Since its revelation in 1930, Pluto (located in the Kuiper belt) has always been an oddity compared to the other planets in our solar system. Beginning from the early 1990s, astronomers started to find more objects surrounding Pluto that were similar in size and composition. Some noticeable ones include Sedna and Quaoar, which were both similar in size to Pluto. Then, in 2005, astronomers discovered an object, later named Eris, that had a size even larger than Pluto. After more “Pluto-like” discoveries, astronomers had to start asking themselves a vital question: if Pluto is considered a planet, does that mean Eris, Sedna, etc. are considered planets as well? They started to discuss what the definition of a planet really was, and how they should classify their new discoveries.
In August 2006, the International Astronomical Union (IAU) formally defined the word “planet” in a new way and further declared that Pluto was no longer one. Simply put, Pluto no longer fit the newly composed definition of a planet and was therefore downgraded to a dwarf planet.
(Image taken from Pixabay, Copyright free)
With the definition of “planet” becoming more strict, Pluto had to fit into these three criteria:
It must orbit the sun.
It must have enough mass and gravity to pull itself into a sphere shape.
It must have a large enough mass to “dominate” its own orbit by clearing the space around its orbital path of other objects, like asteroids and dwarf planets.
While Pluto fits into the first two criteria, it falls short when it comes to the third one. For billions of years, it failed to clear the neighborhood around its orbit. Furthermore, Pluto is only 0.07 times the mass of the objects in its orbit. Meanwhile, Earth is 1.7 million times the mass of the objects in its orbit. Therefore, the IAU decided that Pluto was in fact a dwarf planet instead of a regular one.
Currently, there are five discovered dwarf planets in our solar system: Pluto, Eris, Ceres, Makemake, and Haumea. However, scientists believe that there may be hundreds of undiscovered dwarf planets in the outer solar system. So it turns out, even though Pluto is not a planet, it still is in good company!
Quick facts about Pluto:
Pluto spins backward, from east to west.
Pluto has five moons: Charon, Styx, Nix, Kerberos, and Hydra.
Its composition approximately consists of a mixture of 70 percent rock and 30 percent water ice.
It takes over five hours for light from the sun to reach it.
Links
Mann, A. (2019, March 28). Why Isn't Pluto a Planet Anymore? Space.com. https://www.space.com/why-pluto-is-not-a-planet.html
Kent, L. (2019, August 24). Why Pluto is no longer a planet (or is it?). WPTV.com. https://www.wptv.com/news/science-tech/why-pluto-is-no-longer-a-planet-or-is-it
Choi, C. Q. (2017, November 14). Dwarf Planet Pluto: Facts About the Icy Former Planet. Space.com. https://www.space.com/43-pluto-the-ninth-planet-that-was-a-dwarf.html
EarthSky. (2019, August 24). How did Pluto become a dwarf planet? Earthsky.com. https://earthsky.org/human-world/pluto-dwarf-planet-august-24-2006
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