The following is my NSET (Natural Sciences) 111 paper. Or, as I called that class, NSET 2: Astronomical Boogaloo. The class covered more than astronomy. Biology, vulcanology, evolutionary biology, and environmental sciences. Astronomy is the one I'm more familiar with, so I chose a topic based in that field.
I decided to write about why Pluto was declassified as a planet. As you'll see, it really isn't a planet. It's more of a "dirty snowball," a comet that wasn't given proper fuel. I won't spoil the ending. Instead, I'll allow the research to speak for itself.
Deep-Sixing Pluto—How the Little Guy Lost His Planetary Status
In 2006 The International Astronomical Union made a decision that has impacted both astronomy, and our view of the universe. They gave Pluto the sailor's elbow, and declassified our former ninth planet to mere “Dwarf Planet” status. This was a controversial decision, one that has lead many to say, “How dare they?” But it has also lead astronomers to ask the question, “What constitutes a planet?” Originally, the word planet was simply the Greek word for “wanderer.” But now, a new debate has been created as to the exact definition of planethood. How this is defined will shape our view of the universe.
Tombaugh in 1930. Among his accomplishments Tombaugh was awarded the Rittenhouse medal in 1990.
Pluto was discovered by astronomer Clyde Tombaugh in 1930. Initially, Tombaugh was on the hunt for a presumed gas giant, that was believed to affect Neptune's orbit. Since then Astronomers have learned that such a spare gas giant does not exist. However, Tombaugh did find something else in the process. He found a spec that was moving across the sky at night. It tread step by step, while the stars appeared static. Tombaugh recognized it as another one of these classically-named “wanderers” in the night sky. The result was that soon a new planet was added to our solar system; the first to be discovered by an American. It was dubbed “Pluto,” after the Greek lord of the Underworld. And its distance from Sol, our sun, was such that it was thought to be an icy and barren world.
As it turns out Pluto is indeed icy and barren. Its surface contains frozen methane (natural gas), nitrogen, and even carbon monoxide. Its average temperature is 25K (Kelvin, the coldest temperature scale on record). It also has polar ice caps, much like our own Earth, and our closest neighbor, Mars. However, The temperatures on Pluto fluctuate greatly, yet they are still incredibly frigid. It has often been depicted as a tiny blue dot, with the distant rays of the Sun barely reaching it. It is a lone wanderer, an orphan in a sky full of giants.
When I was a child, I was taught that our solar system had nine planets. But as time went on astronomers began to doubt this. Pluto sits in a region of space known as The Kuiper Belt—a dense band of asteroids and similar objects. It extends from outside of Neptune's orbit at 30 AU (Astronomical units. Codified by the distance from The Sun to planets) to about 1000 AU. The Kuiper Belt functions as both an umbilical, and a nursery. It the birthplace of comets, but also the afterbirth of the Solar System. It is where the ice, dust, and rock that made planets such as our own can be found. It is the leftover material of worlds set adrift. It is a reminder of our past, long before Earth had finished accretion. As early as 1992 objects far larger than Pluto were sighted in this region. Yet, they were also not classified as planets. Why is this so? As it turns out, when it comes to planets, size does matter.
Pluto was originally thought to be nearly the size of Earth. It was also believe to be the possible cause of a perceived eccentricity in Uranus' orbit. But with further observation it was discovered to only be 2300 kilometers in diameter. It could easily fit inside Earth several times over. It would boggled the mind to think how many times it could fit inside of a gas giant such as Jupiter. Further more, its mass far lower than Earth's own moon. It also turned out that the so-called anomalies in Uranus solar orbit were just computational errors. Indeed, size does matter when it comes to planets. To put it bluntly, Pluto just doesn't measure up. It's not tall enough to ride the planetary roller coaster.
Another reason for the mislabeling of Pluto as a planet was lack of knowledge. Little was known about the Kuiper Belt in 1930. Even less was known about the objects beyond Neptune. These so-called trans-Neptunian objects, such as Eris and Makemake are better understood now. But in Tombaugh's time telescopes just weren't sensitive enough to find them. This was an era before anyone had literally set foot in space—let alone sent a space-based telescope such as Hubble. The tools available to Tombaugh at the time were not too dissimilar from those used by Galileo and Sir Edmund Halley. Telescopes, a keen eye, mathematical equations, and a lot of patience were the astronomer's equipment until the space age.
Image from NASA's Hubble Space Telescope in 2012. Pluto and its moons.
Planetary politics also plays a role in why some astronomers won't let Pluto stay in the underworld. NASA has undertaken some very necessary, and expensive, missions in recent years. They range anywhere from Mars colonization projects, to the exploration of Europa (one of Saturn's moons). Most recently NASA sent Rosetta, a robotic hitchhiker that thumbed a ride on a comet, the first mission of its kind. Despite being a success with that mission, and others, NASA has to continue to justify its funding. Congressional purse-strings for a mission like New Horizons (the probe that will observe Pluto) would be firmly drawn shut if Pluto were just a mere hunk of ice. The mission will cost NASA $700 million, and take over a decade. With a bill like that it's no wonder that self-interest will play a factor in future fiscal decisions. The sad truth of scientific funding, and the perpetual lack there-of, comes to the fore. It's not only a matter of prestige for NASA. It is also a matter of job security for a multitude of engineers, physicists, and launch crews.
