Is Pluto a Planet?

Why Pluto Was Demoted

February 4, 2019
Pluto, The Dwarf Planet

Pluto on July, 14 2015. Image taken by New Horizons.

NASA/Johns Hopkins University Applied Physics Laboratory/Sou

When many of us were kids, Pluto was the ninth planet of our solar system. So there was some surprise and even outrage when Pluto was “demoted.” Here’s the skinny on what was once known as our Sun’s smallest planet—and how it got its name!

In 2006, astronomers reclassified Pluto from an official planet to a “dwarf planet.” That said, it is still considered to be one of the largest bodies in the Kuiper Belt, a band of icy objects at the edge of our solar system.

Because Pluto is the biggest object in this region, some call it “King of the Kuiper Belt.” 

Size is relative, perhaps. Pluto is about 1,400 miles (2,380 km) wide. That’s about half the width of the United States, or ⅔ the width of Earth’s Moon.

What Defines a Planet?

Rules have now been created. According to the International Astronomical Union (IAU):

  1. It must orbit the Sun
  2. It must be round (so big enough for gravity to squash it into a round ball)
  3. It must have cleared other objects from its orbit.

Pluto did not meet the third rule. It orbits among other floating icy bodies in the Kuiper Belt.  

While the other planets travel around the Sun in almost perfect circles, Pluto takes an oval-shaped path and the Sun is nowhere near its center. Pluto’s path is also much more tilted than the nice, orderly plane in which most of the other planets orbit.

Image credit: NASA.

What Is a Dwarf Planet? 

A “dwarf planet” has a definition too. It must meet the criteria of a planet above as well as a fourth rule: it must not be a satellite.

There are hundreds of dwarf planets. A couple of the more well-known dwarf planets are Ceres and Eris.

Some folks ask if Pluto is an an asteroid. Actually, it’s called a “plutoid” or an “ice dwarf” because it has an orbit outside Neptune’s (plus, it’s extremely cold).

However, Ceres is so small (especially compared to Pluto) that it is classified as both a dwarf planet and an asteroid. It’s one of the smallest dwarf planets but also one of the largest asteroids, making up approximately a fourth of the mass of the asteroid belt.

    The Endless Debate

    The debate about Pluto’s planetary status goes on, even among astronomers. Some argue that Pluto’s official planetary status should be restored.

    They suggest that the requirement that a planet must “clear” other objects from orbit doesn’t make sense—and that there are plenty of examples. 

    Other astronomers today say that the name is not important. One made the comparison of naming similar to calling any island a continent. 

    Visiting Pluto

    When this article was first written (in 2011), no spacecraft had ever visited Pluto! But a NASA probe named New Horizons launched on January 19, 2006, and arrived in Pluto’s neighborhood in July 2015. Since then, we’ve received many close-up images of Pluto—and we’re also studying the edge of our solar system and how it formed.

    The dwarf planet is much more geologically complex than we had imagined. It’s not just a ball of ice.

    • Pluto has blue skies and mountains as high as the Rockies.
    • There are snow-capped peaks on Pluto—except the “snow” is red! It’s frozen methane!
    • Pluto has a heart-shaped glacier that’s the size of Texas and Oklahoma.

    What would it be like to land on Pluto? See a colorful animation and explanation.

    More Cool Facts About Pluto

    • Pluto orbits the Sun about 3.6 billion miles (5.8 billion km) away on average, about 40 times as far as Earth.
    • We could never live on Pluto. Because it’s so far away from the Sun, its temperature is about 400 degrees below zero Fahrenheit! 
    • If you weigh 100 pounds on Earth, you would weigh only 7 pounds on Pluto.
    • If you lived on Pluto, you’d have to live 248 Earth years to celebrate your first birthday in Pluto-years. 
    • A day on Pluto lasts 153 hours, or about 6 Earth days.
    • Pluto has blue skies and is very hazy, thanks to a thin atmosphere of nitrogen, methane, and carbon monoxide. 
    • Pluto has 5 moons. The largest, Charon, is so big that Pluto and Charon orbit each other like a double planet.

    How Was Pluto Discovered?

    Pluto was discovered in 1930 at the Lowell Observatory in Flagstaff, Arizona, by astronomer Clyde W. Tombaugh. The existence of this planet was actually proposed years before by Percival Lowell, who had theorized that slight disturbances in the orbits of Uranus and Neptune were caused by the gravitational pull of another planet. Lowell even went so far as to make calculations to estimate where to find what is now Pluto. Although he didn’t succeed in finding this object by his death, the search continued until Tombaugh discovered the tiny planet using a new astronomic technique of photographic plates combined with a blink microscope.


    Why Is It Called Pluto?

    • Nope, Pluto’s name is not from Disney character of Pluto (Mickey Mouse’s dog)! Venetia Burney from England, just 11 years old at the time, suggested the name Pluto in 1930. Her grandfather, a librarian, knew many astronomers and sent it to a friend who sent in the name to the Lowell Observatory. She is the only girl (so far) who has named a world!
    • Pluto is the name of the the Roman god of the Underworld (equivalent to the Greek Hades).
    • Pluto’s five moons also have names associated with the underworld. Charon is the name of the river Styx boatman who ferries souls in the underworld; Nix is named for the mother of Charon, who is also the goddess of darkness and night; Hydra is named for the nine-headed serpent that guards the underworld; Kerberos is named after the three-headed dog of Greek mythology; and Styx is named for the mythological river that separates the world of the living from the realm of the dead.
    • Even though Pluto wasn’t named for the Disney character, Pluto the Dog made his first appearance in a movie in 1930, the same year Tombaugh discovered the dwarf planet! No one knows if this was a strange coincidence or not. Take a look at the photo above and it’s a very strange coincidence indeed!

    Then again, perhaps we see what we wish to see.


    Love learning about space? Visit planet Mars, Earth’s neighbor.


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