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Gerardus D. Bouw, Ph.D.


           By now most readers should be aware that Pluto is no longer counted as a planet.  Just how that happened is a matter of emotion and strongly-held opinions.  But to appreciate how that came to pass, we must first look at some astronomical history.


The Original Seven Planets


          Every elementary textbook on astronomy takes pains to explain that the word planet (wandering star) comes to us from the Greeks.  We find that Jude 13 speaks of gainsayers as “wandering stars, to whom is reserved the blackness of darkness for ever.”  The Greek there is asterej planhtai, asterez planetai, i.e., “stars wandering,” to put it literally.  The original definition of a star was any astronomical body’ meaning any object that is above the first heaven, the atmosphere of earth.  That includes what we now mean by the word stars, but also includes asteroids, planets, comets, meteoroids, and, when meteoroids hit the atmosphere, meteors. 

          From that definition, the Greek wandering stars, called “planets” for short, included the sun and moon, as well as the five classical planets, Mercury, Venus, Mars, Jupiter, and Saturn. 


The Copernican Revolution and the Number of Planets


          The Copernican heliocentric model, introduced in 1542, removed the sun and moon from the list of planets but added the earth to the list.  That reduced the number of planets to six. 

          Some fifty years later Galileo trained his telescope on Jupiter and found four satellites accompanying it.  He named them the “Cosmian Planets” after his patron, Cosimo de Medici, but Kepler regarded them as moons and dubbed them satellites, instead.


Herschel’s Planet


          During his lifetime, Sir William Herschel used his telescope to scour the heaven for comets.  In 1781 he discovered what he thought was a comet.  Further observation proved it to be a planet instead.  Some decades of argument later, the world settled on calling it Uranus.  At that point the number of planets was back to seven, namely, Mercury, Venus, Earth, Mars, Jupiter, Saturn, and Uranus. 

          In 1801, Giuseppe Piazzi discovered Ceres, a small body about 580 miles (930 km) in diameter orbiting between Mars and Jupiter.  Piazzi added Ceres (pictured showing its rotation at right) to the list of planets, making eight, but because it only appeared star-like through telescopes, meaning that it was too small to show a disk, Herschel classified Ceres and the other bodies later found forming a ring of objects around the sun and between the orbits of Jupiter and Mars, asteroids.  The name stuck, and Ceres was not counted as a planet.  Significantly, in 1833, Herschel’s son, John, who became the greatest astronomer of the nineteenth century, counted eleven planets: Mercury, Venus, Earth, Mars, Vesta, Juno, Ceres, Pallas, Jupiter, Saturn, and Uranus, which included four asteroids. 


Enter Neptune


          The discovery of Neptune in 1846 increased the number of planets to 12.  However, by mid-century, it was clear that the number of asteroids was too great, and their sizes too small to be counted as planets.  One of the 1853 editions of the Monthly Notices of the Royal Astronomical Society published a list of 23 asteroids under the title “Minor Planets.”  The label was kept and incorporated into the International Astronomical Union’s Minor Planet Center, which keeps track of the hundreds of thousands known asteroids. 

          Removing the minor planets from the count reduced the number to eight: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. 


And One Makes Nine


          In 1931, Clyde Thombaugh used an instrument called a blink comparator (pictured on page 116) to compare two photographs taken some days or weeks apart.  By alternatively showing one then the other in an eyepiece, objects that had moved in the interim appear to jump back and forth.  This was a common method of finding minor planets.  This time, however, it was not a minor planet that jumped back and forth.  It was Pluto.

          At the time of its discovery, Pluto was thought to have a mass about the same as earth’s.  Over time, it was found that Pluto was actually very much lighter and smaller than the earth.  Pluto is now known to be less than 1% the mass of the earth and is smaller than our moon.  Furthermore, Pluto’s or-bit is inclined by 17 degrees to the orbits of all the other planets, that is, the ecliptic.  For part of its year (as is the case at this present time), Pluto is actually closer to the sun than is Neptune.  As a planet, Pluto was clearly an oddball. 


The Kuiper Belt Objects


          In 1992 another object beyond the orbit of Neptune was found.  That was followed by another, and then another, until the number of such objects now runs into the thousands.  Each is a small, icy body that would probably show up as a comet if it approached the sun.  The area is known as the Kuiper Belt. 

          To make matters worse for Pluto, about a hundred bodies in the Kuiper Belt have an orbit that has about the same period as Pluto, namely, 248 years.  That is one and a half times as long as Neptune’s period of 165 years.  This means that these objects, dubbed plutinos, are locked to Neptune in a three-to-two ratio. 

          In 2003 Sedna was discovered.  That trans-Neptunian object was almost as big as Pluto.  The same year, a second body that still bore its temporary designation of 2003 UB313 during the IAU meeting, proved to be even larger than Pluto.  If it is a planet, its naming is the responsibility of the IAU’s group for planetary-system nomenclature.  Otherwise its naming is the responsibility of the Minor Planet Center.[1]  The uncertainty as to which committee should name it helped precipitate the crisis that eventually demoted Pluto.


