Pluto is Two Years Dead

Today is the two year anniversary of Pluto’s demotion from planet status. I had invited several people to contribute to this event. Chris Schierer has remarked that Pluto is really really small and is very very hard to see. Kim Bosco tells us how Pluto was found and why Kansas is key! Jim Cronen opines about classification and lastly, Steph Sisson has composed a planetary haiku for the event:

Like your dark namesake,
return to deepest shadow.
Retrograde motion

I also have an opinion and some comments on this event


You may recall that in 2006, the International Planetary Union decided on a definition of “planet” that did not include Pluto.

The IAU members gathered at the 2006 General Assembly agreed that a “planet” is defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

The clinching criteria was the (c) item. Pluto doesn’t have a large enough mass to have “cleared the neighbourhood”. Unfortunately, as several people have pointed out, if Mercury (a planet) were transported to the Kuiper Belt, it wouldn’t be a planet anymore. The definition is fundamentally arbitrary and therefore hasn’t much scientific usefulness.

This has implications for the ongoing search for “planets” in other solar systems. To date, most of the discovered extra-solar (“not in our solar system”) planets have been large ones. Jupiter-sized or so. These fit anyone’s definition of planet. Several planets close to the size of Earth have also been located. Soon, as techniques and technology improve, even smaller bodies will be found. We’re going to have to pigeon-hole these orbiting piles of rock and ice into our preconceived definitions of what is a planet, and what is not. People will think differently of a planet then they will a moon, or an asteroid, or a Kuiper belt object, or whatever. Don’t forget that the moons Titan (orbiting Saturn) and Ganymede (orbiting Jupiter) are both larger than Mercury, although less massive. Don’t tell me that your school teachers didn’t concentrate more on Mercury than they did Titan or Ganymede because I’ll know you’re lying.

These preconceived notions, while not detrimental in and of themselves, can lead to dangerous blindspots when performing that grandest of tasks: science. While I have faith that eventually these blindspots are exposed and examined, wouldn’t it be better to not force astronomers, astrophysicists and other scientists to pre-ordain orbiting bodies into the categories of “planet” or “not-planet”. A more all-inclusive term is needed for the various piles of rock, ice and gas that circle their central furnaces. “Planet” should probably not be used anymore in science, with an exception:

For the purposes of Grade School science instruction, I think that we should call the most recent 9 planets (including Pluto, for historical reasons) the “official” planets of our solar system and then discard the term in later science instruction, explaining that “planet” is only a convenient label for several large bodies that circle our sun and that it is not a scientific term.

Pluto isn’t a planet anymore, alas. It still has a large place in the hearts of those of us who care.

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8 Responses to Pluto is Two Years Dead

  1. Pluto most certainly IS still a planet. A vote of four percent of the IAU on a definition that makes no linguistic sense by stating that dwarf planets are not planets does not change reality. Facts cannot be changed by decree or fiat. The IAU’s definition departs from the usage of terminology in astronomy, where dwarf stars are still stars (our sun is a dwarf star), and dwarf galaxies are still galaxies. The definition also creates the absurdity where an object of the same size is considered a planet in one location and not a planet in another. If Earth were in Pluto’s orbit, the IAU definition would consider it not to be a planet. The same is true if a Mars-sized object is found in the Kuiper Belt. That would not be a planet while the real Mars is. It makes no sense to define objects solely by where they are while ignoring what they are. Interestingly, within days after the IAU vote (no absentee voting was allowed, which is why 96 percent of the membership did not participate in the vote taken the last day of a two-week conference), 300 professional astronomers led by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto, signed a petition saying they will not use the new definition, which they decried as sloppy. Only a week and a half ago, planetary scientists held a conference titled “The Great Planet Debate” in Laurel, MD, which I attended, where three days of discussion attempted to come up with a better consensus on how to define the term planet. It runs out many planetary scientists, those who specialize in studying planets, are not members of the IAU while most who voted two years ago are not planetary scientists but other types of astronomers. We may yet see the formation of a rival group of planetary scientists who will attempt to create a more inclusive planet definition, where planet is defined as any object with enough self gravity to pull itself into a round shape (hydrostatic equilibrium) that orbits a star. These round objects, Pluto included, are far more like the major planets than like inert, shapeless asteroids, because they have geological processes the asteroids do not have. For more on this issue, feel free to visit my Pluto blog at http://laurele.livejournal.com .

  2. Pluto most certainly IS still a planet. A vote of four percent of the IAU on a definition that makes no linguistic sense by stating that dwarf planets are not planets does not change reality. Facts cannot be changed by decree or fiat. The IAU’s definition departs from the usage of terminology in astronomy, where dwarf stars are still stars (our sun is a dwarf star), and dwarf galaxies are still galaxies. The definition also creates the absurdity where an object of the same size is considered a planet in one location and not a planet in another. If Earth were in Pluto’s orbit, the IAU definition would consider it not to be a planet. The same is true if a Mars-sized object is found in the Kuiper Belt. That would not be a planet while the real Mars is. It makes no sense to define objects solely by where they are while ignoring what they are. Interestingly, within days after the IAU vote (no absentee voting was allowed, which is why 96 percent of the membership did not participate in the vote taken the last day of a two-week conference), 300 professional astronomers led by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto, signed a petition saying they will not use the new definition, which they decried as sloppy. Only a week and a half ago, planetary scientists held a conference titled “The Great Planet Debate” in Laurel, MD, which I attended, where three days of discussion attempted to come up with a better consensus on how to define the term planet. It turns out many planetary scientists, those who specialize in studying planets, are not members of the IAU while most who voted two years ago are not planetary scientists but other types of astronomers. We may yet see the formation of a rival group of planetary scientists who will attempt to create a more inclusive planet definition, where planet is defined as any object with enough self gravity to pull itself into a round shape (hydrostatic equilibrium) that orbits a star. These round objects, Pluto included, are far more like the major planets than like inert, shapeless asteroids, because they have geological processes the asteroids do not have. For more on this issue, feel free to visit my Pluto blog at http://laurele.livejournal.com .

  3. Bill Ruhsam says:

    Thanks for commenting, Laurel! This is exactly the soft of opinion and writing that I like to see!

  4. Pingback: Plutopia! by Schierer Space

  5. Kim says:

    For the record, I both applaud the ‘debate’ as a teachable moment of the Nature of Science and shudder at the mere idea that Pluto will be forever cast out of planetdom.

    Pluto will always be a planet to me. Regardless of what the ‘powers-that-be’ declare.

  6. James Cronen says:

    And crap, I didn’t realize it was the 24th already.

    I have two pages of handwritten notes on the matter that it would be a shame to waste, so I’ll put something together tomorrow and post it on Phun.

    Don’t feel that you need to highlight it here; it’s my own idiotic fault for not actually putting the date on my calendar.

    *grumble* *kicks can* *walks away*

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  8. Pingback: Bookmarks about Pluto

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