Why the Debate Over Pluto isn't Over
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| Portrait of Pluto captured by NASA's New Horizons spacecraft. Image Credit: NASA/New Horizons (Click to Expand) |
I named this blog 'Earth is a Planet' because I believe it is important to maintain perspective about our place in the universe. As I discussed in my previous post, Earth is not the world, located at the center of the universe, but one of many worlds around one of many billions of stars within one of many billions of galaxies. In the vastness of space, the view that our human experience applies to the cosmos at large is simply misguided. It is for this reason that we as Earthlings must look to other planets for knowledge as opposed to vice versa. But what exactly is a planet? While the question may seem simple at face value, the definition of a planet is historically fuzzy and has evolved with our understanding of our Solar System and with our discovery of worlds around other stars. Perhaps the most widely known debate over this very topic in modern times relates to the classification of Pluto after the International Astronomical Union (IAU) updated the scientific community's formal definition of a planet back in 2006. The major consequence of the IAU's resolution, which disappointed much of the public, was the reclassification of Pluto from 'planet' to 'dwarf planet.' We will return to the present definition of a planet later in this post, but it is important that we first discuss the historical context.
The word planet is derived from the ancient Greek word for 'wanderer,' and originally referred to the five 'stars' that appeared to meander across the night sky, separately from the others. These planets were eventually named Mercury, Venus, Mars, Jupiter, and Saturn (Uranus and Neptune, being further away, are not visible with the naked eye and were not discovered until 1781 and 1846, respectively). At this point in history, the Earth was assumed to define the center of the universe, and early astrologers included the Sun and Moon among the original set of planets (Fun fact: the days of the week are named after the seven original planets). Eventually, Copernicus' findings revealed that the Earth (along with Mercury, Venus, Mars, Jupiter, and Saturn) was a planet that orbited the Sun, and that the Moon orbited Earth. Ultimately, this heliocentric model of a Solar System (named after our Sun, a.k.a. Sol) replaced the original geocentric perspective, and a planet was redefined as anything orbiting the Sun.
By the time Pluto was discovered in 1930, our model for the Solar System incorporated the Sun and nine planets. Pluto maintained planetary status until it was later reclassified as a 'dwarf planet' by the IAU in 2006. To this day, many within the wider public remain disappointed and confused by the 'death' of our ninth planet. To fully understand the decision, it is helpful to discuss another former planet with a similar history between Mars and Jupiter. Discovered in 1801, Ceres was labeled as a planet for half a century. Within the following ten years, three similar (former) planets were discovered within the same neighborhood of the Solar System - Pallas in 1802, Juno in 1804, and Vesta in 1807. As the region between Mars and Jupiter continued to fill with newly discovered objects, it became increasingly apparent that labeling them all planets would be unwise given how similar they all were to each other. By the mid-1800s, Ceres, Pallas, Juno, and Vesta were reclassified as asteroids within the asteroid belt, and their fifteen minutes of fame in the classroom were over.
By the end of the 20th century and into the early 21st, Pluto's status as a planet grew fragile with the observation of a number of similar icy objects past the orbit of Neptune. Nevertheless, Pluto remained the largest of these Trans-Neptunian Objects; until another, similarly-sized world could be found in its neighborhood, there was no reason to reclassify it. With the discovery of Eris in 2005, planetary astronomers were having Ceres flashbacks that could no longer be ignored, and a convention was arranged to clarify the definition of a planet. Under the 2006 resolution, a planet is defined as a celestial body that is:
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| Geocentric arrangement of the seven classical planets. Sizes not to scale. Obviously. (Click to Expand) |
By the time Pluto was discovered in 1930, our model for the Solar System incorporated the Sun and nine planets. Pluto maintained planetary status until it was later reclassified as a 'dwarf planet' by the IAU in 2006. To this day, many within the wider public remain disappointed and confused by the 'death' of our ninth planet. To fully understand the decision, it is helpful to discuss another former planet with a similar history between Mars and Jupiter. Discovered in 1801, Ceres was labeled as a planet for half a century. Within the following ten years, three similar (former) planets were discovered within the same neighborhood of the Solar System - Pallas in 1802, Juno in 1804, and Vesta in 1807. As the region between Mars and Jupiter continued to fill with newly discovered objects, it became increasingly apparent that labeling them all planets would be unwise given how similar they all were to each other. By the mid-1800s, Ceres, Pallas, Juno, and Vesta were reclassified as asteroids within the asteroid belt, and their fifteen minutes of fame in the classroom were over.
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| Simplified representation of the Solar System from 1930-2006. Not to scale. (Click to Expand) |
By the end of the 20th century and into the early 21st, Pluto's status as a planet grew fragile with the observation of a number of similar icy objects past the orbit of Neptune. Nevertheless, Pluto remained the largest of these Trans-Neptunian Objects; until another, similarly-sized world could be found in its neighborhood, there was no reason to reclassify it. With the discovery of Eris in 2005, planetary astronomers were having Ceres flashbacks that could no longer be ignored, and a convention was arranged to clarify the definition of a planet. Under the 2006 resolution, a planet is defined as a celestial body that is:
- in orbit around the Sun,1
- massive enough to be spherical (as opposed to a shape more like that of a lumpy potato, like most asteroids), and
- gravitationally dominant within its orbit, such that it has cleared the neighborhood.
Pluto, Eris, and Ceres, having not met the third criterion, were thus classified as dwarf planets.2 Similarly to the gradual discovery of the asteroid belt, the observation of additional icy objects within the same region of our Solar System led to the recognition that Pluto was one of many objects within the Kuiper Belt.
Setting aside the classification intricacies, however, it is clear that the definition of a planet is far from fixed, and it will likely have to be refined with future discoveries in our own Solar System and beyond. The rapidly growing catalog of extrasolar planets around other stars (a.k.a. exoplanets) has made it even more difficult to classify the diversity of observed worlds. Planetary systems are far more complex than anyone could have predicted even just a century ago, and there are not always distinct categories that individual objects can be sorted into. Perhaps there will be a day when a formal definition for the term 'planet' becomes cumbersome and unnecessary. Regardless of how Pluto is ultimately classified, New Horizons has shown us that it is a pretty awesome world in our Solar System that is just as worthy of admiration and exploration as any of the eight major planets.
Fun Links:
- Scale model of the Solar System
- All about the seven new planets discovered around TRAPPIST-1
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| Simplified representation of the Solar System after 2006. Not to scale. (Click to Expand) |
Setting aside the classification intricacies, however, it is clear that the definition of a planet is far from fixed, and it will likely have to be refined with future discoveries in our own Solar System and beyond. The rapidly growing catalog of extrasolar planets around other stars (a.k.a. exoplanets) has made it even more difficult to classify the diversity of observed worlds. Planetary systems are far more complex than anyone could have predicted even just a century ago, and there are not always distinct categories that individual objects can be sorted into. Perhaps there will be a day when a formal definition for the term 'planet' becomes cumbersome and unnecessary. Regardless of how Pluto is ultimately classified, New Horizons has shown us that it is a pretty awesome world in our Solar System that is just as worthy of admiration and exploration as any of the eight major planets.
Fun Links:
- Scale model of the Solar System
- All about the seven new planets discovered around TRAPPIST-1
1 Does not apply to extrasolar planets
2 Pallas, Juno, and Vesta are not dwarf planets because they do not meet the second criterion. Satellites like our Moon are not considered dwarf planets.
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