Why Pluto Is No Longer a Planet
For more than 75 years, children learned that our solar system had nine planets, with Pluto being the smallest and most distant. However, in 2006, Pluto was officially reclassified as a dwarf planet, reducing the number of recognized planets in our solar system to eight. This decision sparked debate among scientists and the public alike.
Pluto’s Discovery
Pluto was discovered on February 18, 1930, by Clyde Tombaugh at the Lowell Observatory in Arizona. It was named after the Roman god of the underworld and quickly became known as the ninth planet.
For decades, very little was known about Pluto because of its great distance from Earth.
Why Was Pluto Reclassified?
The change came after the International Astronomical Union established a formal definition of a planet in 2006. According to the IAU, a celestial body must meet three requirements to be considered a planet:
- It must orbit the Sun.
- It must have enough gravity to become nearly round.
- It must have cleared the neighborhood around its orbit, meaning it is gravitationally dominant in its orbital region.
Pluto satisfies the first two requirements but fails the third.
What Does “Clearing the Neighborhood” Mean?
A planet’s gravity should be strong enough to either:
- Pull nearby objects into itself,
- Capture them as moons, or
- Push them away over billions of years.
Pluto shares its orbital region with countless icy bodies in the Kuiper Belt, a vast ring of frozen objects beyond Neptune. Because Pluto has not become gravitationally dominant in this region, it does not meet the IAU’s third criterion.
What Is a Dwarf Planet?
A dwarf planet is an object that:
- Orbits the Sun.
- Is nearly spherical due to its own gravity.
- Has not cleared its orbital neighborhood.
- Is not a moon of another object.
Other well-known dwarf planets include:
- Ceres
- Eris
- Haumea
- Makemake
Why Did Scientists Change the Definition?
In the early 2000s, astronomers discovered many Pluto-sized objects in the Kuiper Belt. One of them, Eris, is slightly more massive than Pluto.
Scientists faced two choices:
- Recognize dozens—or eventually hundreds—of new planets, or
- Create a clearer definition of what qualifies as a planet.
The IAU chose the second option, leading to Pluto’s reclassification.
What Did the New Horizons Mission Reveal?
New Horizons flew past Pluto in July 2015, giving humanity its first close-up view of the dwarf planet.
The mission revealed that Pluto has:
- Towering mountains made of water ice.
- Vast plains of frozen nitrogen.
- A thin atmosphere.
- Blue skies caused by atmospheric haze.
- At least five known moons, including the large moon Charon.
These discoveries showed that Pluto is a surprisingly active and complex world.
Is the Decision Still Controversial?
Yes. Some planetary scientists argue that Pluto should still be considered a planet because it is geologically active and has many planet-like features.
However, the official classification used by the International Astronomical Union remains that Pluto is a dwarf planet, and this is the standard taught in schools and used in most scientific literature.
Fun Facts About Pluto
- Pluto takes about 248 Earth years to orbit the Sun.
- A single day on Pluto lasts about 6.4 Earth days.
- Pluto’s orbit is more elliptical and tilted than those of the eight major planets.
- Sunlight at Pluto is about 1,600 times dimmer than on Earth.
Conclusion
Pluto was not removed from the solar system—it simply received a new classification. It remains one of the most fascinating worlds known to science. Although it is now officially called a dwarf planet, Pluto continues to provide valuable insights into the formation and evolution of our solar system, reminding us that scientific knowledge evolves as new discoveries are made.
