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The Pluto Controversy


Is Pluto really the ninth planet of our solar system or just one of many objects orbiting our sun in the Kuiper Belt beyond Neptune?  The great debate is raging among planetary astronomers.  The verdict of history is often the triumph of reason over passion and truth over prejudice.  So it will eventually be with Pluto.


The Discovery of Planet X


Clyde Tombaugh was a young man when he discovered Pluto.  His work was done at Lowell Observatory, an observatory founded by a wealthy amateur astronomer, Percival Lowell.  Lowell was not what you would consider a mainstream astronomer in his day, but rather is testimony to the triumph of well-financed enthusiasm over academia.  His original pursuits of proof for life on Mars met with defeat during his lifetime.  He was convinced that he could see canals on Mars and invested in the largest refractor of the day and commissioned his agents to explore the western United States to find a mountaintop with perfect seeing to build an observatory for that fine instrument.  In this manner, Lowell Observatory was founded on Mars Hill on the outskirts of Flagstaff, Arizona.  It was drawings made of Mars that landed Clyde Tombaugh a job at Lowell Observatory in 1929.  Clyde had made these drawings at the eyepiece of his 9inch Newtonian on the farm in Burdett, Kansas.  One of those early Tombaugh drawings still hangs in the clubhouse of the University of Kansas Astronomy Club on the KU campus in Lawrence.  The drawings and Clyde’s enthusiasm impressed the people at Lowell Observatory.  On 15 January 1929, Dr. V. M. Slipher welcomed Clyde to Lowell Observatory and put him to work on the new 13-inch telescope taking photographic plates in search of Planet X, a proposed ninth planet that Percival Lowell was trying to find before his death on November 16, 1916.  No more planet searching had been done for the 13 years since Lowell’s death.


Clyde was given the use of the new 13” scope and assigned the task of getting it working and then using it to take glass plate images of suspected parts of the sky where Planet X was computed to be.   He then used the Carl Zeiss Blink-Comparator to scan the pairs of plates produced a few days apart.  After months of rough equipment work, the actual search began in April 1929.


On February 18, 1930 Clyde found Pluto on plates taken January 23 and 29.  The discovery was later confirmed on new observations and on March 13, 1930, Percival Lowell’s birthday, the discovery was announced to the world.  In less than a year Clyde had done what others had spent decades trying to do.  However, the discovery was the result of new technology being applied to produce vast quantities of data.  Clyde’s work was the result of a year of concentrated effort, almost Herculean by most accounts.  He literally thrashed the sky.  But by the age of 24 he was famous the world over.


This great discovery allowed Clyde to go to college and earn an undergraduate degree at the University of Kansas and later a Masters Degree from Northern Arizona University.  Clyde finished his career teaching at New Mexico State University and died at home in Las Cruces, New Mexico on January 17, 1997, four days before his 91st birthday.   The Clyde Tombaugh story ranks right up there with the stories of Horatio Alger as an inspirational story of achievement from humble beginnings.  The naming of Pluto was as much a convenient tribute to Percival Lowell (PL = Percival Lowell) as it was a mysterious name of the god of the underground keeping with the naming conventions of the astronomical community.  It was at its time a triumph of amateur astronomy and professional amateur collaboration. 


However, Tombaugh himself continued his search for planet X until 1943, because he was not convinced that this was the object he was searching for.  There was some discussion at Lowell that this might be a distant satellite of another planet and an urgent search was made in the region of Pluto before an announcement was made.  Tombaugh did not find any other TNO’s because the relative albeido of Pluto was 60% vs. the 4-5% of most of the other TNO’s. 


Some Early Ironies


There are three early characteristics of Pluto and its discovery that lie at the source of later controversies about the object and emerge later in arguments regarding questioning its status as a planet.  


