So Is It A Planet?

And Why Not???

Pluto From New Horizons

Pluto From New Horizons

From CNN on-line:

It was three years ago Monday that the International Astronomical Union demoted Pluto from a planet to a dwarf planet, a decision that made jaws drop around the world.

An outcry followed, textbooks had to be rewritten, long-held beliefs were shattered, and many people felt our cosmic neighborhood just didn’t seem the same with eight — instead of nine –planets in the solar system.

Well, even though I was working on the New Horizons Mission to Pluto at the time, my jaw didn’t exactly drop. The debate had been going on for awhile. Besides. It’s just nomenclature. As it was, ever since Arthur C. Clarke pointed out that Europa was every bit as interesting as Jupiter, astronomically speaking, many of us had sort of realized early on that there was more to the Solar System than just planets, comets and asteroids.

Not that planets were downgraded, mind you. It was more like everything else was upgraded in importance. The icing on that particular cake was that astronomers began to realize that even the planets were more varied than originally supposed. There weren’t just two kinds; rocky like the Earth and gaseous-giants like Jupiter. It seems better now to recognize that Uranus and Neptune might be yet a third class – with interiors that are much different than the others, and with unique formation-histories to boot. The discovery of large ice-balls in the outer solar system, of which Pluto is  the earliest known (and probably best) example, is merely the last step in that march.

So, planet or no, Pluto is going to be a great place to visit.  Too long a commute to live there, however.

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8 Comments on “So Is It A Planet?”


  1. I would have no great problem having a 50-planet solar system. Of course, the distinction “must be dominant enough to clear away objects in its orbital space” seems arbitrary now, but wait until you live there. Inhabitants will have names for the minor celestial bodies that cross their orbit, but they probably won’t allude to Disney characters. They’ll call them “The Brochure Never Mentioned This” and “Certain Death Eventually.”

    Fast forward to 2909. Pluto is reclassified into a group of bodies deemed “Too small to think about” and derided by the Perfect Ellipse Society as “Perturbed as if by seasonal mating hormones”, a decade before it gets hurled out of its orbit by Comet Obama and wipes out all life on Earth except for penguins.

  2. joe Says:

    Heh! I do like the way you think, Michael ;>

    The penguins survive because of Linux (Fedora 882, I would imagine), right?

    I’m sure we’ll find out in 2015 that Pluto is “Mostly Harmless”.


  3. Pluto is a planet. It is just a different type of planet than the terrestrials, gas giants, and ice giants. There is no reason schools or textbooks need to change anything, as the decrees by the IAU do not change what is out there. Only four percent of the IAU even voted on this, and most are not planetary scientists. Their decision was immediately opposed by hundreds of professional astronomers led by New Horizons PI Alan Stern. The IAU planet definition makes no sense because it says dwarf planets are not planets at all and because it classifies objects by where they are while ignoring what they are.

    A better, more inclusive definition is that a planet is a non-self-luminous spheroidal body orbiting a star. The spherical part is important because it means an object is large enough for its own gravity to pull it into a round shape, a state known as hydrostatic equilibrium and characteristic of planets and not of shapeless asteroids.

    Teaching only one interpretation–that of the IAU–when there is clearly an ongoing debate is disingenuous and a disservice to students of all ages.

  4. joe Says:

    That’s the crux of the argument, Laurel; Is the IAU being arrogant when they chose to define “planet” their way? I mean, it’s been a perfectly cromulent word for millennia, so who elected them “Webster’s”?

    But then again, the term should have some scientific precision to it, shouldn’t it? The definition you provided allows every Tom, Dick and Ceres to be deemed a planet (and that’s alright, especially since Ceres [em]and[/em] Vesta were deemed to be planets in 1805…) but there’s going to be a lot of little, round ice-balls out beyond Neptune that’ll want the title, and they won’t be distinguishable from cometary nuclei. And I’m not sure that this is even a problem!

    Just to name drop, I worked with Alan Stern for a bit, and he’s adamant. Pluto is a planet by tradition, and that’s as good an argument as any.

    I’d vote to call them Shakespearean Bodies myself – a rose by any other name…

  5. Kevin from NV Says:

    Pluto is a planet and should be considered as such, for another reason I haven’t heard discussed but should be: it has its own moon (perhaps two) orbiting it.

    This is a distinction that will affect how the body is approached, settled and populated in the centuries to come.

    Of course, this is more of an engineering point of view, not astronomical but as we spread out into our Home System I think its relevance will become clearer.

    Kevin Greene, P.E., M. ASCE

  6. joe Says:

    Actually, Kevin, it looks like Pluto has 3 moons. Nix and Hydra were photographed in 2005.

    I’m still recovering from the discovery of Charon in 1978!

    Did you know that there are asteroids with “moons”? Perhaps more accurately known as “double asteroids” (because the bodies are so similar in size), it’s worth considering that Pluto and Charon are in a similar state!


  7. Stern’s argument that Pluto is a planet is not based on “tradition.” It is based on his legitimate conviction, which many scientists share, that the primary factor in distinguishing planets from asteroids is that planets are large enough for their own gravity to pull them into a spherical shape. Very few Kuiper Belt Objects or objects in the asteroid belt meet this requirement. Cometary nucleii, which are clearly much smaller than any of these large objects, are almost certainly not in hydrostatic equilibrium.

    If using the spherical criterion results in there being hundreds of planets in our solar system, so what? Why do so many astronomers have such a problem with having a large number of planets? We have billions of stars and billions of galaxies. We don’t limit the Periodic Table of the Elements so there will be a small enough number to memorize. If our solar system has hundreds of planets, then that is what it has. It was not “designed” for our convenience.

    We can easily distinguish the many types of planets through use of subcategories, such as terrestrial planets, gas giants, ice giants, dwarf planets, super Earths, hot Jupiters, etc. Such a system is used to classify stars, and there is no reason it cannot also be used for planets.


  8. I agree that it’s just nomenclature; reclassification of the thing doesn’t change the thing. It’s not like we’re going to demolish Pluto because it’s less significant now.

    Science progresses in occasional leaps followed by long periods of quiet. Generations got used to there being nine planets, but with confirmation of the Kuiper Belt, that simple view has pretty much been smashed. Look how long it took us to adopt the Copernican model of the solar system. People died. (Now that was debating.) There’s a lot of nostalgia coming from the Nine-ists, but the revolution has come and there’s no turning back. The textbooks will need changed. When I was 8, I had a textbook that said Saturn had 20-30 moons. It’s out-of-date now. Just pick a definition that won’t outdate itself before the ink dries. For me, I’m not even comfortable talking about “eight planets”. I think it sets up the next generation for more Pluto-esque disappointment and gnashing of teeth. I’d rather say to kids “There are many hundreds of different bodies in our solar system, of which about 8 are relatively big.”

    If you really want to offend people, take Earth out of the definition of planet.


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