Who is to blame for the demotion of Pluto?
The former 9th planet in our solar system has been an extraordinarily controversial topic since 2006 when it was demoted from planet to “dwarf planet”. The demotion raised many questions about modern science, such as “What specific properties does an object need to have to be classified as a planet?” and “How have these properties changed since the discovery of Pluto?” The answer is that they haven’t changed a bit; in fact our knowledge of Pluto has increased greatly since its discovery. The evolution of modern science is the direct cause of Pluto’s much publicized demise.
Some would say that Pluto was doomed from the start. Astronomers were looking for a relatively large planet outside the orbit of Neptune, enticing them to make any object they found to fit into their theory, despite its actual size. Pluto was once imagined to be much more massive than it is in reality, which is the prime factor in its demotion. There is no doubt that mistakes about the physical properties of Pluto were made for a very long time, so the question becomes not “Why was it demoted?” but rather, “Was it ever truly a planet to begin with?”
There is no room for sympathy or nostalgia in science. Pluto staying a planet just because it has been a planet for a while would undermine the integrity of science as a whole. I myself used to be very upset about the demotion of Pluto, it felt like they were taking a part of my childhood away. It also gave me this uneasy feeling that something you were taught in school growing up could be completely wrong. Then I realized that nobody was intentionally lying to us because Pluto indeed was a planet at that time. It made me curious as to what in the world happened to convince scientists that we should remove one of our 9 planets? Without proper research, it is easy to just blame it on grumpy scientists, and some scientists did get “dragged through the mud” during the demotion process.
In order to understand why Pluto was demoted, we must first understand exactly what Pluto is. Pluto lies in an area of space about 50 AU from the sun called the Kuiper belt. It is an area filled with many smaller objects, similar to the asteroid belt. The Kuiper belt is far more massive than the asteroid belt though, nearly 20 times as wide. Pluto was thought to be the largest object in the Kuiper belt until the discovery of 3 other objects in 2005 that are roughly the same size as Pluto. These dwarf planets were named Eris, Makemake, and Haumea. The objects in the Kuiper belt are so far away from Earth that none were even seen until the 1970’s. We now have counted over 1100 Kuiper belt objects, although there are surely more. The immense distance from the sun is an important factor in why Pluto was thought to be larger than it actually is. An object that far away was extremely hard to measure with any precise accuracy up until very recently.
The surfaces of many of the Kuiper belt objects are often similar. They are mostly an icy mix of water ice, ammonia, and light hydrocarbons such as methane. The composition of the Kuiper belt objects are nearly identical to the composition of comets. Pluto is no different; it has a surface of mostly frozen water with some frozen methane. The density of Pluto is what you would expect from a world of frozen water, about 2100 kg/m cubed. Because of the presence of frozen methane, we can assume that the surface temperature of Pluto is no greater than 50 degrees Kelvin.
There is a chance that Pluto may have a very thin atmosphere comprised of methane due to the frozen methane that it’s surface contains. There are also hints that Pluto may have bright polar caps. We won’t know more about the make-up of the solar caps or their nature until 2015 when the New Horizons mission arrives near Pluto. The New Horizons mission was a mission sent out by NASA in 2006 to explore Pluto and other Kuiper belt objects.
The orbit of Pluto is one of the most peculiar traits it has. It is very elongated and it is also tilted at a 17.2 degree angle to the ecliptic plane. Because of this irregular orbit, Pluto is actually closer to the sun than Neptune for a period of about 20 years. When Pluto is at perihelion, it is 29.7 astronomical units away from the sun, which is inside the orbit of Neptune. It takes a considerable amount of time for Pluto to orbit the sun; a year on Pluto lasts 248 Earth years!
In the diagram above you can see how odd Pluto’s orbit actually is. Even though it looks like the orbits of Pluto and Neptune intersects, the planets are never at risk of colliding with one another because they keep a constant 3:2 resonance. This means that Pluto’s orbital period is exactly 1.5 times Neptune’s orbital period. Due to Pluto’s lengthy trip around the sun, it will not cross Neptune’s path again until the middle of the 23rd century.
Pluto also has three known moons, Charon, Nix, and Hydra. Charon is by far the largest, and was discovered nearly 30 years before Nix and Hydra. Nix and Hydra were both discovered in 2005 and were named in part because of the initials of the New Horizons mission that I mentioned earlier. Pluto’s moons are unique because they all orbit Pluto in extremely compact orbits. Charon and Pluto have even been considered ‘double planet’ because it is larger compared to its primary than any other planet or dwarf planet in our known solar system. The moons were also thought to be made the same way our own moon was created, through impact theory.
One of the most interesting stories in the history of Pluto is the way it was discovered, the discovery also alludes to reasons why it was demoted. From the late 1800’s into the early 1900’s, astronomers began to agree that Neptune’s mass was not enough to account for all the influence it had over Uranus’ orbit. In 1908, William Pickering was one of the first astronomers to suggest that beyond Pluto laid a planet that was roughly twice the size of Earth. His predictions were largely ignored, until the American Percival Lowell predicted the same thing, that there much is another very large planet beyond Neptune.
Earlier in his career, Lowell had convinced himself that Mars was home to intelligent life similar to that on Earth. He called the planet he was expecting to find beyond Neptune ‘Planet X’ and began his search for it by carefully observing the motion of Uranus. Lowell died in 1916 never ended up finding ‘Planet X’ himself.
