statement, Ceres, Eris, Quoar, Sedna, and a number of other Trans Neptunian objects have a hydrostatic equilibrium. Infact there are currently 15 so called dwarf planets that have a hydrostatic equilibrium, not 3.

Its an issue of classification. Do we teach that we have 8 planets in our schools, or do we teach that we have 23? The same argument could be made for moons too. Titan has an atmosphere thicker than the earths, but its still considered a moon and not a planet. Ganymede is almost the size of Mars, and both are much larger than Mercury or any known dwarf planet.

Personally I wouldn't dwell on on categorizations. The category of dwarf planet simply helps to contour our perception of a solar system, not just our own.

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