The final nail in Pluto's coffin (yes, pun intended, as Pluto was lord of the underworld) was its own orbit. The International Astronomical Union used this as their criteria to define a planet. It wasn't Pluto's distance from our Sun, as such. It was more a case of Pluto chugging along with little fuel in its engine. As codified by the IAU a planet must “Clear its orbital neighborhood.” To explain further I must first give the IAU's full definition of a planet. Three criteria were chosen for this now technical definition. Ancient Greek philosophers like Aristotle would have loved to argue this definition, just as astronomers and the public do today.
First, a planet must orbit a star (not another planet). This is why large moons like Titan and Europa are not considered planets. They orbit Saturn, not the sun.
Second, it must attain hydrostatic equilibrium. This means that it has a mass large enough to sustain gravity, and accreted into a spherical shape (planets just aren't square or rectangular).
Third, and this is the key point, it must be “gravitationally dominant.” In doing so it must clear it's own neighborhood. That means that there are no objects larger than itself nearby. There are several trans-Neptunian objects that dwarf Pluto several times over. Also Pluto takes a ridiculously long time to orbit Sol (our local star, the Sun). It takes 248.6 Terran years for Pluto to orbit the sun. This means that it wasn't even one Earth year old when it was declassified as a planet. In cosmic terms it was barely out of its adolescence. Though Pluto does have natural satellites (Charon, Nix, Hydra, P4, and P5) the mere presence of them is not enough to elevate Pluto to planetary status. The presence of these natural satellites doesn't count for much. Mercury and Venus have no satellites, and yet they are full-fledged planets. What Pluto does qualify as is a “dwarf planet.”
The IAU defines a dwarf planet in similar, but slightly altered terms.
First, it must orbit a star. Okay, that's pretty straightforward there.
Second, it must have a spherical shape due to hydrostatic equilibrium. Pluto is good on that criterion as well.
Third, it must clear it's orbit.
Fourth, it must not be a satellite.
In other words; Pluto is defined as dwarf planet by what it is not, not what it is. The main differences between the two classes of planets are not necessarily size-based (per the IAU's definition) rather it is based on orbital path. The size issue is more one that astronomers use are part of their anti-Pluto arguments. One that does make sense, as Pluto is basically a small comet without a tail. If it were ignited, it would have been Comet Pluto, and been a more rare visitor than Hally's eponymous comet. It may be a dwarf planet to some, but to call it that would be refer to it as a planet at all.
In conclusion I agree that Pluto is not a planet, or even a dwarf planet. It should properly be referred to as a trans-Neptunian object. That tells us immediately what it is, and what it is not. It is an object that is beyond the orbit of Neptune. It does not clear it's own orbit. And it is part of the Kuiper Belt. It's greater claim to fame should not be that it was once thought of as a planet, but that it is residue from the creation of the solar system. Pluto is a bit of left-over building material that was not used to make a rocky, Earth-like planet, or serve as the core of a Jovian gas giant. Instead, it is there as a reminder of what could have been...and what once was. In that instance further study of this trans-Neptunian object is required. By understanding Pluto we will understand the nature of the our solar system. We will have a sort of baby picture of where we came from...and a preview of where we might be going.
Comins, Neil F. Discovering The Essential Universe (Second edition). pp. 138-140. New York.
W.H. Freeman and Company. 2004. Print.
“IAU 2006 General Assembly.” International Astronomical Union Website. IAU. N.D. Web.
“Pluto, Perception & Planetary Politics.” Jewitt, David and Luu, Jane X.
Daedalus, Vol. 136, No. 1, On Nonviolence & Violence (Winter, 2007), pp. 132-136. MIT Press American Academy of Arts & Sciences. http://www.jstor.org/stable/20028095
“The Problem with Pluto: Conflicting Cosmologies and the Classification of Planets.” Messeri, Lisa R. Social Studies of Science, Vol. 40, No. 2 (April 2010), pp. 187-214. Sage Publications, Ltd. Stable http://www.jstor.org/stable/25677402
Piantadosi, Claude A. Mankind Beyond Earth: The History, Science, and Future of Human Space Exploration. Columbia University Press, January 2013. Digital.
Yeomans, Donald K. Near-Earth Objects: Finding Them Before They Find Us. New Jersey. Princeton University Press, November 2012. Digital.
Blogger's Note: The images in this post did not accompany my original paper. They were added by me for some extra flair. My final grade: 10 out of 10!
Text Copyright R.J.X. Joyce 2015
All images are taken from NASA.
All images are taken from NASA.