Lowell was a good astronomer, but I can’t help but think that there is something inherently erroneous about a scientist searching for something that he already believes is there. It makes him much more prone to disregarding small facts to the contrary in order to prove that he is right. It is similar to ghost hunting shows, they are expecting to find ghosts, and therefore any bump or strange noise that they hear must be a ghost because that is what they are looking for. It may be a key trait for conspiracy theorists, but I believe it to be a poor quality for a scientist.
Fourteen years after Lowell’s death another American astronomer Clyde Tombaugh discovered the elusive ‘Planet X’ using Lowell’s observatory. He found it remarkably close to where Lowell was originally looking also, only 6 degrees off. Although, in the 1980’s, calculations concluded that Pluto was not at all massive enough to have caused the irregularities in Uranus and Neptune. The prediction of Pluto being there was correct, but the size of it was wildly inaccurate. Below is a picture of Percival Lowell in his famous observatory.
The demotion of Pluto in 2006 came as a shock to many pedestrians, but in the realm of astronomers, it was sort of a long time coming. It was demoted from a planet to a new classification called a ‘dwarf planet’ on August 24, 2006 by the International Astronomical Union because it did not have all 3 characteristics an object needs to be considered a planet. If you look back to its discovery though, you have to ask yourself “Was it even deserving of planetary status to begin with?” It was less than half as large as it was expected to be, and was not the factor for the irregularities in the nearby planets obits. We may have been too hasty to call it a planet to begin with.
The controversial demotion became even more controversial in 2010. When the 3 other dwarf planets were discovered (Eris, Haumea, and Makemake) they initially thought that Eris was larger than Pluto, which made the demotion a little easier to swallow. It wouldn’t be fair for Pluto to be a planet if there was a larger object near it. Then in November 2010 Eris passed in front of a distant start which cast a shadow and made a more precise reading of the size of Eris possible. It turns out that Eris is almost certainly smaller than Pluto, making Pluto the largest of the Kuiper belt objects still.
Public outcry by Pluto lovers worldwide has been heard since its demotion. This data stirred the pot once again by suggesting that perhaps Pluto should be reinstated as a planet. What needs to be understood is that what makes a planet is not simply its size alone. Saturn’s moon Ganymede is larger than Mercury for instance, but I never hear any public outcry that this should be its own planet. What makes a planet a planet is more complicated than some may think.
The first thing that an object needs to be considered a planet is the fact that it orbits around our sun. This has also stirred some controversy because it only applies to our solar system. Any planet that is found not orbiting our sun cannot, by definition, be a planet until the definition is amended. Regardless of this loophole, Pluto does in fact orbit the sun.
The second characteristic that an object must have is that they must have enough mass to create enough gravity to render them round, called hydrostatic equilibrium. Most objects in the solar system with a size greater than 400km obtain this characteristic. This happens to icy objects at an even lower mass, which is what Pluto is. Pluto has this characteristic as well, the 3rd characteristic a planet must have is where Pluto and the other dwarf planets run into trouble. Going by just the first 2 criteria Eris, Haumea, and Makemake would also have to be planets, and I think people would have gotten even more upset if the IAU added 3 formerly unknown planets to our solar system.
The third characteristic states that an object must “clear its neighborhood” of the smaller objects that are near its orbit. The International Astronomical Union inherited this criterion in 2006 along with the first two in order to make it easier to decipher real planets from dwarf planets. What clearing its neighborhood means is that the body has become gravitationally dominant and there are no objects of comparable size within its proximity. There is no mathematical equations for the term, so it’s mainly subjective and up to astronomers to decide exactly what it means, and they decided that Pluto does not meet this final criterion.
In all the hoopla surrounding Pluto’s demise one thing is clear; Pluto was not demoted without reason. Many people feel nostalgic about Pluto and can’t understand how something like this could happen. This is a mistake on their part though, it’s not that they cannot understand what happened, it’s that they do not want to. They don’t go out and do the necessary research that would tell them exactly what happened to Pluto.
People need to think about Pluto in the large scale of things. Would you rather science not change something just because the majority of the population believes it to be true? If this were the way that science worked it would not be science anymore and we will still believe that the earth is flat. Correcting mistakes is a cornerstone of proper implementation of the scientific method and I, for one, am happy that they got this one right.
I loved Pluto as much as anyone else who felt a sting when they first heard the news that it was demoted and no longer a planet. As time goes by I’m realizing just the opposite is happening, Pluto has by no means “fallen off the map.” It is more popular now than it ever was as a planet, the smallest, furthest away planet to boot. It would have never been talked about this much had it been left to be, nor would it have been truly a planet still. Revoking its planetary title was the right decision in every way.
Pluto has been praised in numerous different ways since its demotion. In 2006 the American Dialect Society voted the word “plutoed” the word of the year, beating out “YouTube” by over 40 votes. The word plutoed means to be demoted or devalued, as happened to Pluto earlier that year. Two years later in 2008, the IAU decided to create a term commemorating Pluto. Any dwarf planet similar to Pluto is now referred to as a “plutoid”.
This tells me that the population’s feelings towards Pluto are misguided and not well thought out. Like Shakespeare said, “A rose by any other name would smell as sweet.” Pluto is still there, and it can still be remembered and seen through a telescope. If anything, Pluto has now reached legendary status, and legends never die. Pluto has made all of us feel something that we never before thought we could feel for a planet. Human emotions, pity, regret, nostalgia, these are not normal feelings to have for an inanimate object. In some ways Pluto’s demotion has morphed it from a planet, into a friend. Who doesn’t want to give Pluto a big metaphorical hug and say “Don’t worry buddy, we’ve all been there.” Three cheers for the smallest guy on the team, the black sheep, the runt of the litter. Pluto may have been eradicated from the textbooks, but it will forever remain in our hearts as the planet that was.