Facts About Pluto: Diameter & Orbit

Instructor: John Williams
Pluto is the tenth largest planetary body in our solar system, and was one of the original nine planets until 2006. Much about Pluto is still unknown, and it is a major area of exploration for scientists. This lesson discusses some facts and characteristics about this celestial body.


Growing up, you probably were taught that our solar system contained nine major planets. These planets, starting with Mercury, extended from the sun to the very last planetary body, known as Pluto. Scientists have been able to explore the characteristics of most of these bodies pretty easily. However, they have been limited in exploring Pluto due to its distance and other factors. This lesson will address what is known about Pluto, what information has been suggested, and where we stand today in understanding this interesting planetary body.

Pluto's Size

Pluto was discovered in 1930, and it is the smallest of the nine original planets. It is even smaller than many of the moons in the solar system. It has a diameter of approximately 2,306 kilometers, which makes it about two-thirds of the size of our moon.

In 1992, scientists determined that Pluto may be a part of a group of orbiting bodies at the very edge of our solar system known as the Kuiper Belt. This fact is important, in that it has been the source of much debate regarding Pluto's planetary status.

Demotion of Pluto

Scientists identified several other orbiting bodies in the Kuiper Belt that were similar in size and mass to Pluto, which opened the door for debate regarding whether or not Pluto should be considered a major planet. In 2006, scientists officially demoted Pluto to the level of dwarf planet, or one of the orbiting bodies that circle the sun due to gravity(attractive force) but also has other objects within its orbit.

For these reasons, we now have 8 major planets. As a side note, scientists have also identified another dwarf planet, known as Eris, that is larger than Pluto, making Pluto the second largest dwarf planet.

The current solar system
Solar System

Pluto's Physical Characteristics

Pluto's surface contains nitrogen ice and small amounts of methane and carbon dioxide gases. Fifty to seventy percent of the internal layers of Pluto are composed of rock, while 30-50 percent of the mass is made of ice. These ranges show the lack of certainty that scientists have been able to establish regarding the composition of Pluto. However, some information regarding mass became available after the discovery of Pluto's largest moon, Charon. By calculating the gravitational effect of Charon on Pluto, scientists were able to estimate the size and mass of Pluto as part of this binary (two-part) system.

Pluto and its moons

Pluto's Orbit

Pluto's orbit takes 248 Earth years, which means that it takes almost three centuries for it to make the trip around the sun. Pluto's orbit path is elliptical (oval-shaped), which is different from the circular orbits of other planets in the solar system. It is interesting to note that Pluto will periodically cross paths with Neptune in the course of its orbit. Scientists believe that because Pluto is a smaller body, and because of its irregular orbit, it may actually change patterns in the future. This means that our estimations of Pluto's future orbit is uncertain.

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