Pluto has an orbit inconsistent with the other planets.   The orbit of Pluto is inclined or tilted by 17 degrees to the plane of the other eight planets.  In addition it is so elliptical that for the 20 years from 1979 to 1999 Pluto has actually been the 8th planet of the Solar System, closer to the Sun than Neptune. This would indicate that Pluto may have had an origin different from the other eight planets or was the victim of some massive cosmic collision.  For many years the popular theory on the origin of Pluto was that it was a moon of one of the gaseous planets that had been knocked out of orbit by a collision or near miss with some other object.  This strange orbit did not seem so peculiar while it was unique to Pluto, but rather became one of those interesting oddities that made the ninth planet more interesting and special. 


The math was wrong.  Neptune had been discovered as a result of mathematical calculations made to explain the inconsistencies in the orbit of Uranus.  At the time Newton had just introduced his latest mathematical explanations of the mechanics of orbits and these were applied to the observations of Uranus and proved that another massive body existed that was causing Uranus to speed up or slow down in its passage around the sun.  Several competing astronomers worked to predict and then find the eighth planet that was causing this phenomenon.  And so Neptune became the first planet discovered as a result of the application of mathematics.  Astronomers were interested in determining if Neptune explained all of the inconsistencies in Uranus’s orbit or was there some other object out there?  As fortune proves, if you look hard enough you will find what you are looking for.  Uranus was still being perturbed by something else besides Neptune.  It was being slowed down and speeded up by some foreign body, called Planet X.  And the search was on.  Hence, Clyde got his job at Lowell Observatory.    After the interplanetary spacecraft made measurements sent to the outer planets all of these minor errors were shown to be computational errors and another body was not affecting Uranus.   However, by then much time had passed. 


Pluto isn’t very big.  In addition to showing that the math was wrong, the new observations confirmed that observations proved that Pluto is smaller than our own moon and has a mass just two tenths of one percent of the Earth. That's just a fraction of what would be needed to account for the presumed motion of Uranus that started astronomers looking for a ninth planet in the first place.   Clyde just got lucky. 


The fact is Pluto is smaller than many objects in our solar system.  For now it is the largest of the TNO’s, but that status could change any day.


The Kuiper Belt


Gerard P. Kuiper proposed a theory in 1951 that a disk of cometary objects existed beyond the orbit of Neptune and that these icy objects were the source of many of the comets that entered the inner solar system.  Among Kuiper’s other discoveries were the atmosphere of Titan (1944), the carbon dioxide atmosphere of Mars (1948), Uranus’s satellite Miranda (1948), and Neptune’s satellite Nereid (1949).  Kuiper’s theory was not really proven until a decade after his death.


Kuiper was actually the first one to question the planetary status of Pluto in the 1950’s.   He suggested it might have been an escaped moon of Neptune.  The press picked up on this at the time saying Pluto was not a planet.


In the 1960’s in “The History of Astronomy”, Patrick Moore suggested, “Pluto may be just the largest member of an outer asteroid belt.”


Around 1988, astronomers David Jewitt (University of Hawaii) and Jane Luu (University of California at Berkeley) began searching for members of the Kuiper belt with a CCD camera coupled to the 2.2-meter reflector on Mauna Kea, Hawaii. The equipment was capable of detecting objects as faint as magnitude 25.  After nearly 5 years of systematic searching they found a distinct image on August 30.46, 1992, which was subsequently designated 1992 QB1. This was followed on March 28.42, 1993 with the discovery of 1993 FW.  By June 19, 1995, 28 objects had been discovered. 

How many objects exist within the Kuiper Belt? The Hubble Space Telescope Kuiper Belt Search Team obtained 34 10-minute exposures of a single 4-square-arcmin field during the period of August 21.7 to 23.0, 1994. Final analysis of the images, which showed objects as faint as magnitude 28, revealed 59 possible candidates that would be equal to or greater than the size of Halley's comet. From this number they estimated that 60,000 such objects would exist in every square degree.

The long-thought asteroid Chiron, which in recent years has begun showing cometary activity, is the largest member of the Centaur objects, which orbit between Jupiter and Neptune. It is now generally accepted that these objects were tugged out of the Kuiper Belt sometime in the past. Another recently suggested possibility is that Pluto and its moon Charon may be the largest known object in the Kuiper Belt, and that some of the icy moons of the outer planets may be also be former members that were captured.

