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Why Is Pluto Not a Planet?

History of Pluto's Discovery

Pluto was previously considered a planet since it’s discovery in 1930 by Clyde Tombaugh. Pluto had little resistance to its classification as the ninth planet upon its discovery. This may have been a slight case of confirmation bias since the presence of a “ninth planet” or “planet X” was hypothesized by Percival Lowell. Lowell believed that the apparent discrepancies of Uranus were the causation of another planet beyond Neptune. Pluto was within only 6 degrees of the predicted location of the ninth planet as predicted by Lowell. However, Lowell predicted that this planet would be around 6.6 the mass of Earth.

As telescopes improved, we were able to learn more about Pluto. In 1950, Gerard Kuiper was able to observe Pluto as a spherical world using a 200-inch telescope at Mount Palomar. Kuiper estimated that Pluto’s diameter was around 5,900 kilometers. Pluto was observed as it passed near a star in 1965. This was an opportunity to gather more information about the size of Pluto and reasonable inference of its mass. Pluto would either cover the star and cause an occultation or Pluto would barely miss the star if it is a small body. Pluto missed passing in front of the star.

Based on this observation, Pluto had to be at least less than 6,800 kilometers across, verifying Kupier’s measurements. This size meant that Pluto was smaller than Mars. In 1988, using spectroscopic analysis, we found that Pluto is covered in frozen methane. Frozen methane is highly reflective. Pluto appeared dimmer than what it should have been considering it’s the size. This meant that it had to be a little smaller than we initially thought.

At this point, astronomers had a rough estimate regarding Pluto’s size. But what about its mass? That information came with the discovery of Charon in 1970 by James W. Christy with the help of Robert Sutton Harrington at the United States Naval Observatory. Using Charon’s orbital period, Harrington was able to calculate the mass of the Pluto. Pluto’s mass turned out to be significantly less than what was previously estimated, about one-twentieth the mass of Mercury, making it the smallest planet by a wide margin.

Furthermore, Pluto’s estimated mass, along with its orbital eccentricity and high orbital inclination are also factors that differed significantly from the other planets. Scientists began to consider the possibility that Pluto may not be a planet in the traditional sense after all.

Pluto's Fall from Planethood

Pluto’s descent from planethood continued into the ’90s. With the discovery of Quaoar, Sedna, Eris, Makemake and other trans-neptunium objects in the Kuiper Belt with similar characteristics as Pluto, the debate began amongst astronomers as to the true nature of these planetary objects. Astronomers believe there could be as many as 200 similar objects. Either these objects had to classified as planets, or Pluto had to recategorized.

Robert Harrington estimated the satellite’s orbit using the information given to him by Christy regarding Charon. Robert then used this information to compute Pluto’s mass. It was the first time astronomers could make a direct calculation like this — Charon had been the breakthrough they needed. The mass of Pluto was estimated to be just two-tenths of one percent of the Earth. There was no way Pluto could have disturbed Uranus’s motion to cause the observed discrepancies.

Nearly 30 years after it’s discovery, astronomers determined that Pluto was too small to be responsible for orbital discrepancies seen in Uranus and Neptune.

Pluto's Present Day Classification

Pluto was finally classified as a dwarf planet in 2006 during their 26th General Assembly of the International Astronomical Union by failing to meet the IAU’s new classification of planets. The new criteria were: 1. The body must orbit around the Sun, has to have sufficient mass to have a nearly round shape (hydrostatic equilibrium) and has to have”cleared the neighborhood” around its orbit.

Pluto fails to meet the third condition. Pluto’s mass is substantially less than the combined mass of the other objects in its orbit, in contrast to Earth, which is 1.7 million times mass in its orbit (excluding the Moon). The IAU further determined that bodies that meet criteria one and two but do not meet condition three would be called dwarf planets.

Both the public and the astronomical community have resisted the reclassification. Alan Stern, the principal investigator with NASA’s New Horizons mission to Pluto, continues to the debate the topic. Stern points out that, by the terms of the new definition, Earth, Mars, Jupiter, and Neptune, all of which share their co-orbits with asteroids, would be excluded. He also argued that all big spherical moons, including the Moon, should likewise be considered planets.

Public reception to the IAU decision was mixed. Many accepted the reclassification, but some sought to overturn the decision with online petitions urging the IAU to consider reinstatement. A resolution introduced by some members of the California State Assembly facetiously called the IAU decision a “scientific heresy.”

The New Mexico House of Representatives passed a resolution in honor of Tombaugh, that declared that Pluto will always be considered a planet by the state. This resolution also made March 13, 2007, Pluto Planet Day. The Illinois Senate passed a similar resolution in 2009, on the basis that Clyde Tombaugh, the discoverer of Pluto, was born in Illinois. The resolution asserted that Pluto was “unfairly downgraded to a ‘dwarf’ planet” by the IAU.”

Final Thoughts on Pluto

I agree with the IAU. There is no possible way that Pluto should be considered a “planet” with equal status as the eight planets. The dwarf planet seems like an accurate designation since it acknowledges that Pluto is a planet of sorts. Pluto shares more characteristics with other worlds of its size, such as Ceres, Eris, Makemake, and Haumea. Even if we were to go back to classifying Pluto as a planet, we’ll still to draw a distinction in-between “major” planets and “minor” planets. Classifying Pluto as a dwarf planet is a cleaner destination, especially since Pluto has more in common with Ceres and Eris than it does with Earth or Saturn.

My only disappointment with IAU is not taking the opportunity to expand upon the standards of what it means to be a planet. I believe size and mass has a significant part of the equation. This criterion is inferred with the last criterion, but we need to leave little to interpretation. Other characteristics separate Pluto from a planet such as Neptune. As Mike Brown, discoverer of several trans-neptunium objects, mentioned, “Nostalgia for Pluto is really not a very good planet argument, but that’s basically all there is.”

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