In August 2006, astronomers at the International Astronomical Union (IAU) democratically voted to reclassify Pluto as a dwarf planet, but 20 years later, with our increased knowledge and understanding of planetary systems, is it time to reconsider this controversial decision? Should Pluto be called a planet again? For several years before the IAU vote, there were agitators among the astronomical community pushing for the nine worlds in our solar system to become eight and for Pluto’s planetary status to be revoked. In 2000, Neil DeGrasse Tyson went so far as to remove Pluto from the planet exhibit at New York’s Hayden Planetarium. Questioning Pluto’s status began even earlier, in the 1990s, with the discovery of distant objects in the Kuiper belt, beyond Neptune, that had orbits similar to Pluto’s. It all came to a head in July 2005 with the discovery of Eris, which is a Pluto-sized body even farther from the Sun than Pluto. Was Eris the tenth planet? If it weren’t a planet, how could we still call Pluto a planet? A year later, the question was discussed at an IAU meeting, where astronomers from around the world gathered to discuss various puzzles. Numerous proposals and counterproposals were put forward, some of which even increased the number of recognized planets to 14. But in the end Pluto became a dwarf planet. Although Pluto was officially “reclassified”, to many in the public it seemed as if Pluto had undergone a demotion. This point of view was promoted by those scientists who were against the decision, such as Alan Stern, who was the principal investigator of the New Horizons mission to Pluto. Stern’s spacecraft would launch in 2005 on a journey to a planet, but would arrive at a dwarf planet in 2015. Some argued that Pluto was still some kind of planet, just small. This argument is somewhat undermined by the fact that the IAU gave Pluto a minor planet number (134340), which is normally reserved for asteroids. Nationalist arguments by some in the United States said that Pluto should remain a planet because it was discovered by an American astronomer, Clyde Tombaugh, at Arizona’s Lowell Observatory in 1930. And when New Horizons reached Pluto, it revealed not a minor body, but rather a complex world in itself. You may like What is Pluto? However, for 20 years, Pluto has remained a dwarf planet, but on the anniversary of that fateful IAU vote, is it time to reconsider that decision? Paul Byrne, a planetary astronomer at Washington University in St. Louis, certainly thinks so. “Yes, Pluto is a planet,” he told Space.com. And not just Pluto, but also the other dwarf planets: Ceres, Eris, Makemake and Haumea, not to mention Sedna and Quaoar, which are dwarf planets in all but name. “Ultimately, I think they’re all planets and I don’t think there’s any benefit to not including Pluto as a planet,” Byrne said. To some extent, this whole debate is semantic, and Byrne accepts this view, reminding us that: “Nature doesn’t care what we call Pluto.” However, perception is another matter, which is where Byrne thinks the IAU’s 2006 decree ultimately failed us. Byrne describes how, when he gives public talks about the planets, audience members don’t ask questions about Pluto’s amazing active nitrogen glaciers or what formed its gigantic heart shape. Instead, they want to know why he was demoted. The whole issue has hindered the presentation of science to the public. What to read next “We’ve done the public a disservice from a science communication perspective,” Byrne said. “The messages have hampered the attempt to understand what Pluto is.” Pluto’s active nitrogen glaciers are typical of planetary processes. (Image credit: NASA/JHUAPL/SwRI) One of the arguments in 2006 was that models of solar system formation predict that there could be hundreds of objects like Pluto and Eris that remain undiscovered in the Scattered Disk and Oort Cloud, which is a vast unexplored region that could extend up to a light-year from the Sun. “One of the questions people asked was: How would children remember the names of 200 planets?” Byrne said. “But look how many Pokémon there are whose names kids know. If there are finally 200 planets in the solar system, so what?” The IAU definition of a planet. Semantic or not, the way we categorize celestial objects has to be based on some kind of scientific thinking. In 2006, the IAU came up with a three-point plan for what constitutes a planet. First, a planet had to be in hydrostatic equilibrium, meaning that its gravity had pulled it into a spherical shape. Furthermore, its interior will have been differentiated, with the heavier elements sunk to the core. Everyone seems to agree with this part of the planetary definition. Pluto’s gravity has turned it into a spherical object. (Image credit: NASA/JHUAPL/SwRI) Second, it must be orbiting the sun, so a moon is not a planet because a moon doesn’t orbit the sun, it orbits a planet (although Byrne points out that if he had his way, he’d call large moons planets, too). Strictly speaking, this rules out exoplanets because they don’t orbit our