A few years ago, the International Astronomical Union formally defined the term planet and reclassified Pluto. Instead of the last-discovered, smallest, coldest, and pretty much least of the planets, they decided Pluto is the first-discovered, innermost and one of the largest of the Kuiper Belt objects, a dwarf planet.

There was a gigantic uproar, because the media presented the change as Pluto being demoted from planet status. The truth is that Pluto wasn’t demoted, simply classified correctly.

The reclassification took Pluto from last, smallest, and least of the planet to first, possibly the largest, and innermost of a whole new class of planetary bodies — Plutoids. Just typing that word makes me happy. Plutoids: smile.

The Hayden Planetarium in New York, headed by Dr. Neil deGrasse Tyson, was the first institution to notice that Pluto doesn’t really fit in with the planets, but until the discovery of actual bodies in the theoretical Oort cloud, we didn’t have anyplace else to classify it. But Pluto looks a lot like Haumea (2004) and Makemake (2005), not to mention Eris (2003), who started the whole mess.

Eris is a dwarf planet about the same size as Pluto, but apparently about 27% more massive, which would imply it’s denser than Pluto. It’s also quite a ways farther out, about 3 times as far away from the sun as Pluto. It even has a moon, called Dysnomia.

Eris (originally dubbed Xena) was considered the 10th planet in the Solar System for a few years, until that 2006 IAU meeting, when the world’s astronomers actually defined what a planet is (about time, people). They also defined dwarf planets, Plutoids, and Plutinos (objects which have a 3:2 orbital resonance with Neptune). It was a busy meeting.

And then the hate mail started, almost all of it from the US. Dr. Tyson received so much of it he compiled it all into a book called The Pluto Files, published in 2009 — which hit the NYT extended bestseller list.

We Americans love our Pluto, king of the underworld (and Mickey’s dog!), first among the Kuiper Belt objects, with two classes of solar system objects named after it. Now isn’t that more interesting than being a lowly planet?

3 comments so far

  1. Laurel Kornfeld on

    Unfortunately, your article contains several erroneous statements. The IAU definition did NOT “classify Pluto correctly,” and furthermore, the media has misrepresented it as a consensus among the world’s astronomers,when this is not the case. It is just one view in an ongoing debate.

    It was not “the world’s astronomers” but four percent of the IAU, most of whom are not planetary scientists, who voted on this in violation of the IAU’s own bylaws, on the last day of the two-week 2006 General Assembly. No electronic voting was used, meaning anyone not in a particular room on that day could not vote. The controversial decision was opposed in a formal petition by hundreds of professional astronomers led by New Horizons Principal Investigator Dr. Alan Stern. You can find that petition here:

    Dr. Stern is the person who first created the term “dwarf planet,” but he intended for it to refer to a third class of planets, small objects large enough to be rounded by their own gravity but not large enough to gravitationally dominate their orbits. He never intended for dwarf planets to not be considered planets at all. The four percent of the IAU who voted on this misused and misappropriated the term Stern created.

    In astronomy, dwarf stars are still stars, and dwarf galaxies are still galaxies. The IAU definition, which says dwarf planets are not planets at all, makes no sense. It is flawed in many ways, mostly because it defines objects solely by where they are while ignoring what they are. If Earth were in Pluto’s orbit, it would not “clear that orbit” either. A definition that takes the same object and calls it a planet in one location and not a planet in another makes absolutely no sense.

    Neither Pluto nor Eris were “classified correctly” by this four percent of the IAU. That is because according to the equally legitimate geophysical planet definition, which many astronomers adhere to, a planet is simply any non-self-luminous spheroidal body in orbit around a star. The spherical part is key because it means the object is large enough and massiv enough to be squeezed into a round or nearly round shape by its own gravity. Not distinguishing the overwhelming majority of asteroids and Kuiper Belt Objects, which are tiny rocks and ice balls shaped only by their chemical bonds, from complex worlds that are rounded by their own gravity, is simply bad science.

    Like Earth, Pluto is geologically differentiated into core, mantle, and crust. It has geology and weather. These make it a complex world and a small planet. There is no reason Pluto and Eris cannot be dually classed as small planets and as Kuiper Belt Objects. The first tells us what they are; the second tells us where they are.

    Since writing his book, Dr. Tyson has moderated his position to the point that he now acknowledges that the definition of planet and the status of bodies such as Pluto and Eris remain a matter of debate. This is clear from the DVD he made, also titled “The Pluto Files,” in 2010.

    In 2009, planetary scientists asked the IAU General Assembly to reopen the planet definition discussion in order to create a broader, more inclusive definition. The IAU leadership refused, leading the planetary scientists to boycott the conference. As a result, there is a growing schism between astronomers who uphold the IAU definition and those who do not. But there is nothing that makes the IAU definition more “official” than the alternative, more inclusive one.

    Almost no one uses the word “plutoids,” which was created in a closed door backroom deal in 2008. Also, the Oort Cloud is not the same as the Kuiper Belt. It is much further out, and we have yet to discover any Oort Cloud objects.

    Pluto and Eris are not “lowly” planets. They are simply members of a third planetary class.


    • ValRoberts on

      It’s not an article, Laurel. It’s an essay, which is essentially an opinion.

      I thought it was nice that the first American-discovered planetary body was given its own class as the first, biggest and best of something, instead of being the weird little redheaded stepchild that can’t even orbit in the same plane.

      Obviously you disagree, but that’s how opinions work.


      • Laurel Kornfeld on

        Opinion is more accurate for both your post and my comment. It’s sometimes harder to distinguish objective, “news” posts from opinion posts online than it is in print, where one knows to find opinions on the “op-ed” page. There are varying opinions among astronomers about what a planet is, and neither view is more or less scientifically worthwhile than the other.

        Orbiting in the same plane is not a “requirement” for an object to be a planet. Many exoplanet systems have been discovered with multiple giant planets orbiting the star all in different planes. Pluto’s orbit is stable, meaning it will not be eventually thrown out of the solar system or crash into another solar system body.

        Pluto is actually not a weird red headed stepchild. It turns out that dwarf planets, which some consider a third class of planets, are the most numerous in our solar system. I’m referring only to those objects large enough to be in hydrostatic equilibrium–rounded by their own gravity. It’s just that until recently, we did not know that the others existed. Some astronomers believe that dwarf planets will be the most common type in exoplanet systems as well, but it will be quite some time before we have the technology to detect them.


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