Is Pluto a Planet? Learn 12 Interesting Facts about Pluto

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Updated on: Educator Review By: Michelle Connolly

What is Pluto?

Pluto is the ninth-largest and tenth-most-massive known object in our Solar System that directly orbits the Sun. It was considered the smallest planet in the Solar System until 2006. In that year, a large group of scientists decided that Pluto was not a true planet. They called it a dwarf planet instead.

Dwarf planet Pluto is a member of a group of objects in our Solar System beyond Neptune called Kuiper Belt. Kuiper Belt is an area that consists mainly of many comets, asteroids, small icy bodies, and other remnants from when the Solar System formed. Pluto is the largest body in the Kuiper Belt.

Discovery of Pluto

Pluto was discovered in 1930 by the American astronomer Clyde Tombaugh. He was a young astronomer working at Lowell Observatory in Flagstaff, Arizona. His task was to take pictures of the night sky every two weeks and compare them to see if any object had shifted. In February 1930, when he compared some pictures, he noticed an object that was moving against the backdrop of distant stars. It was a small, dim, remote body in our Solar System. This object is now called Pluto.

Why is Pluto Not a Planet?

In 2006, the International Astronomical Union (IAU), an organization of professional astronomers, created a definition for the term “planet”. The IAU set three conditions for an object in the Solar System to be considered a planet:

  1. It must be in orbit around the Sun.
  2. It must be massive enough so that its gravity pulls it into a spherical (or rounded) shape.
  3. It must have cleared the neighborhood around its orbit.

Pluto orbits the Sun, but while it is large enough to be spherical, it is not large enough to clear the neighborhood around its orbit. To clear its neighborhood means that the gravity of the planet left no other large bodies around it other than its moons. So, Pluto only meets two of these conditions.

The IAU also decided that bodies which meet criteria 1 and 2 but do not meet criterion 3, would be called “dwarf planets.” So, the IAU downgraded the status of Pluto from a planet to a dwarf planet.

Structure of Pluto

  • Inside Pluto: Pluto probably has a rocky core surrounded by a mantle of water ice. Some scientists think that Pluto may have an ocean beneath its surface.
  • Surface: Pluto consists of rocks and frozen gases. Some areas of Pluto’s surface are very bright, while others are dark. The bright areas are probably a frozen gas called nitrogen. Pluto’s surface consists of mountains, valleys, plains, and craters. The mountains on Pluto are big blocks of water ice, and they sometimes have a coating of frozen gases like methane. Valleys on Pluto are so long that they can reach 370 miles (600 kilometers) in length. The most prominent plains on Pluto appear to be made of frozen nitrogen gas.

Pluto’s Atmosphere

Pluto has a thin and soft atmosphere that expands when it comes closer to the Sun and collapses as it moves farther away. It consists mainly of nitrogen, methane and carbon monoxide. When Pluto gets close to the Sun during its orbit, the ice on its surface sublimates (changes directly from solid to gas) and rises to form the thin atmosphere.

Temperature of Pluto

Pluto is very far from the Sun. The average distance between Pluto and the Sun is 5.9 billion kilometers. As a result, Pluto receives only a little sunlight, and it takes the sunlight more than five hours to reach Pluto. So, the temperature on Pluto’s surface is very low. Scientists believe that the average temperature on its surface is about −387° F (−233° C).

Pluto’s Orbit and Rotation

Like other planets, Pluto has two types of motion: orbit and spin. Pluto spins about its center slowly, and it is tilted so it spins nearly on its side. Pluto’s orbit around the Sun is unusual compared to other planets. It is tilted and egg-shaped. Because it is so far away from the sun, Pluto completes one orbit around the Sun every 248 Earth years.

Pluto’s Colors

Pluto has a range of colors, including off-white, light blue, yellow, orange and large patches of deep red. The dark reddish patches are due to the interactions between the sunlight and the gases of methane and nitrogen on Pluto. Overall, Pluto appears to be ruddy or brownish from a distance.


Pluto Moons

Pluto has five known moons: Charon, Styx, Nix, Kerberos, and Hydra.

Charon is the closest one to Pluto and the biggest one of them. It is about half the size of Pluto itself. Because it is very close to Pluto, Charon and Pluto are often referred to as a double planet. Some astronomers have considered the two objects a binary system rather than a planet and satellite.

Pluto’s other four moons are much smaller. They are less than 100 miles wide. Their shapes are irregular, not spherical like Charon. Unlike many other moons in the solar system, these moons are not tidally locked to Pluto. They all spin and do not keep the same face towards Pluto.

Pluto’s Name

The name Pluto was proposed by Venetia Burney, an 11-year-old schoolgirl in Oxford, England. She was interested in classical mythology and astronomy. The name Pluto was of the Roman god of the underworld. Venetia thought that it was a good name for a dark and cold world like this dwarf planet. She suggested it when she was talking with her grandfather Falconer Madan. He was a former librarian at the University of Oxford’s Bodleian Library. Madan passed the name to Professor Herbert Hall Turner. Turner told this proposed name to the astronomers in the United States. The astronomers voted on a list of three names including Pluto. Pluto received all votes, and then the dwarf planet was officially named Pluto.

12 Interesting Facts about Pluto

  1. Clyde Tombaugh was just 25 years old when he discovered Pluto.
  2. Pluto’s interior is warmer that some think there could be an ocean deep inside.
  3. Pluto’s surface is one of the coldest places in the Solar System, at roughly minus 387 degrees Fahrenheit (minus 232 degrees Celsius).
  4. Pluto’s axis of rotation is tilted, so it spins almost on its side.
  5. Pluto’s rotation is retrograde. It spins from east to west like Venus and Uranus.
  6. Pluto completes one orbit around the Sun every 248 Earth years. That means that a year on Pluto lasts 248 Earth years.
  7. Pluto has yet to complete a full orbit around the Sun since its discovery.
  8. Pluto completes one rotation in about 6.5 Earth days, so a Pluto day lasts about 6.5 Earth days.
  9. It takes Pluto 153 hours to complete one rotation. So, one day on Pluto takes about 153 hours.
  10. Charon’s orbit around Pluto takes 153 hours, which is the same time it takes Pluto to complete one rotation. This means that Charon neither rises nor sets. The same side of Charon always faces the same spot on Pluto’s surface.
  11. At its closest point to the Sun, Pluto passes inside the orbit of Neptune, the eighth planet in the Solar System.
  12. Pluto gets closer to the Sun than Neptune during its orbit, even though it is actually further from the Sun.

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