The Kuiper Belt is also our closest link to the circumstellar disks found around other main sequence stars, and an understanding of the physical processes operative in the Belt (both now and in its early stage) will mark a key step forward in understanding the problem of planetary formation.

The known Kuiper Belt objects fall into three distinct dynamical classes: the Resonant KBOs, the `classical' KBOs, and the `scattered' KBOs.   The Resonant KBOs are objects, which lie in or near the mean motion resonances of Neptune. Some have also been found in the 4:3 resonance at 36.4 AU and the 5:3 resonance at 42.3 AU. However, the 3:2 resonance at 39 AU (which counts as its members Pluto and its cousins, the `Plutinos') is by far the most populated, contributing to 1/3 of the known KBO’s. 

There are now 366 KBO’s with many more being discovered every day.  Some of the more recent discoveries have been quite large; although some astronomers now believe these objects should not be called Kuiper Belt Objects, but rather Trans Neptunian Objects.  Many astronomers, soon after the discovery of Pluto, were considering its probably small mass and its non-major-planet-like orbit as indications that this was an unusual object akin to comets or minor planets shows that this was a logical way of thinking. Astronomers realized that "major planet" categorization did not make complete sense, and this sensibility evidently remained through the 1930s, 1940s, and 1950s as one astronomer after another made brief comments inferring the meaning of Pluto in terms of a family of objects beyond Neptune. So it may be unwise to call these objects anything other than "trans-Neptunian objects", or "cubewanos", "plutinos", and "scattered-disk objects".

In 1996, the TNO 1996 TO66 was discovered and its size was estimated at 600km in diameter. (NATURE, June 9, 1997)  This compares to a diameter of 2300 km for Pluto and 1200 km for Pluto's satellite Charon.   This discovery and the likely discovery of even more and larger TNOs contributed to a proposal by Brian Marsden, International Astronomical Union's Central Bureau for Astronomical Telegrams and Minor Planet Center, that Pluto be given joint status as a planet and a minor planet.  This proved to be one of the most inflammatory episodes in the Pluto controversy.

