Welcome Back, Pluto

Science may be, should be, rethinking its decision to kick Pluto out of the planetary club
by Gordon Campbell

Pluto is on the comeback trail. The respected Scientific American science journal says so, here.

As you’ll recall, the one-time ninth planetary member of our solar system got kicked off the team thanks to a controversial vote taken by a meeting of astronomers in Prague in 2006. Well, Pluto has just received a fresh vote of confidence from science. The Hubble telescope has recently determined that Pluto has five moons. If having moons counts as a planetary credential that puts Pluto well ahead of Mercury and Venus (none) Earth (one) and Mars (two). Pluto has also been determined to have an atmosphere. More precise measurement has also confirmed that Pluto (and not Eris, as formerly believed) is indeed the largest object orbiting our Sun beyond Neptune. Yes, it is small – but as Pluto’s defenders keep on pointing out, a dwarf star is still a star, and Tom Cruise is still a person.

In other words, the grounds on which Pluto was kicked out have always seemed contrived, largely to justify its eviction. A couple of years ago, Werewolf published a long and detailed account of the finding, naming, and casting out of Pluto from the planetary club. It also contained examples of the songs and parodies written in Pluto’s defence, and excerpts from the laws passed by state governments in California and New Mexico to re-instate Pluto as a planet, at least while it was passing over their territory.

As the Werewolf article also noted, Wellington’s Evening Post newspaper had contended at the time of Pluto’s discovery (in a March 22, 1930 report headlined “ A New Planet?”) that those rich and excitable Americans had probably got the whole thing wrong. Even after it did finally concede that Pluto did exist, the Evening Post lavished far more space ( see EP, May 27, 1930) on what this discovery might mean for people who believe in horoscopes. From the Evening Post report :

Mrs Elizabeth Aldrich, editor of “The Astrologer” writes that Planet X will enable her to step up her accuracy from 75% to 85%…There are two schools of thought among astrologers. One contends that the planet has always been exerting an influence, for good or evil, on mankind. The other holds that a planet remains absolutely neutral or harmless until it has been discovered, whereupon it begins to operate. This school does not indicate the means by which the planet is advised of its having been ‘spotted’ on Earth.

Some “mild disagreement” did exist, the Evening Post further noted, as to the identity of the new entrant :

“Evangeline Adams says it is known as Isis, the Rev. Arthur W. Brooks says its name is Vulcan or Lilith, Mrs Aldrich calls it The Great Unknown, and Jacob Levie refers to it as The Invisible.”

I’m not going to repeat the entire Werewolf article, but some of its content may be worth boning up on. Especially if Pluto is getting back its planetary status – and what better time to put things right than July 2015 ? That’s when Pluto will finally be reached by NASA’s New Horizons space probe, which will be winging its way past Pluto that month, before heading off into deep space. That timing is a karmic co-incidence. The New Horizons probe was launched in 2006, in the very same year that Pluto was being struck off the planetary books. Wouldn’t it be entirely appropriate that the outrage of 2006 should be put right by a man-made visitation fired into space at the same time that the original injustice was being perpetrated ? Truly, science works in mysterious ways.

To be rigorous about this, it comes down to the classification criteria. Planetary status depends on who’s looking, and with what pre-conceptions. After all…from the viewpoint of Jupiter and the other gas giants, the solar system would probably look like a bunch of smallish, rocky planets close to the sun, with another group of smallish icy planets out in the other direction, and with neither group looking like they have much in common with the big guys.

The Prague meeting that expelled Pluto had used three qualifying criteria for planethood, namely :

1. The object must be in orbit around the Sun.

2. The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of hydrostatic equilibrium.

3. It must have ‘cleared the neighbourhood’ around its orbit. i.e it must dominate all other objects on its orbital path.