Problems Confronting the IAU Nomenclature Committee


          It is informative to list the problems and concerns that confronted the IAU Committee last August:


1.        Children may not be able to learn the names of 50 planets.  On the other hand, they learn fifty states and their capitols; at least, they were able to do that forty years ago.

2.        New technology such as the Hubble telescope has revealed factors that could not have been foreseen in prior centuries.  This precipitated the issue in the first place.

3.        Little children love Pluto; don’t demote it.  Then, too, there is tradition to consider. 

4.        If astronomers in the 1930s knew Pluto was actually smaller than the moon, they would not have called it a planet.[2]

5.        If we rank the solar system bodies by size, is there a break in the distribution that we could use, and if there were several, would we break at Pluto’s size, or smaller, or larger?

6.        A 19-member committee after two years of debate could not define the word, “planet.”  Could the present committee in several weeks come up with a definition that had escaped the prior committee?


It was the last issue—the definition of planet—that the nomenclature committee decided to tackle.  There are two ways to define a planet.  One method involves its shape, and size.  The other method is dynamic, by how it interacts gravitationally with other bodies in the solar system.  The size ranges from Jupiter to specks of dust.  Some of the asteroids exhibit structural and dynamics of both planet and asteroid. 


The Shape of a Planet


          The committee, chaired by astrophysicist and historian of astronomy Owen Gingerich, considered the spheroidal shape of an object as a possible defining property for a planet.  If a body is large enough that its gravity pulls it into a sphere, that could define a planet.  Rocky bodies become spheres above 0.1% earth’s mass, which happens at a diameter of about 500 miles (800 km).  Icy bodies, such as Pluto and its moon, Charon, become spheres at about half the size of rocky bodies. 

          Indeed, the dirty ice balls are much more numerous than rocky ones.  They all reside beyond the orbit of Neptune and have orbits that are eccentric and significantly inclined to the ecliptic plane.  For that reason, Dr. Gingerich proposed that these objects be called plutons.  In addition, a pluton must have a period in excess of 200 years, a boundary that also divides short-period comets from long-period comets.  In that way, the name would preserve Pluto’s historic role as the first-discovered object as well as solve their difference to the major planets.  Though the proposal removed Pluto from the list of planets, it would add Ceres since that body has been confirmed to be spherical, as can be seen from the figure on page 115.  The proposal made a lot of sense. 

          But sense is in short supply these days.  Dr. Gingerich prepared a press release to head off the possibility of misunderstanding.  He used the clause, “eight classical planets, Ceres, and a growing number of plutons.”  But the writers of the press release rejected the accurate wording and wrote their own, which was not related to the committee’s work.  They announced that Pluto would still be a planet and that there would be twelve planets in all.  In addition to the traditional nine, the list included the soon-to-be-renamed 2003 UB313, and Pluto’s largest moon, Charon.

          The complaints stormed in.  The nomenclature was getting too complicated for the children.  The press covered the dissidents.  Objections arose from many fronts.  Several members of the dynamics faction, feeling ignored by the largely structuralist approach, put forth a resolution to add to the definition that a planet must be “the dominant object in its local population zone.”  They believed that Ceres and the icy bodies, including Pluto, should be eliminated from the list. 

          As a result of the strategic blunder of the press release, neither the original proposal nor the dynamic faction’s proposal could garner a majority vote.  As if the matter of hurt feelings was not enough, multiculturalism reared its ugly head when Andrea Milani objected that in Romance languages, Pluton is the name of Pluto.  Never mind that it was the Americans who discovered and named the planet, and that it was not their fault that the Romance languages decided to use their own spelling instead.  Nevertheless, the attendees overwhelmingly rejected the plutons label for the dirty ice balls.  They also rejected plutonians.  


Back to the Drawing Board


          With the public relations storm raging about them, the committee honed in on a simplification of the definition that would assuage some of the critics of the original proposal.  Even with the endorsement of the world’s largest group of planetary scientists, the Division of Planetary Sciences of the American Astronomical Society against them, the dynamics faction persisted in their vehement opposition to the proposed wording.  They insisted that the wording acknowledge the dynamical “evolution” of the solar system. 

          Some of the dynamics faction were thus added to the committee, including the aforementioned multiculturalist, Milani.  The net result of that debate added the contentless and ambiguous wording that a planet must be “the dominant object in its local population zone.”  Before the final resolution was composed, that ambiguous wording was replaced by an even worse one, “cleared the neighborhood around its orbit.”


Definition of a “Planet” in the Solar System

          Here is the text of the resolution, complete with footnotes, entitled “Definition of a ‘Planet’ in the Solar System,” that was adopted by the IAU in Prague August 24, 2006:

          Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding.  This applies, in particular, to the designation “planets.”  The word “planet” originally described the “wanderers” that were known only as moving lights in the sky.  Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.

          The IAU therefore resolves that “planets” and other bodies in our solar system be defined into three distinct categories in the following way:

1.        A “planet”[3] is 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,[4] and (c) has cleared the neighborhood around its orbit.

