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.
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.
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:
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.
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.
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).
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 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 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.
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.
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