Pluto 101 | National Geographic

• [Instructor] At the edge of the solar system, Pluto pushes the boundaries of our understanding of the universe. Nestled within the far-flung Kuiper belt, the dwarf planet is believed to be one of the countless celestial objects left over from the formation of the solar system. While it is one of the Kuiper belt's largest bodies, Pluto is only half as wide as the contiguous United States and about two-thirds the size of Earth's moon. Making it easily dwarfed by the solar system's eight true planets.

This tiny world takes 248 Earth years to orbit the Sun, and it does so from an average distance of over 3.6 billion miles, or 40 times the space between Earth and the Sun. Such a distance from the solar system's main source of heat causes extreme temperatures on the dwarf planet's surface, between negative 375 and negative 400 degrees Fahrenheit. The frigid temperatures play a crucial role in shaping the geological composition of Pluto. Its core, likely made of rock and metals, is encapsulated by a mantle made of ice. The dwarf planet's crust is made of rock material and other types of ice, such as frozen carbon monoxide, methane, and nitrogen.

These frozen gases cover Pluto's approximately 6.4 million square miles of surface area, which is barely the size of Russia. The icy terrain is much like Earth's, with polar ice caps, valleys, plains, and craters. It even has glaciers made of frozen nitrogen, and frozen water on the surface makes giant floating mountains. (peaceful music) Such varied terrain is influenced by the presence of weather patterns or an atmosphere. Pluto's atmosphere is thin and reaches a high altitude due to the dwarf planet's low gravity, which is only about 6 percent as strong as Earth's.

The atmosphere's comprised of nitrogen and methane gases, plus red hydrocarbon particles that scatter sunlight to give Pluto blue skies. Parts of the atmosphere may even freeze and fall like snow. Floating high above the dwarf planet's atmosphere are five moons. The largest, Charon, is about half the size of Pluto. The four other moons—Nix, Hydra, Kerberos, and Styx—are much smaller.

While the eight true planets were discovered by the mid-1800s, Pluto was not spotted until 1930. Pluto, named after the Roman god of the underworld, was considered the ninth planet. But in 2006, Pluto lost this status. At the time, worlds similar to Pluto were being discovered deeper in the Kuiper belt. This initiated close scrutiny of Pluto and the definition of a planet.

The International Astronomical Union evaluated Pluto based on the characteristics necessary to be a true planet. It must orbit the Sun, is not a moon, and has enough mass and gravitational pull to assume a round shape. However, it was the fourth characteristic that compromised Pluto's status: its inability to clear its orbit of debris. So Pluto was reclassified and arguably demoted from being a true planet to being a dwarf planet.

Pluto's story represents our evolving understanding of the universe. There is always more to discover, and we are continuously reaching toward it.