Pluto

Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet located in the Kuiper belt, a distant region filled with icy bodies beyond the orbit of Neptune. It was discovered in 1930 by Clyde W. Tombaugh and was originally considered the ninth planet of our solar system. However, in 2006, after discovering many similar objects in the Kuiper belt, scientists created a new definition of a planet, and Pluto was reclassified as a dwarf planet.

Pluto is much smaller than the inner planets and has about one-sixth the mass of Earth's Moon. It is made mostly of ice and rock and orbits the Sun in a path that sometimes brings it closer to the Sun than Neptune, though the two never collide due to a special pattern in their orbits.

Pluto has five known moons, the largest being Charon, which is more than half the size of Pluto itself. In 2015, the spacecraft New Horizons became the first and only spacecraft to visit Pluto, sending back detailed images and data that helped scientists learn much more about this fascinating world.

History

Discovery

Further information: Planets beyond Neptune

In the 1840s, a scientist named Urbain Le Verrier used math to predict where the planet Neptune might be, after noticing strange movements in the orbit of Uranus. Later, in 1906, Percival Lowell began searching for another possible planet he called "Planet X". He and his team looked for years but didn’t find it until after his death. In 1930, a young astronomer named Clyde Tombaugh discovered 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”. Over time, this symbol changed in how it looked. 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 that Lowell had predicted. They kept looking for a bigger planet, but later found that there wasn’t one. Instead, Pluto turned out to be 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, a group of 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, but the official rule says otherwise.

Orbit

Pluto takes about 248 years to orbit the Sun. Unlike the planets, which have nearly circular orbits close to a flat plane called the ecliptic, Pluto's orbit is tilted and stretched out. This means Pluto's path sometimes brings it closer to the Sun than Neptune's orbit, even though it does not actually get closer to Neptune.

Pluto and Neptune never collide because of a special pattern in their orbits. For every two orbits Pluto makes, Neptune makes three. This keeps their paths separate and safe from collisions. Even though Pluto's orbit crosses Neptune's path when viewed from above or below, Pluto is always far enough north to avoid any meeting. This pattern has stayed the same for millions of years, keeping both planets safe.

Rotation

Pluto's day is equal to 6.387 Earth days. Like Uranus and 2 Pallas, Pluto spins on its side with an axial tilt of 120°. This causes extreme seasons, where one part of Pluto has continuous daylight while another part stays dark.

Researchers from the University of Arizona think this unusual tilt might be due to frozen nitrogen building up in shadowed areas. This buildup can cause Pluto to shift its orientation, similar to how Earth's Antarctic ice sheet affects our planet.

Geology

Main articles: Geology of Pluto and Geography of Pluto

Pluto's surface is made mostly of nitrogen ice, with some methane and carbon monoxide. Its mountains are made of water ice, and 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 even be a hidden ocean of liquid water deep inside Pluto. The surface shows signs of many kinds of geological activity, including glaciers and possible volcanic activity.

Mass and size

Pluto is a small, distant world with a diameter of about 2,376.6 kilometers and a mass of (1.303±0.003)×10²² kg, which is about 17.7% of the Moon’s mass. It is slightly larger in size than the continent of Antarctica, but much smaller in mass. Compared to Earth, 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 have measured Pluto’s size and mass using telescopes, spacecraft like NASA’s New Horizons, and observations of its moon, Charon. These methods helped them learn more about this fascinating object far from the Sun.

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. This atmosphere changes as Pluto moves around the Sun, sometimes getting thicker and sometimes thinner. Even though Pluto is very cold, its atmosphere can stay gaseous because of the way these gases behave.

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

Natural satellites

Main article: Moons of Pluto

Pluto has five known natural satellites. The largest and closest to Pluto is Charon, first identified in 1978. Charon is special because it might be in a state where it balances itself in shape. 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 orbit Pluto in nearly circular paths.

Pluto's place in our Solar System was unclear for a long time. It wasn't until 1992 that astronomers found small icy objects beyond Neptune, similar to Pluto. These objects are part of the Kuiper belt, a group of bodies between 30 and 50 AU from the Sun, where Pluto is the largest member. Like other objects in the Kuiper belt, Pluto is thought to be a leftover piece from when planets formed around the Sun.

Observation and exploration

Pluto is very far from Earth, making it hard to study closely. To see Pluto, you need a telescope 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, being 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.