Why Pluto Is No Longer a Planet

In 1930, an astronomer Clyde W. Tombaugh discovered Pluto. The discovery was so significant that the planet was named after the mythological guardian of the underworld. It will interest you to know that an eleven-year-old English girl was the person who gave this planet its name. Equally as interesting is that Pluto was unique for being the tiniest and ninth planet in the ecosystem. Its size was just 2400km. Perhaps you might not know, but Pluto is no longer a planet. In fact, this has been so for the past 11 years. Pluto never collapsed nor was it attacked.  But, why then did it lose its status as a planet? We have highlighted the major reasons below:

  • The discovery of Kuiper Belt objects: Even though Pluto existed for many years, many astronomers were never in support of it being among the list of planets. They felt it was rather too tiny to be a planet. For instance, at the Haydn Planetarium in New York in 2000, an exhibit featuring only eight planets was unveiled. The event was followed by criticisms from astronomers who felt insulted by the exhibit. However, later developments verified the opinion of the exhibit. The most important of which was the discovery of a population of small, icy bodies beyond the orbit of the Neptune. These masses called Kuiper Belt Objects were estimated to be 70,000 in number. Interestingly, these objects were found to have sizes roughly comparable to that of Pluto.
  • The discovery of Eris: In 2005, an astronomer Professor Mike Brown and his team discovered a Pluto-like mass. The object named Eris was said to be 2600km in size, about 200km more than the size of Pluto. The argument on whether Eris was a planet or just one of the Kuiper Belt Objects led to review of Pluto as a planet. If Eris is as big as Pluto, it also deserves to be a planet – don’t you think so? If Eris was to be listed as the tenth planet in the ecosystem, what then will happen when similar objects are discovered? Probably, the ecosystem is going to have as much as hundreds of planets in decades to come. To finally solve these puzzling developments, the International Astronomer Union (IAU) took an important step to define a planet at one of its meetings.
  • IAU’s definition of a planet: Defining the term “planet” took the central stage at the IAU’s 26th General Assembly held in Prague, the Czech Republic from August 14, 2006 to August 25, 2006. After several open discussions and debates from top astronomers, the Union agreed on these three definitions of a planet: A planet must be round; A planet must be massive enough to be pulled into a shape of a sphere by its gravitational forces; A planet must have cleared the “other objects” in its orbit. That is, as it travels, its gravity must be able to sweep and clear the space around it of other objects. In short, all objects in its orbit must be under its influence (like a moon).
  • The IAU’s resolution on Pluto: All the pre-existing planets met the first two criteria. However, Pluto and Eris failed to meet the third criteria. Pluto on its own was only 0.07 bigger than the mass of the “other objects” in its orbit. In comparison, the Earth, for example, is 1.7 million times the mass of the “other bodies” in its orbit and thus fits the definition of a planet perfectly. In fact, Pluto’s moon, “Charon” was found to be half of Pluto’s size. Thus, the Union resolved that Pluto doesn’t have the needed capacity to sweep and clear the space around it of these other objects. They were of the opinion that until Pluto crashes into many of the “other objects” in its orbit and gains mass, it will remain out of the list of planets. As a result, the IAU unanimously voted for the removal of Pluto from the list.
  • The New Classification: IAU resolved that there were eight planets in the ecosystem and thousands of Kuiper Belt objects. Some of these Kuiper Belt Objects were also found to have roughly comparable sizes as well as similar attributes. These unique objects were classified as “Dwarf Planets” and included Pluto, Eris, Sedna, Quaoar, and Ceres.


In spite of all these events, Pluto remains a fascinating concept to a lot of astronomers. In the same year, it was demoted, NASA launched a mission to Pluto called the New Horizon to study Pluto further. The mission took nine years and ended in July 2005. We believe next time you talk about Pluto, you will speak of a “dwarf planet” that was listed as a “planet” for 76 years.

Photo Credit: Xkcd