Pluto << PLOO toh >> is a dwarf planet that orbits far from the sun. It shares the region of its orbit, known as the Kuiper belt, with a band of icy bodies called Kuiper belt objects (KBO’s). Pluto has a diameter of about 1,470 miles (2,370 kilometers), less than a fifth that of Earth. It is smaller than all the planets and than several planetary satellites (moons) in the solar system.
Orbit and rotation.
Pluto orbits the sun at an average distance of around 3,670,050,000 miles (5,906,380,000 kilometers), almost 40 times as far as Earth. Pluto travels around the sun in an elliptical (oval-shaped) orbit, completing one orbit every 248 Earth years. Most of the time, Pluto lies beyond the orbit of Neptune, the outermost planet. For about 20 years of each orbit, however, Pluto’s irregular orbit brings it closer to the sun than is Neptune. Pluto last entered Neptune’s orbit in 1979 and remained inside it until 1999.
Pluto spins on its axis about once every six Earth days. Relative to its orbit, Pluto’s axis is tilted 122.5° from perpendicular. As a result, Pluto appears to rotate on its side. The tilt of the axis causes Pluto to have seasons. Unlike on Earth, however, Pluto’s poles receive more sunlight on average than its equator.
Mass and density.
Pluto has only a fraction of a percent of Earth’s mass. Pluto is about a third as dense as Earth and is primarily composed of rock and ice. Pluto’s gravity is 1/15 the strength of Earth’s gravity. Thus, an object weighing 100 pounds (45 kilograms) on Earth would weigh only about 7 pounds on Pluto (the equivalent of about 3 kilograms on Earth).
Surface and atmosphere.
Pluto’s surface is one of the coldest places in our solar system. The temperature ranges from -387 to -369 °F (-233 to -223 °C). Much of the surface is covered in different ices and frosts. The area around Pluto’s equator is dark red. It is made up of water ice and of methane frost that has been broken down by solar radiation. This dark belt is broken by a bright, heart-shaped basin composed of methane ice, nitrogen ice, and frozen carbon monoxide. Other areas on Pluto appear reddish brown and are covered with frozen methane.
Pluto’s surface consists of many different types of terrain, including ridges, mountains, canyons, and flowing ice plains. Some landforms are found only on Pluto. One example is the so-called “bladed terrain,” covered in steep, bladelike ridges of frozen methane. Parts of the dwarf planet, such as the dark regions around its equator, are heavily cratered. Scientists think these areas formed around 4 billion years ago. The brightest regions of Pluto have no visible craters and are younger than 10 million years old. Any large craters there were erased by flowing ices, likely in a process called cryovolcanism. Cryovolcanism resembles volcanic activity, but it is driven by ice rather than magma. Some cryovolcanoes may still be active on Pluto today.
Scientists have found evidence that an ocean of liquid water exists deep beneath Pluto’s surface. The presence of such an ocean would explain how certain geological features formed. Scientists expect this subsurface ocean to be covered by ice almost 200 miles (320 kilometers) thick.
Pluto has a thin, hazy atmosphere made up of mostly nitrogen and methane. Its atmospheric pressure can be as low as 1/100,000 that of Earth. Pluto’s atmosphere includes at least 20 distinct layers of haze up to 125 miles (200 kilometers) above the surface.
Satellites.
Charon, Pluto’s largest satellite, has a diameter of about 750 miles (1,200 kilometers). Charon is so close to Pluto in size that the two actually orbit a point above Pluto’s surface. This point of balance between two orbiting objects is called a barycenter. Pluto and Charon are doubly tidally locked, meaning the same sides of both bodies face each other at all times, and Charon revolves around their barycenter once per Pluto day. Pluto also has four smaller satellites. All of them are less than 40 miles (65 kilometers) in diameter and are irregularly shaped.
Pluto formed about 4.6 billion years ago along with the rest of the solar system. Scientists think that Pluto collided with another large body early in its history. The collision formed Charon and the other satellites. A similar collision between Earth and a large body early in the planet’s history is thought to have formed Earth’s moon.
History of Pluto study.
In 1905, Percival Lowell, an American astronomer, thought that the force of gravity of some unknown object was affecting the orbits of Neptune and Uranus. He began an intensive search for it from his observatory in Flagstaff, Arizona. He used a telescope to photograph the area of the sky where he thought the object was. He died in 1916 without finding it. In 1930, Clyde W. Tombaugh, an assistant at the Lowell Observatory, used predictions made by Lowell and other astronomers and spotted Pluto by examining photographic plates. The discovery turned out to be a coincidence, however, because the location of Pluto could not have been determined by Lowell’s calculations. The planet was named after the Roman god of the dead. The name also honors Percival Lowell, whose initials are the first two letters of Pluto.
In 1978, astronomers at the United States Naval Observatory substation in Flagstaff discovered a large satellite of Pluto. They named it Charon. In 1988, astronomers detected an atmosphere around Pluto when it passed in front of a distant star, partly blocking the star’s light. Using the Hubble Space Telescope, astronomers found four additional satellites that are much smaller than Pluto and Charon. Hydra and Nix were found in 2005. Kerberos was discovered in 2011 and Styx in 2012.
From its discovery in 1930, Pluto was widely considered the ninth planet of our solar system. But its small size and irregular orbit led many astronomers to question whether Pluto should be grouped with such worlds as Earth and Jupiter. The debate intensified in the 1990’s with the discovery of the KBO’s, which had many similarities with Pluto. The discovery of roughly Pluto-sized KBO’s led the International Astronomical Union (IAU), a widely recognized authority in naming heavenly objects, to create a new category called dwarf planets in 2006. The IAU assigned Pluto to this category. But some astronomers consider the term dwarf planet to be poorly defined and have rejected its use.
In 2015, the United States National Aeronautics and Space Administration (NASA) probe New Horizons flew by the Pluto system. Before the flyby, scientists could only study Pluto as a dim, grainy blur in photographs taken by Earth-based or orbiting telescopes. New Horizons took many stunningly detailed images of Pluto and its moons and studied its terrain and atmosphere. 
