In 2006, the International Astronomical Union (IAU) voted on the definition of a planet. Famously, Pluto no longer met the criteria and was demoted to a dwarf planet. Things have been a bit of a mess since then — so is it time to redefine the planet?
To be fair, Pluto had it coming. The word “planet” never had an official definition, and astronomers had always played fast and loose with its use. To the ancient Greeks, a planet was any “wandering star,” which included the sun and the moon. With the Copernican revolution, the definition changed: Earth was considered a planet in its own right, the moon was demoted to a satellite, and the sun got promoted.
This worked for over 200 years, until William Herschel discovered Uranus and Giuseppe Piazzi discovered Ceres, the largest object in the main asteroid belt between Mars and Jupiter. Initially, both Uranus and Ceres were classified as planets. But once more objects were found to share similar orbits with Ceres, astronomers had to rethink things — surely, planets lived alone, after all. Herschel proposed the term “asteroids” for the small objects between Mars and Jupiter, while Uranus remained a planet (a situation that certainly benefited Herschel’s own legacy).
Astronomers were comfortable with those classifications even when Clyde Tombaugh discovered Pluto in 1930. But that new planet was a stretch — it had a really wonky orbit and was far smaller than the other planets, so it didn’t take long for astronomers to start wondering if they should start reexamining how objects were classified.
Starting in the 1990s, astronomers began to find more objects that shared similar orbits with Pluto. But the real nail in the planetary coffin came in 2005, when astronomer Mike Brown discovered Eris, an object about the same size as Pluto orbiting beyond Neptune.
So, in 2006, when astronomers gathered at the IAU meeting in Prague, a large contingent moved to have that body define what a planet should be. There were two camps: the geophysicists who argued that planets should be defined by their appearance, and the dynamists who believed that planets should be defined by their properties.
In essence, the geophysicists argued that a planet should be anything that is large enough that its own self-gravity pulls it into a nearly spherical shape. The dynamists countered that a planet should be anything that can dominate and mostly clear its orbit of any debris. The first definition would allow Pluto, along with Ceres and…
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