As Werewolf pointed out in 2011, that third condition is the really contentious one. Arguably, even Jupiter hasn’t ‘cleared its neighbourhood’, given how many asteroids are trailing before and aft, along its orbital path. Many entities cross over Neptune’s orbit, including Pluto, which has clearly worked out an accommodation with it. In other words, the condition of ‘clearing the neighbourhood’ turns out to be a highly subjective notion. And if you picked a different criterion – does it have a moon in orbit around it? –then Pluto qualifies hands down. As the astronomer Alan Stern pointed out in 2010, Pluto also has a few other relevant attributes:

In fact, Pluto, and its cohorts, are planets. They have all the attributes of planets. Let me give you some examples. They have cores. They have geology. They have seasons and atmospheres. They have clouds. They have polar caps in many cases. They have moons. And I can’t think of a single distinguishing characteristic that would set apart Pluto and other things that you’d call a planet, other than its size. So, I like to say, a Chihuahua is still a dog.

Finally…does it really matter all that much? More to the point, why does Pluto’s treatment matter to the people for whom it clearly matters a lot? in the New Yorker last month, Adam Gopnik tried to get to grips with the nature of Plutonic Love:

Whatever Pluto is…seems secondary to our understanding of why it’s a weird spot in the solar system. Science doesn’t only or even primarily ask “What is?” questions; it asks “How come?” questions. Not “What is gravity?” but “How come apples fall down instead of up?” What is man? Well, there is no answer, because he is not an essence but a long story, with lots of damn-close-to-a-man, sort-of-a-man, just-about-as-good-as-a-man, might-as-well-call-it-a-man, before we get to man—and we’re altering, too, even as we argue. The question of whether Pluto is or is not a planet is interesting only because it is, so to speak, the popular residue of an ever more interesting argument. It is really a shorthand way of asking, How was the solar system formed, and how many kinds and categories of objects can its history provide?

This kind of reasoning causes a panic because it seems that if you don’t have a definition, or have one that can be altered every few years, you must just have a guess. The resentment against science that is current now when it comes to subjects like climate change is not that scientists know too much; it is that they are not in a sufficient state of hysteria about not knowing for certain. They admit their fallibility, and still insist that they are worth listening to. This is the best guess we’ve got, they say, and the best guess we’ve got is the best guide to go on.

As Gopnik says, the authority-anxious types have been playing this kind of story out for a very, very long time :

The odd thing is that, historically, acting on the best guess available has turned out to work out rather well, while relying on the sure thing known for certain since the beginning of time and revealed in a holy book has turned out to be a pretty poor strategy for finding out how this planet whirls among the stars while creating prosperous and peaceful places on it. Between anarchy and authority falls argument, and the arguments, even the seemingly remote ones, are always where the cool (if not truly, Plutonianly cold) stuff can be found.

Fine. Yet Gopnik seems to be begging the question here, somewhat. After all, who are the medieval dogmatists here – the people who drummed Pluto out of the solar system because it did not conform to their arbitrary agenda, or the people who’ve been clamouring to re-establish the old planetary order? And the answer is, surely : both of them. In these postmodern times, we have good reason to be wary of both the rationalists and the traditionalists.

In the meantime, Pluto has done nothing to deserve being treated as an outcast. We’ve done it wrong, and it is time to make amends.


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  1. tussock, 29. July 2014, 16:52

    Dude, no. Science. You’ve got to be consistent. So Alan Stern also thinks that Moon, Io, Europa, Ganymede, Callisto, Titan, and Triton are planets, as well as Ceres, Haumea, Makemake, and Xena the Warrior Princess.

    You can’t pick and choose. If you ignore orbital clearance, you get all the dwarf planets. If you ignore co-orbiting bodies you get Charon as well as Pluto. There’s almost no reason to leave out Titan and Triton either, they’re just captured dwarf planets. Moon is huge, and really easy to see, count it too, the ancients did.


