Pluto: Still Not a Planet

Eris, the icy ball bigger than PlutoLast night was the conference banquet and after party. Many of us stayed up too late. Many of us looked at our conference program this morning in hopes that maybe something boring was going on that we wouldn’t mind sleeping through. All of us dragged ourselves out of bed and came to the 8:30am Sunday morning talk. Michael Brown, the premier finder of small icy-rocky things in the outer solar system, is giving a talk titled, “How I killed Pluto and why it had it coming.” How can anyone sleep through a talk like that?

He just showed us digitized versions of the original discovery plates for Pluto and it is now talking about how odd Pluto was known to be even at the moment it was found by Clyde Tombaugh. Everyone was looking for another Neptune or Uranus sized object. Instead, they found something smaller than the moon! This wasn’t expected, and astronomers at the time of its discovery didn’t have the right noun to attach to this new object. It wasn’t an asteroid – It reflects too much light and isn’t located in the parts of the solar system we expect to find asteroids. It wasn’t a comet – It doesn’t come into the inner part of the solar system grow a tail, and go back out (although we now know that if it came into the inner part of the solar system for some reason it would indeed grow a tail). When your only choices are comet, asteroid or planet, Pluto becomes a planet.

But today, we have more bins to throw objects in: Kuiper Belt objects, Oort Cloud objects, asteroids, planets (and various sub-divisions of each). And with these new bins it becomes important to consider the labels we give things.

But more on that later.

So Clyde found Pluto. If Pluto belongs to a non-planetary bin, where are the other objects. Well – Brown is finding them today. Next question: Why did it take so many decades to find other objects in the outer solar system? Well, the technology made it hard. Brown pointed out that Tombaugh’s images covered roughly the area on the sky of a hand (palm fingers and all) held out at arms. Modern images, however, cover an area the size of a small freckle on a hand held at arms length. When you are searching the sky a freckle at a time, it is hard to find things quickly. So, we are more sensitive to things we were never sensitive to before, but it is more time consuming to find them.

Currently there are 1100 other icy objects known to exist in the outer solar system. Some (or at least 1 named Eris) are bigger than Pluto. Most are smaller. They mostly have these crazy orbits that are tilted and stretched into ellipses. Kuiper Belt objects orbit in a swarm, buzzing about Neptune.

So, when you look at Pluto as just one of many objects darting around Neptune, and when you recognize Pluto’s not the largest of these objects, you need to work on defining bins. If Pluto is a planet, what other smaller objects are also planets? Or perhaps Pluto is not a planet? According to the International Astronomical Union, none of these tiny objects are planets. They are Kuiper Belt objects, plutions, dwarf planets… They are many things other than planets.

So what is a planet? Well, if you want the scientific reasons, go listen to our Astronomy Cast podcast on the topic. Let’s instead imagine, as Brown is telling us to imagine, that you are an alien flying into solar system. As you fly through you see this swarm of small icy things in the outer solar systems followed by these giant gassy blobs. Between the round gas blobs and the rocky worlds are potato shaped giant rocks. The rocky worlds then orbit the sun. An alien would dump things into 4 general bins, big round rocks orbiting the sun, small randomly shaped rocks forming a belt, gas giants, and icy swarming objects. Since the terrestrial planets and gas giants both orbit in orbits with nothing else (other than their moons), you can see these as two parts of the same (we call it planet) bin. At the same time, the asteroids and Kuiper Belt objects – the stuff that swarms – can be seen as two parts of the same (we call it minor planet) bin.

Pluto, left to an outside observer, just wouldn’t get called a planet. This, to me, is the best explanation of why it is silly to call Pluto a planet. One of these objects just isn’t like the other ones. That would be Pluto in a picture of the planets. Put it in a picture of Kuiper Belt objects and now it fits.

Pluto, get over yourself. You’re not a planet and you never were. Someone just misfiled you.

Interesting parting bit: Brown has been hinting at a new large (think Sedna) object may be getting announced in the not too distant future. Gotta love hints.

So, is anyone still wondering “Why is Pluto not a planet?

9 Comments

  1. Michael Welford August 5, 2007 at 6:00 pm #

    First off: “original discovery plates for Jupiter”?

    Second: Not all transneptunians have wild orbits. In the Kuiper main belt, eccentricities are typically lower than for Mars.

    Third: The IAU planet definition isn’t crappy because it excludes Pluto. It’s crappy because of the Orbital Bully Clause ( also known as “clearing the zone” ).

    More after recharging…

  2. Michael Welford August 7, 2007 at 6:13 pm #

    I’ve objected to the IAU planet definition before. ( Here and on other sites ) But don’t worry about me endlessly repeating myself posting on that subject. I’ll be raising a different objection each time.

    According to a recent theory, there was a major reconfiguration of the outer system at the time of the Late Heavy Bombardment ( possibly including Neptune and Uranus switching orbits ). This theory involves Neptune migrating out toward its current orbit and encountering the early version of the Kuiper Belt. But, this early belt was hundreds of times more massive than at present. According to the IAU, Neptune was a nonplanet until it tossed most of this mass away. And it’s possible that it had a more or less clear orbital zone before migrating out. This would have Neptune being a planet in good standing for half a billion years before the Late Heavy Bombardment, then not being a planet for a couple of hundred million years during the Late Heavy Bombardmdnt, than becoming a planet again afterwards. To me that’s just screwy.

    With regard to a possible new dwarf planet, something with the mass of Sedna would be a yawner. Something with an orbit like Sedna would be exciting.

  3. pamela August 8, 2007 at 2:25 am #

    Hi Michael,

    Can you please provide links to peer reviewed journal articles containing this theory and to a statement explaining why Neptune wasn’t a planet? I’m afraid I have never encountered these arguments.