sun, but common sense dictates that they’re fine because they orbit their own star instead. However, what about rogue planets wandering interstellar space alone? Astronomers are beginning to discover many of these floating worlds. You could argue that most of these formed in orbit around a star only to be ejected, so it’s fine to keep calling them planets, but some rogue planets appear to have formed directly from the collapse of a gas cloud without having a parent star. These planets do not meet the gaps in the IAU definition. It is the third point that draws the most ire from critics. He claims that to be a planet, an object must have cleared its neighborhood. In other words, it must be dominant in the region around its orbit, since its gravity has swept away every other body. Pluto fails in this because its orbit actually crosses that of Neptune and is accompanied by other Kuiper Belt objects in neighboring orbits. But then you could say that Neptune must not be a planet because it has Pluto infringing on its orbital realm. And what about the existence of near-Earth asteroids? Does its existence mean that Earth is not a planet because it hasn’t eliminated them yet? There certainly seems to be some sloppiness in this part of the planet definition, perhaps as a result of the somewhat rushed nature of the vote 20 years ago. Planetary processes But if the IAU decision were rejected, what should replace it? How should we classify the planets? Byrne has some ideas. “Being round and having some differentiation makes a lot of sense to me,” he said. “Geophysically, if they are large enough to become balls, then they are large enough to have undergone some type of internal chemical processing and are capable of having some type of active geology, in terms of degassing of volatiles to form an atmosphere, volcanism, surface geomorphological processes, even a short-lived magnetic field. That’s why I count the moon as a planet because it has had these planetary processes.” Byrne acknowledges that gas giants don’t have these properties since they have no solid surface, but no one doubts Jupiter’s planetary credentials, and when it comes to Uranus and Neptune, we don’t really know if there’s some kind of super-Earth lurking deep within them. The nickname “dwarf” also doesn’t impress Byrne. To him, it seems to imply that there is a hierarchy of planets with the gas giants at the top and the dwarf planets at the bottom. The New York Times article on the discovery of Pluto. (Image credit: The New York Times Archive) “There is no natural hierarchy; they are not formed according to one,” he said. And although Pluto, Eris, and the other dwarf planets are made of detritus left over from the construction of the solar system, they still merged in the same way as the other planets, in that the building blocks came together to form these larger objects. However, a counterargument to Pluto being a planet is that if it is only one in 200, then it is nothing special; would be forgotten and ultimately treated almost incidentally. As a dwarf planet, it is the standard bearer of its species, the guardian of an entire realm of worlds of similar size. It’s a choice between being a big fish in a small pond or a small fish in a big pond. The other 200 planets? The real test will come when astronomers begin to discover some of these supposed 200 planets, approximately, far from the sun. As long as they are all about the size of Pluto, there will be no debate, but what will happen when astronomers discover one the size of Mars, or even as big as Earth? Surely we would think of an object as large as a planet. Perhaps this world would have formed closer to the Sun but was ejected through gravitational interactions with one of the giant planets, but on the other hand perhaps it formed there in the outer solar system from the same debris as Pluto. “By the IAU definition, it couldn’t be defined as a planet because it wouldn’t have cleared its neighborhood,” Byrne said. “If we found such a planet, it would be another nail in the coffin of trying to distinguish these things solely based on where they are and not what they are.” Whatever the case, if Pluto is to be a planet again, it must scientifically deserve to be so. Nostalgia, nationalistic pride, or giving children planet names to remember are not sufficient reasons to overturn the IAU’s original decision. Revisiting the name “dwarf planet” and finding a more appropriate category might be the way to go. Or maybe we just don’t have the words to describe the things we’re discovering. Concepts like planets have their roots in history dating back millennia, when it was not understood what these wandering “stars” really were, so it is perhaps not surprising that we now encounter difficulties when trying to apply the term “planet” to our modern understanding of different types of worlds. If there is no way to categorize these objects that is consistent and satisfactory to everyone, then maybe we should stop worrying about it so much. Regardless of what we decide to call it, Pluto will always be Pluto.
