Pluto

Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet located in the Kuiper belt, a distant area filled with icy objects beyond Neptune. It was found in 1930 by Clyde W. Tombaugh and was once called the ninth planet of our solar system. In 2006, scientists changed the rules for what counts as a planet, and Pluto became a dwarf planet.

Pluto is much smaller than the planets closer to the Sun. It has about one-sixth the mass of Earth's Moon. It is made mostly of ice and rock. Pluto travels around the Sun in a path that sometimes brings it closer to the Sun than Neptune, but the two never hit each other because of how their orbits are arranged.

Pluto has five known moons. The biggest is Charon, which is more than half the size of Pluto. In 2015, the spacecraft New Horizons visited Pluto. It was the first and only spacecraft to go to Pluto. It sent back pictures and information that helped scientists learn more about this interesting world.

History

Discovery

Further information: Planets beyond Neptune

In the 1840s, a scientist named Urbain Le Verrier used math to find where the planet Neptune might be. In 1906, Percival Lowell began looking for another planet he called "Planet X". He and his team searched for years. In 1930, Clyde Tombaugh found a moving object in the sky. It was later named Pluto.

Name

The name Pluto was chosen because it matched a Roman god of the underworld. Many people suggested names, but “Pluto” was picked because it started with the same letters as astronomer Percival Lowell’s name. An eleven-year-old girl in England named Venetia Burney was one of the first to suggest the name.

Symbol

After Pluto was named, a special symbol was created using the letters “PL”. The name Pluto also inspired other things, like a character in cartoons and an element called plutonium.

Planet X disproved

After Pluto’s discovery, scientists realized it was too small to be the mysterious Planet X. They later found that there wasn’t a bigger planet. Instead, Pluto is part of a group of objects called the Kuiper belt.

Classification

Further information: Definition of planet

As more objects like Pluto were found, scientists debated whether Pluto should still be called a planet. In 2006, scientists made a new rule: a planet must clear its area of other objects. Pluto didn’t meet this rule, so it was called a dwarf planet instead. Some people still think Pluto should be a planet.

Orbit

Pluto takes about 248 years to go around the Sun. Unlike the planets, Pluto's orbit is tilted and stretched out. This means Pluto sometimes comes closer to the Sun than Neptune's orbit, but it never hits Neptune.

Pluto and Neptune never crash into each other because of a special pattern in their orbits. For every two times Pluto goes around the Sun, Neptune goes around three times. This keeps their paths separate and safe. Even when Pluto's orbit crosses Neptune's path, Pluto is always far enough away to stay safe. This pattern has stayed the same for millions of years.

Rotation

Pluto's day is 6.387 Earth days long. Like Uranus and 2 Pallas, Pluto spins on its side. This gives Pluto very different seasons, with one part getting constant sunlight and another part staying dark.

Scientists at the University of Arizona think this might happen because of frozen nitrogen building up in dark areas. This can change how Pluto spins, just like how Earth's Antarctic ice sheet affects our planet.

Geology

Main articles: Geology of Pluto and Geography of Pluto

Pluto's surface is mostly made of nitrogen ice, with some methane and carbon monoxide. Its mountains are made of water ice. The surface looks very different in brightness and color, ranging from dark black to bright white. One famous feature is called Tombaugh Regio, or the "Heart," a bright area on Pluto.

Scientists think Pluto has a rocky core surrounded by a layer of water ice. There might be a hidden ocean of liquid water deep inside Pluto. The surface shows signs of geological activity, including glaciers.

Mass and size

Pluto is a small, faraway world. Its diameter is about 2,376.6 kilometers. That’s a bit larger than the continent of Antarctica, but its mass is much smaller. Pluto’s surface gravity is very weak—only about 0.063 times that of Earth’s.

Pluto is more than twice as wide as Ceres, the largest object in the asteroid belt. But it is less massive than another dwarf planet called Eris. Scientists measured Pluto’s size and mass using telescopes, spacecraft like NASA’s New Horizons, and observations of its moon, Charon.

The mass of Pluto compared to other dwarf planets (Eris, Haumea, Makemake, Gonggong and Quaoar, plus Pluto's moon Charon and the former dwarf planet Triton (Neptune I)). The unit of mass is ×10²¹ kg.

1. Triton 21.39
2. Eris 16.38
3. Pluto 13.03
4. Haumea 3.95
5. Makemake 2.69
6. Gonggong 1.75
7. Charon 1.59
8. Quaoar 1.21

Atmosphere

Main article: Atmosphere of Pluto

Pluto has a very thin atmosphere made mostly of nitrogen, methane, and carbon monoxide. The atmosphere changes as Pluto moves around the Sun, sometimes getting thicker and sometimes thinner. Even though Pluto is very cold, its atmosphere can stay as a gas because of how these gases act.

Scientists have found that Pluto's atmosphere has layers of haze high above its surface. These layers are made by winds moving over Pluto's mountains, which help keep the atmosphere a little warmer than the surface below.

Natural satellites

Main article: Moons of Pluto

Pluto has five known natural satellites. The biggest and closest is Charon, found in 1978. Beyond Charon are four smaller moons: Styx, Nix, Kerberos, and Hydra. Nix and Hydra were found in 2005, Kerberos in 2011, and Styx in 2012. These moons move around Pluto in almost circular paths.

Pluto's place in our Solar System was unclear for a long time. In 1992, astronomers found small icy objects beyond Neptune, like Pluto. These objects are part of the Kuiper belt, a group of bodies far from the Sun, where Pluto is the largest. Pluto is thought to be a piece left over from when planets formed around the Sun.

Observation and exploration

Pluto is very far from Earth, so it is hard to study closely. You need a telescope to see Pluto because it looks like a star, even in big telescopes. Early maps of Pluto were made by watching how its brightness changed when its largest moon, Charon, passed in front of it.

The New Horizons spacecraft flew by Pluto in July 2015. It was the first and only spacecraft to explore Pluto directly. It sent back lots of important information about Pluto and its moon Charon. Scientists hope to send more missions to learn even more about Pluto and its surroundings.