  2. Gordon Campbell, 30. July 2014, 10:58

    Don’t give lectures about the need for consistency. The point I made in the article was that orbital clearance has been used inconsistently to rule out Pluto, despite neither Jupiter nor (arguably) Neptune having cleared the objects in their orbital paths.
    Once you recognise orbital clearance isn’t an absolute, then no, you don’t “get all the dwarf planets” . The status of Ceres was resolved in the 19th century when the asteroid belt between Mars and Jupiter was defined as a special category, and only the expulsion of Pluto has re-opened that debate. You also don’t need to include the moons of other planets, which takes care of many contenders on your list. Do the others (Haumea, Makemake, Xena etc) have several orbiting moons, as Pluto does ? I’m curious.
    Just because relative smallness shouldn’t necessarily rule you out, it doesn’ t mean that relative largeness necessarily rules you in. The size factor is interesting, though. When Eris was briefly thought to be bigger than Pluto, the anti-Pluto crowd were happy to use that as ammunition against Pluto.. Now that science has found Pluto to be definitely bigger, this is suddenly irrelevant to the argument.
    In sum, I’d argue that planetary status is a subjective taxonomy. Relative orbital clearance, relative size are considerations but obviously, they’re not absolutes ; and the existence of moons is a contributing but not decisive factor. On that basis, I’d argue the subjective case is for the inclusion, not exclusion of Pluto.

  3. Raj Pillai, 30. July 2014, 19:55

    Gordon… excellent article.. 100% agree..
    Highly recommend all those interested in a serious debate about Pluto’s status, its science etc should join the Facebook open group “Society of Unapologetic Pluto Huggers” ..
    Gordon: I post a lot on that page and its only about Pluto.. Feel free to post about Pluto on that page too..
    We do have many serious planetary scientists/ Pluto lovers (incl: Dr Alan Stern, PI of New Horizons) following this page and contributing…
    Love to see pics of Pluto taken from NZ by amateurs… Just an FYI..
    Raj Pillai

  4. Mike Wrathell, 31. July 2014, 3:12

    All the dwarf planets should be planets. No one says there are too many rivers to all be rivers. The sun is a yellow dwarf star, yet dwarf stars are considered to be stars by the IAU. Talk about consistency. Dwarf planets should be a subcategory of planet. The Earth could not clear its orbit if it were as far out from the Sun as Pluto. Ceres and Charon should be planets, too, as well as Makemake, Haumea and Eris as they are confirmed dwarf planets by the IAU. I recognize their authority, but they have to use their power in a responsible way. The well-documentary shenanigans at the 2006 General Assembly in Prague that led to the demotion of Pluto were shameful. I watched video the the sesssion in which the rancid resolution was ramrodded. Pluto huggers were cut off in mid-sentence. One man was threatened with the destuction of his career if he voted for Pluto and admitted it on Facebook. I have more horror stories, too.

  5. Mike Wrathell, 31. July 2014, 3:16

    I just posted a long comment and it disappeared. I run a Facebook group dedicated to replanetizing Pluto. If you love Pluto, please join. Society of Unapologetic Pluto Huggers. The scientific case for replanetization is solid. You can learn the case and learn all about Pluto if you join. For one thing, the Sun is a yellow dwarf star, yet still a star. Why, then are dwarf planets not a subcategory of planets? Talk about consistency.

  6. Laurel Kornfeld, 31. July 2014, 8:26

    What is wrong with including all the dwarf planets as planets? The idea that we cannot have “too many planets” because kids won’t be able to memorize their names has no scientific value. The solar system has whatever number of planets it has. Memorization is not important for learning. We don’t ask kids to memorize the names of all the rivers or mountains on Earth or of Jupiter’s 67 moons. The important thing is that kids understand the different subtypes of planets and their characteristics.

    Ceres actually is a planet too, according to the geophysical planet definition, which states that a planet is any non-self-luminous spheroidal body orbiting a star, orbiting another planet, or free floating in space. 19th century astronomers did not have telescopes powerful enough to resolve Ceres into a disk. Today, we know it is spherical and therefore a small planet. Like Pluto, it may even harbor a subsurface ocean.

    As for the spherical moons, Dr. Alan Stern, Principal Investigator of the New Horizons mission, suggests we refer to them as “satellite planets.” This idea was first proposed in the 19th century. Moons that are spherical have the same characteristics as planets–geology, weather, and differentiation into core, mantle, and crust. Moons not large enough to be spherical are not rounded by their own gravity, meaning they are shapeless rocks that do not have the complexity the spherical ones do. These should not be considered planets.

    But Europa, Enceladus, Titan, Triton, even Earth’s moon–what is wrong with considering them satellite planets. They too may have subsurface oceans and are complex worlds, some of which are top contenders for hosting microbial life.


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