S04E07: The Apology Insufficiency

Tonight’s show had a special guest star, Dr. Neil deGrasse Tyson, Director of the Hayden Planetarium in New York City.   As we learned from Sheldon,  Dr. Tyson was instrumental in defining a new class of solar system object, the “dwarf planet” — and then demoting Pluto into it.

Dr. Neil deGrasse Tyson, astrophysicist, guest starred on tonight's show as Dr. Neil deGrasse Tyson.

Several objects nearly the same size as Pluto share approximately the same orbit, one even larger.   If Pluto is a planet, then so must be the other five or so large round blobs orbiting our Sun (that aren’t moons).   Up until a few years ago you could recall their names in order with a simple mnemonic:   “My Very Elegant Mother Just Served us Nine Pizzas”  (for Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto).

Eris is bigger than Pluto and orbits the Sun at nearly the same distance. But it was never called a planet.


But as Dr. Tyson points out at the Hayden, let us not simply count planets.  More important,  the planets seem to fall into different classes. The first four are small and rocky with iron cores, typically called the terrestrial planets.  The next four are the gas giants.  Having two different types  of planets is not accidental.  As the solar system formed out of a blob of gas about 5 billion years ago, the innermost material was much hotter due to the proximity of the Sun.  So small dusty and metallic grains could form and coalesce, but nothing that would be gaseous.  Only out around the distance of Jupiter and beyond was it cool enough that methane and water would crystallize into ice and coalesce into the gas-giant planets.

At least so goes the theory.  We can’t watch our own solar system being formed.  But over the last two decades we started taking actual data, because we have the ability to observe planets around other stars. We are now on the verge of being able to measure the actual distribution of terrestrial and gas-giant planets around many different stars.  We’ll see what the data tell us.

We would probably have a fifth rocky planet but the constant gravitational influence of Jupiter as it orbits the Sun keeps the material beyond Mars from coming together and instead we find the Asteroid Belt.  Some are still large enough to forms spheres under the influence of their own gravity, such as the asteroid Ceres.   Ultimately the definition of a planet included the new criterion that it  have mostly cleared its orbit of other material.  Ceres and  Eris fail.  And so too went Pluto.

Would we have called Ceres a planet too? (windows2universe.org) Do definitions really matter?

Beyond Neptune we find a different kind of objects.  Out near Pluto are thousands of objects composed of rock and ice.  Their composition is similar to that of comets.  And it is no surprise. That is where many of the short period comets  (say 50-200 year orbits) originate from.  These objects form a third type of object, beyond the terrestrial planets and the gas-giant planets and form what is called the Kuiper Belt, a large collection of objects that also orbit the Sun in roughly the same plane as all the planets.  These are some of the objects of Raj’s research in previous seasons,  “Trans-Neptunian Objects”.

I think having the public ruckus over Pluto was good for science.  After all, we showed our benefactors that our ideas are not written in stone.  We demonstrated  a hallmark of science, that when a better idea comes along we are willing  to change our definitions and theories.

What really amuses me though is that this happened with astronomers, who otherwise cling onto old nomenclature more than any other field I know:

Let’s start with “planetary nebulae”.  A nebula is the word that astronomers give to any cloud-like object that astronomers find. So far so good.  Astronomers long ago found some of them around of stars.  At the time, astronomers thought that this was the gas and debris that lay in a planetary disk,  hence the name.  Astronomers now know, that these nebula are actually formed by stars in their death throes.  Gas is thrown off as the star ends its life.  It has nothing to do with forming planets. As long as astronomers are keen to fix up the definition of planet, why not fix up “planetary nebula” to something else as well?

The "helix nebula" is a cloud of gas puffed out by a star 700 light years away and is an example of a "planetary nebula". If you look carefully you can find it on the wall in Leonard & Sheldon's apartment.

Another pain left over from 2000-year old astronomy is the classification of the intensity of the stars in our sky, called apparent magnitude.   Now you might think that a larger magnitude is brighter, but you’d be wrong.  That’s backwards.  Fine, we can live with a minus sign.   The dimmest star you can see with your naked eye is 6.   The star Vega was chosen as the calibration point, 0.  Well actually Vega is magnitude -0.03.  But why change the scale?   Ever?     Once per 2000 years?  No.  Was Vega chosen because it was the brightest star in the sky?  That would be sensible.  Well close. It is fifth brightest.  It was the first star to be photographed, however, and that set the scale forever after.

To make matters worse, take a look at magnitude quantitatively.  Out here in California, we are used to talking about Richter scale for magnitudes of earthquakes.  And it is pretty simple.  A “magnitude 6” earthquake  is about 10 times the shaking as a magnitude 5 quake.   A quake with magnitude 7 is 100 times more shaking than 5.  That’s a good way to measure when the distributions vary so widely.  It is called a logarithmic scale, or ratio scale.   So is a magnitude=5 star 10 times as dim as a magnitude=4?   No.  What is it then?   A factor of 2.5119.   Why?   Because that’s the fifth root of 100.   Ask a silly question, you’ll get a silly answer.     This scale is left to us from the Ancient Greeks.   There’s nothing wrong with it, but it does make teaching it to non-science majors needlessly difficult.

Stars can classified by their surface temperature.   From hottest to coolest they are tagged with letters.  By now, you are not expecting A,B,C,D,….  Right.  It is O,B,A,F,G,K,M.   It is a relic of a previous attempt to classify stars based on the strength of the absorption of light by hydrogen.   It is easy to remember though: “Oh, Be a Fine Girl (Guy), Kiss Me!”.

At least the reclassification of Pluto is progress.  Astronomy will be around at least another 2000 years to slowly fix the others.   From now on we only need concern ourselves with “My Very Elegant Mother Just Served Us Nachos”.