2.        A “dwarf planet” is 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, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.

3.        All other objects[5] except satellites orbiting the Sun shall be referred to collectively as “small solar-system bodies.”


The Idiocies of Multiculturalism and Evolutionism


          There you have it.  The greatest example of the fruit of inclusionism, evolutionism, and multiculturalism you have ever seen.  I have never seen an amalgam of any two of those, let alone all three, that produced anything that deserved a better rating than stupid, and these definitions are no exception.  Let us examine some of the problems inherent in the definitions. 

To start, consider the definition of a dwarf planet.  We have dwarf people, but they are still people.  Astronomy has dwarf stars and dwarf galaxies, but a dwarf star is still a star and a dwarf galaxy is still a galaxy, yet a dwarf planet is not a planet.  It is a “celestial body,” not a planet; read the definition.  Recall, too, that Pluto falls into the “dwarf planet” category and that the press releases told us that Pluto is no longer a planet.  There you have it, I’ve exaggerated nothing. 

Another problem occurs in definition (1)(a) which says that planets orbit the sun.  By design, it excludes all the planets detected around other stars.  Can we still call them planets?  They cannot be called dwarf planets [definition (2)(a)].  Maybe we can call them planetoids.  It is clear that the definition is far from complete. 

Then there are the grammatical errors.  Consider definition (3), where it says “All other objects except satellites orbiting the Sun….”  The only satellite I am aware of that meets this definition is our moon, which from the sun’s perspective always follows a path that is concave to the sun.  This is not the case for other satellites.  So, according to (3), the moons of all the planets are classed as “small solar system bodies,” but not our Moon, which is left unclassified.  That is what a couple of missing commas can do.[6] 

Finally, consider the addition insisted on by the dynamics faction— those who wanted to incorporate evolutionary ages into the text via orbital evolution definitions (1)(c) and (2)(c).  Mark Sykes of the Planetary Science Institute noted, “The problem with this definition is that it is too simple and leads to nutty consequences.”  A Scottish newspaper carried the headline, “Pluto Row Could Lead to Neptune Losing Planet Status.”  The reason?  Neptune has so far failed to clear the “dwarf planet” Pluto from its neighborhood.  That makes Neptune a “dwarf planet” according to definition (2)(c).  Then there is earth.  It had not yet cleared out of its way the earth-grazing asteroids.  Indeed, one of those, Apophis, may pass within 25,000 miles from the surface of earth in 2029.  It follows that the earth is not a planet.  Jupiter is accompanied by two clusters of asteroids called Trojans.  Jupiter also has an entourage of comets, one of which crashed into Jupiter in July of 1994.[7]  There is no chance that Jupiter will ever be clear of these so Jupiter does not qualify as a planet either.  We expect bodies at the Lagrangian points of all planets, but we have not looked at those points for all planets.  Probably, when all the observations are complete, there will be no planets left in the Solar System; at least, not according to the IAU’s 2006 definition.  Clearly, evolution enstupifies. 

The best we can say for the IAU members who voted for this idiocy is that they were probably so tired of the childish bickering that they voted for anything—even be it idiotic—just so they could adjourn and go home.  At worst, the remaining members actually thought they had a working definition and really believed in it.  I prefer to believe the former and, if I, and the saner IAU members are right, when next the IAU reconvenes in 2009, they will scrap this piece of idiocy and come up with a working model, one that will also address issues skirted by the adopted resolution. 

The two issues not covered in the proposed definition are the maximum mass issue and exoplanet issue. 

There is a point at which a planet is so massive it starts to noticeably shine by its own light.  The energy source may be gravitational collapse, chemical, or nuclear.  At some point a line must be drawn where a body is no longer a planet but becomes a brown dwarf, that is, a star.  This is probably the most difficult issue at hand.

The exoplanet issue deals with the hundreds of planets that have been discovered orbiting other stars.  This issue can be solved by using the mass definition, but if dynamic considerations are thrown into the mix, the solution is rendered so artificial as to be impossible. 


[1] Note added in proof: in September, after the IAU’s ruling on Pluto, UB313 was officially named Eris after the Greek goddess of discord and strife.  Its moon is called Dysnomia after the goddess of lawlessness.  Eris’s earlier nickname, Xena, was dropped.  So far, more than 300 planetary scientists have signed a petition protesting Pluto’s demotion. 

[2] This is a specious argument.  It only makes sense in hindsight, knowing that there were other, smaller bodies beyond it.  If Pluto were unique, it would still have been called a planet, even if astronomers had known it was smaller than the moon. 

[3] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

[4] An IAU process will be established to assign borderline objects into either dwarf planets and other categories.

[5] These currently include most of the solar-system asteroids, most trans-Neptunian objects, comets, and other small bodies. 

[6] It should effectively read, “All other objects, except satellites, orbiting the Sun….”

[7] Unruh, J. T., 1994.  “Jupiter and the comet encounter of July 16-22, 1994,” B.A. 4(70):5.