    There are currently as many theories for planetary formation as there are known solar systems (the theories may actually exist in larger numbers). It would be interesting to see what data this theory has substantiating it.

  4. Krishna August 8, 2007 at 10:54 am #

    Hi! Not being an expert, professional, or even a particularly good amateur astronomer, I don’t have the knowhow to engage in this discussion at the same level as the rest of you, but how about a “layperson’s” perspective:

    Even if I were to accept the IAU’s definition of a planet as excluding Pluto, I would argue that it’s ok for us to call Pluto a planet anyway for historical and cultural reasons (that is, to make an exception). There are plenty of examples of silly conventions in science and mathematics that get carried on simply for historical reasons, even when knowledge should make them obsolete. One that comes to mind is how we indicate the direction of current. Everyone knows which way the electrons are really moving, but like mules staring down an approaching avalanche, we continue to draw the arrow the other way, probably because we like using our right hands instead of our left hands.

  5. Michael Welford August 8, 2007 at 9:01 pm #

    Pamela you asked:
    Can you please provide links to peer reviewed journal articles containing this theory and to a statement explaining why Neptune wasn’t a planet? I’m afraid I have never encountered these arguments.

    Theories connecting the Late Heavy Bombardment to planetary migration have been around for a few years.

    The idea that Neptune temporarily lost its planetary identity was my own. Based on the IAU planet definfition and my perhaps limited understanding of those theories.

    The basic story is this. 4 1/2 billion years ago planetary migration was a stately process. Objects from a ( hypothesised ) massive outer disc of protoplanetary debris would drift into the gravitaional influence of the 4 giant planets and get tossed about randomly. Frequently such objects would end up being tossed out of the solar system by Jupiter. As these objects got tossed around they exert a gravitional influence on the objects doing the tossing. ( They may have been small but collectively they ( are assumed to have ) had significant mass. The net result was that Jupiters orbit gradually migrated in towards the Sun while Saturn, Uranus and neptune migrated out.
    Then about 4 billion years ago the migration brought Jupiter and Saturn into a 1:2 orbital resonance. This altered their orbits to the point that the orbits of Uranus and neptune were disrupted and Neptuned barreled into the outer disc. This sent even more material from the outer disc inward, which speeded the migration but also tended to circularize the orbits of the giants. Jupiters in-migration sent objects from the “asteroid main belt” to the inner solar system, which was the Late Heavy Bombardment.

    Here is a letter to Nature on the subject:
    ^ Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets,
    Gomes, Levison, Tsiganis, Morbidelli – Nature 26, May 2005.
    http://www.nature.com/nature/journal/v435/n7041/pdf/nature03676.pdf

    There are some more links here:
    http://www.psrd.hawaii.edu/Aug06/cataclysmDynamics.html
    and at Wikipedia
    http://en.wikipedia.org/wiki/Late_heavy_bombardment

    Pamela you also said:
    It would be interesting to see what data this theory has substantiating it.

    As far as I know it’s mostly computer simulations so far. The theory will be constrained as we learn more of the orbits of transneptunians. Also orbits in Jupiters Trojan regions would not be stable during this migration, so the present day trojans would have to come from the outer belt.

  6. Laurel Kornfeld May 3, 2008 at 2:12 am #

    “Pluto, left to an outside observer, just wouldn’t get called a planet. This, to me, is the best explanation of why it is silly to call Pluto a planet. One of these objects just isn’t like the other ones. That would be Pluto in a picture of the planets. Put it in a picture of Kuiper Belt objects and now it fits…Pluto, get over yourself. You’re not a planet and you never were. Someone just misfiled you.”

    I’m not wondering; I know the definition adopted by four percent of the IAU is wrong and that Pluto IS a planet. The obvious split among astronomers on this issue makes it clear that different observers will come to differing conclusions. Yes, Pluto is different that the other eight “classical planets” but that does not disqualify it from being a planet. Rather, it tells us we need to make “planet” a broad category with multiple subcategories.

    We have the terrestrial planets, the gas giants, and the ice giants. Why not add a new category, the ice dwarfs? This category would apply to any KBO that has achieved hydrostatic equilibrium, meaning it has enough self gravity to have pulled itself into a round shape. Most KBOs do not fit this description. In fact, only ONE KBO larger than Pluto has been confirmed, and that is Eris. There is no reason that these large, round KBOs cannot fall into two categories and be both KBOs and planets.

    With new discoveries both in our solar system and others, we are likely to find that there are far more types of planets than we previously thought. That is all the more reason to broaden our conception of what a planet is. They are not all alike even in our solar system. Earth and Jupiter are very different. And none has totally “cleared its orbit” of neighboring asteroids. Neptune certainly has not “cleared its orbit” of Pluto.

    When a new definition like the IAU’s is immediately met with a protest petition by over 300 professional astronomers who argue the vote was “hijacked” in a backroom deal and describe it as “sloppy science that would never pass peer review,” something is very wrong. It’s the four percent of the IAU, most of whom are not planetary scientists, who came up with the linguistic nonsense that a dwarf planet is not a planet at all, that are misleading people and doing a disservice to the public and to astronomy.

    I choose to follow the much more sensible planet definition put forth by my astronomy instructor Al Witzgall: “A planet is a non-self-luminous spheroidal object orbiting a star.” That’s it. It differentiates planets from asteroids yet leaves room for a wide range of objects to qualify as planets.

  7. tezza October 21, 2008 at 12:35 am #

    hi i honestly think that i love ya

  8. kevin September 25, 2009 at 1:09 pm #

    pluto is a planet or earth is not a planet

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