Thinkin' About Space: Pluto, the largest Kuiper Belt object

Clyde William Tombaugh was just a farmer with a penchant for astronomy. When his parents’ need for help with damaged crops kept him from attending college, he spent his time making telescopes and digging trenches to use them in. He sent detailed drawings of planets to the Lowell Observatory and was promptly hired. Percival Lowell, the namesake of the observatory, had calculated discrepancies in Uranus’s orbit unaccounted for by Neptune, which led him to believe there existed a massive body beyond Neptune. So Lowell put Tombaugh to the task of finding “Planet Nine.”

In January of 1930, Tombaugh used the observatory’s astrograph to take photographs over two different nights. Viewing the photos through an apparatus called a blink comparator, he was able to discern a small speck of light shifting between the two frames. This speck came to be known as Pluto.

Or rather, Kuiper Belt Object 134340 Pluto. Almost immediately, Pluto’s status as a planet came into question. Not only was its orbit highly inclined to the plane of the other planets, its size was determined to be far below that of even the Moon with every new calculation. Its mass was also constantly being lowered, the first major breakthrough being a calculation of its albedo. The albedo of an object is a measure of how much light (in this case sunlight) is able to be reflected. Pluto’s albedo is around that of methane ice, meaning it was more luminous than massive, and therefore could not be more than 1/100th the mass of Earth. The discovery of Pluto’s moon Charon cemented its mass as much too small to have any measurable effect on Uranus’s orbit. Also, the flyby of Neptune by Voyager 2 more accurately determined Neptune’s mass and accounted for the orbital perturbations.

More than 70 years later, astronomer David Jewitt discovered an object beyond Neptune, in a similar orbit to Pluto. This was the second object in the Kuiper Belt. Mike Brown (you might know him as “Pluto Killer”) discovered another speck floating beyond Neptune. And another. And another. One of these was even more massive than Pluto. Pluto was not alone in its orbit. In fact, many, many objects orbit the Sun at similar distances. Currently, there are thousands of known KBOs, with hundreds of thousands more thought to exist. The Kuiper Belt is much like the Asteroid Belt, except more icy than rocky and generally more massive (the largest asteroid, 1 Ceres, is about a quarter of Pluto’s diameter). The evidence against Pluto’s planetary status was building.

In 2009, the International Astronomical Union made the decision that Pluto was not gravitationally dynamic enough to have cleared its orbit of the entirety of the Kuiper Belt and thus was not a planet. Most of the world is unhappy with the decision. I stand by it wholeheartedly, and I’ve loved Pluto half of my life. I spent nine years waiting for New Horizons to make a flyby and ever since have had Pluto on my mind. But Pluto still isn’t a planet. Science changes over time as we learn more and more about the universe around us. We used to think the Sun orbited the Earth and that nothing was smaller than an atom. Understanding that these were false increased our knowledge immeasurably, just as understanding that Pluto is the largest body in a region called the Kuiper Belt allows us to think more broadly about the outer solar system.

Ethan Gower is a sophomore studying astrophysics at Ohio University. Please note that the views and opinions of the columnists do not reflect those of The Post. Do you still consider Pluto a planet? Let Ethan know by emailing him at eg662511@ohio.edu or tweeting him at @ThinkinAbtSpace.