The International Astronomical Union demoted Pluto to a dwarf planet in 2006, leaving scientists searching for an explanation for many years.
The International Astronomical Union (IAU) demoted Pluto from a planet to a dwarf planet in 2006, causing many astronomers to still want to reconsider the matter many years later. One of them is to clarify the parameters that distinguish planets from other celestial bodies.
So why is Pluto no longer considered a planet? The problem begins with the definition of a planet – or lack thereof. Before 2006, there were no strict criteria for a planet. Instead, planets are loosely considered objects larger than asteroids orbiting the Sun. For example, in the mid-1800s, more than a dozen objects once considered planets were now considered asteroids.
Image of Pluto taken by NASA’s New Horizon spacecraft in 2015. Photo: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
When did Pluto become a planet?
When Clyde Tombaugh discovered Pluto in 1930, scientists were searching high and low for an unknown celestial body to explain some irregularities in its orbit. Tombaugh, a junior astronomer at the Lowell Observatory in Arizona, was tasked with determining the cause. After several months, he successfully located a round, rocky object beyond Uranus that he believed might be contributing to its orbital wobbles. This celestial body was eventually named Pluto, after the Roman god of the underworld. Although smaller than some known moons, it is still considered large enough to be considered a planet.
However, it was soon discovered that Pluto was not large enough to exert the gravitational pull necessary to influence Uranus’s orbit. Furthermore, in the 1990s, astronomers discovered that Pluto is surrounded by several similarly sized objects; it belongs to a region of the solar system later named the Kuiper Belt.
This sparked debate about Pluto’s place in the planetary system, and culminated at a 2006 meeting in Prague.
That year, the IAU tasked a small subcommittee with developing a definition of “planet.” They came up with three criteria:
+ It must revolve around the Sun.
+ It must have enough mass to pull itself into a circle.
+ It must clear all other celestial bodies, except its own moons, from its orbit.
Based on the third claim, the committee declared that Pluto no longer qualified as a planet because of its location in the object-filled Kuiper Belt, where thousands of objects lie beyond the orbit of Neptune. Therefore, Pluto is not a gravitationally dominant object in its neighborhood – and is therefore not a planet, according to the new definition.