26 Responses to “S04E07: The Apology Insufficiency”

  1. kuro Says:

    i liked pluto as well sheldon 😛
    i think i found it (the helix nebla). Is it on the window below the action figures?

  2. tudza Says:

    Hey, the nacho line is a good one. For us older folks it will be:

    My very educated mother just served us nine … oh never mind!

  3. Arturo Quirantes Says:

    Great to see Neil DeGrasse Tyson, one of my heroes, on TBBT! This season is turning out great Now we even have on it the classical Neil-just-killed-Pluto. Now we just need Spock and Salzberg doing cameos ;-))

  4. Roy Thompson Says:

    http://mcaf.ee/f8de4 You’ll find this a tad amusing.

  5. Gilles Lemagnen Says:

    /nit-picky on : 2nd paragraph lack the word “Saturn” in the brackets.
    /nit-picky off

    I learned a few things while liking the humorous tone of this post. Thanks !

  6. dmk Says:

    The mnemonic I learned in the 60s was: “My Very Educated Mother Just Showed Us Nine Planets”. I loved that it used the word “Planets” for “Pluto”.

    • Andrew from Vancouver Says:

      Ditto! That is the exact mnemonic I learned.

    • Ted Seeber Says:

      I learned my science from science fiction, thus Robert Heinlien’s menomic from _Have Space Suit Will Travel_:
      Mother Very Thoughtfully Made (a) Jelly Sandwich Under No Protest.

      T for Terra instead of Earth, and the “assorted asteroids” are in there. Same book included NEWTONIAN physics calculations for spaceships under various constant accelerations, and the distance of each planet from the sun in AUs.

  7. feldfrei Says:

    Thanks for this non-ideological comment on Pluto’s lost planetary status. In fact the debate about “Is Pluto a planet?” lead to the more general question “What is a planet?” and forced the scientist to find a (for now) quite general definition. In terms of units astronomers not only like magnitudes and parsecs but also the cgs system (since they rarely deal with electric quantities 😉
    Btw – weren’t planetary nebulas named that way because they look like a distant planet (as the new planet Uranus found by Herschel)?

    The lack of a planet between Mars and Jupiter reminds me my (BF) PhD exam when it was the turn of the theory professor (TP):
    TP: Do you know the Titius-Bode series?
    BF: Yes, it decribes empirically the radii of the planetary orbits.
    TP: Then you know that there’s a planet missing between Mars and Jupiter.
    BF: Yes, but there are many small ones.
    TP: Correct. Now I tell you, that once there was a planet and then some kind of an Auger effect took place (Auger effect was a subject of my thesis) kicking it out of the solar system – would you believe that?
    BF: hmmm…. there’s the swing-by mechanism (gravitational slingshot) for probes that can acquire enough energy passing by a planet to leave the solar system.
    TP: That’s very special. What about a real planet?

    Has anybody here an idea?
    (As an awkward matter of fact it took me a while to get the point … 😉

    • David Saltzberg Says:

      Nebula is latin for “cloud” or “mist” so that is the origin of the name. The word really just describes how it appears, not what it is. A nebula can be anything from a nearby dust cloud, to a distant galaxy which are very different objects. The point is that a nebula is the opposite of a point-like star because it is an extended object. (Planets and comets are also excluded from the category nebula, even though they are extended objects.) Herschel catalogued many nebulae (over 1000) in addition to having discovered Uranus. But I know of no etymological relation and don’t see why there would be.

      I don’t know who was the first to use the word “nebula” for these objects. If someone knows, leave a comment.

  8. feldfrei Says:

    According to this source http://www.brighthub.com/science/space/articles/23516.aspx it was Herschel who introduced first the term “planetary nebula” (just because of the apparent similarity to giant planets).
    Inspecting the Wikipedia article on nebulae http://en.wikipedia.org/wiki/Nebula I found this: http://www.eso.org/sci/publications/messenger/archive/no.49-sep87/messenger-no49-42-43.pdf
    Edmond Halley published an article in 1716 about six nebulae:

  9. Tejaswy Says:

    I think the highlight was the metric system and the failure of American system to implement it. In retrospect it caused the crashing of the mars climate mission

  10. Ben H. Says:

    Great episode!

    Check out this very recent and relevant scientific update. It turns out Eris might actually be smaller than Pluto. The reason we thought it was larger was due to it’s mass and the assumption that the compositions are similar. We are finding that Eris may actually be much more dense than we thought.


    also at the planetary society:


  11. psychtld Says:

    This thing would seem to be an issue of classification, and how it’s done, really. And this is an issue that cropped up in every discipline that i studied on my way to my M. Ed. degree (and beyond): psychology, mathematics, physics, archaeology and ethnology/anthropology). A key matter in deciding on classifications in all of these issues seems to have been how to make a classification… usually, it’s done statistically, but there has to be some sort of prototype for a category (or it’s impossible to actually classify anything), and there has also to be some understanding of dimensionality within the concept that defines the category. Understanding of continua (or near-continua) is essential… even in psychology (should that be ‘especially’ in psychology?).

    The ‘demotion’ of Pluto is not a demotion, per se: it is merely a reclassification. Not exactly something to have to apologise for!

  12. Tradução: “S04E07: The Apology Insufficiency (A Insuficiência do Pedido de Desculpas)” « The Big Blog Theory (em Português!) Says:

    […] feita por Hitomi a partir de texto extraído de The Big Blog Theory, de autoria de David Saltzberg, originalmente publicado em 4 de Novembro de […]

  13. Anonymous Says:

    Did you get a chance to speak with Dr. Tyson?What do two physicists talk about?

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