Marsden’s Proposal

In a special Minor Planet Center bulletin M.P.E.C. 1999-C03 Issued 1999 Feb. 4, 16:04 UT Brian Marsden proposed that Pluto be given dual status as both a planet and as minor planet number 10,000. 
According to the circular,  the principal reasoning for this is the recognition during the past few years that Pluto was the first discovered and largest known member of the "Transneptunian Belt" (sometimes called the "Kuiper Belt" or "Edgeworth-Kuiper Belt") of small objects beyond Neptune that possess some similarity, at least dynamically, to bodies in the Cisjovian Belt.”  
Marsden went on to say that this was not to be considered a “demotion” of Pluto from planetary status, but rather that this would make Pluto the king of the Kuiper Belt.  Marsden voiced an opinion that he doubted that a KBO larger than Pluto would ever be found.  He finished his circular with a request for the astronomical community to send him email with their opinion as to whether or not they felt like this was a worthy proposal.  He allowed three weeks for these responses to be made.
This editorial circular published by Dr. Marsden sparked a heated commentary that raged for months and ultimately became so venomous that the International Astronomical Union made the decision not to give Pluto dual status as minor planet 10,000.   This was despite the fact that a significant majority of the scientific community polled by Marsden were in favor of the dual status.  This created the beginnings of a rift between the IAU authorities and the scientific community that eventually widened further over the following months.  
The popular press picked up the story and it became a topic in newspapers, magazines and on radio and television talk shows.  Eventually, this became such an emotional topic that school children were spot lighted for their opinions on whether their favorite “Disney character” should be a planet.  Long time friends and supporters of Clyde Tombaugh voiced outrage at the idea that Pluto might lose its planetary status.  
At one point "Kevin Zahnle, also a space scientist at NASA Ames, said Pluto is a true-blue American planet, discovered by an American for America."  --Leonard David,, 30 January 2001
The media circus played itself out over the course of a year and a half and finally died down to a low rumble.  Meanwhile, more TNO’s of larger size were being discovered every month.  
Asteroid number 10000 was assigned to 1951 SY, an asteroid discovered on September 30, 1951 by A. G. Wilson at Palomar.  The citation says:
The Greek word for ten-thousandth, Myriostos honors all the astronomers, past and present, from around the world, professional and amateur, observer and orbit computer, who participated, over an interval of 198 years, in the achievement of accumulating 10,000 minor planets with orbit determinations of the highest quality. (Minor Planet Circular No. 34632)   The citation goes on to quote the essential text of M.P.C. 33615 regarding Pluto.  
Hayden Planetarium’s Exhibit of the Solar System sans Pluto
In February of 2000, the Rose Center for Earth and Space in New York, a division of the American Museum of Natural History, opened a new exhibit of the solar system in the lobby of the world famous Hayden Planetarium.  The exhibit exhibited scale models of the planets of our solar system, but only eight planets were on exhibit.  Pluto was missing.  This minor oversight went unnoticed and unannounced for almost ten months.  Finally in January 2001, newspapers and the media started writing flaming editorials denouncing Neil de Grasse Tyson, the Director of the Hayden Planetarium for his decision.
His response was:  "There is no scientific insight to be gained by counting planets.  Eight or nine, the numbers don't matter."
--Neil de Grasse Tyson, Hayden Planetarium, 27 January 2001
The strangest development of all was when Dr. Mark Sykes, Chairman of the Division of Planetary Sciences at the American Astronomical Society and an astronomer at the Steward Observatory in Arizona decided to pay Dr. Tyson a visit.  Their meeting was documented and transcripts were provided to the media.  
Dr. Sykes visit and Dr. Tyson’s reaction can be summarized by their opening exchange:
Dr. Sykes:  “When people come in, they are expecting to see what astronomers think. What you’ve got up here is not what astronomers think.”
Dr. Tyson:  “It’s what some astronomers think.”
The end result of all of this is Dr. Sykes wrote some negative comments about the Hayden exhibit and Dr. Tyson put up a plaque on the railing saying 

Recent significant discoveries of TNO’s
2000 WR106 was discovered November 28, 2000 by Bob McMillan and Jeff Larsen, of the Spacewatch project at the University of Arizona using a telescope on Kitt Peak (Minor Planet Circular 2000-X02 dated December 1, 2000).  The excitement over this discovery was due to the immense size of the object.  It is now thought to be as large or larger than Ceres, the largest known asteroid.    It may still be a little early to say for sure how big 2000 WR106 is.  It depends on what its albiedo is.  
2000 WR106 was assigned number 20,000 and later named Varuna, after one of the oldest of vedic deities, the maker and upholder of heaven and earth.  (Minor Planet Circular 42368)  
The second largest known member of the Kuiper Belt now has a minor planet number.
1998 WW31
On December 22 and 23, 2000 three astronomers using the 3.6-m Canada-France-Hawaii Telescope were observing 1998 WW31 and discovered it had a moon.  They appear to be 40,000 km apart.  (Minor Planet Circular 2001-G29)  This discovery eliminated another distinction that Pluto was the only object in the Kuiper Belt with a moon.
There are many astronomers who believe that the designation of Pluto as a planet is wrong.  The majority of the serious researchers in the field are of the opinion that Pluto is one of thousands of objects circling the sun in resonant orbits and otherwise in the Kuiper Belt.  As new discoveries are made of larger TNO’s and TNO’s with moons similar to Pluto and Charon, there is growing enthusiasm for this view.
There is continuing debate regarding the definition of what a “planet” is.  Past arguments that it circle the sun, be spherical, and have a moon, now do not exclude other objects like 1998 WW31 and may exclude Mercury and Venus.  Research on planetary systems of other stars are turning up great variety of systems vastly different from our own.  
The Pluto debate is an emotional one for those who grew up with Clyde Tombaugh and Walt Disney.  It may cause a rethink of the way we define the elements of star systems.  Indeed the term “planet” may be as obsolete as the concepts of astrology in due time, and probably as slow to die.