Pluto was reclassified as a dwarf planet in 2006 and lost the title of ninth planet. The demotion of Pluto was a controversial event and provoked serious discussions in the scientific community and among people.

Percival Lowell, an American astronomer, first proposed the existence of Pluto in 1905 when he observed strange deviations in the orbits of Uranus and Neptune. Lowell thought there must be another object whose gravity was affecting the ice giants, causing them to misalign their orbits. Lowell predicted the position of the mysterious planet in 1915, But he died 15 years before its discovery. Finally, based on the predictions of Lowell and other astronomers, Clyde Tamba discovered Pluto in 1930 at the Lowell Observatory in Arizona.

Pluto was named by the suggestion of Venisha Burney , an 11-year-old child from England. With the news of the discovery of the ninth planet, Venisha suggested to her grandfather that the name of the god of the underworld in Roman mythology be placed on it. His grandfather then passed the suggested name on to the Lowell Observatory. Pluto is also considered to be a tribute to Percival Lowell because it contains the first two letters of Percival Lowell’s name.

Because Pluto is so far from Earth, little was known about the dwarf planet’s size or surface condition until 2015, when NASA’s New Horizons spacecraft flew past it. New Horizons showed that Pluto, with a diameter of 2,370 kilometers, is less than one-fifth the size of Earth and only about two-thirds the size of our planet’s moons.

New Horizons’ observations of Pluto’s surface revealed various surface features; Among the mountains whose height reaches 3500 meters and are comparable to the Rocky Mountains on Earth. Although frozen methane and nitrogen cover most of Pluto’s surface, these materials are not strong enough to support such high peaks; As a result, scientists believe that the mountains were formed on a bed of water ice.

Pluto’s surface is covered with an abundance of frozen methane, But New Horizons scientists have observed dramatic differences in the way light reflects off this icy surface across the surface of the dwarf planet. They have observed features similar to Earth’s ice sheets or erosion features in Pluto’s mountainous regions. These surface effects are much larger on Pluto; As it is estimated that their height is 500 meters; While the size of ground samples is only a few meters.

Another distinctive feature on Pluto’s surface is a large heart-shaped region known informally as the Tamba region. The left side of this area (the area that takes the shape of an ice cream cone) is covered with frozen carbon monoxide. Scientists have detected other changes in the composition of surface materials in the “heart” of Pluto.

At left center of the Tamba region is a very flat area that the New Horizons team has informally named the “Sputnik Plateau” in honor of Sputnik, the first artificial satellite to orbit Earth. This region of Pluto’s surface does not have craters caused by meteorite impact; A feature that shows that the Sputnik Plateau is geologically a very young region that is not more than a hundred million years old. It is possible that this area is still being formed and changed by geological processes.

Pluto’s ice plains show dark streaks several kilometers long that lineup. It is possible that these lines were formed by strong winds that blow on the surface of the dwarf planet. The Hubble Space Telescope has also obtained evidence that Pluto’s crust could contain complex organic molecules.

Pluto’s surface is one of the coldest places in the solar system. The temperature there can drop to minus 226 to minus 240 degrees Celsius. Comparing images taken of Pluto at different times showed that the dwarf planet appears to have become redder over time, possibly due to seasonal changes.

Pluto may have a subsurface ocean now or may have had one in the past. However, there is still no consensus on this. If an ocean had already formed under the surface of Pluto, it could have greatly influenced the history of this dwarf planet. For example, scientists believe that the Sputnik Plateau region grew so heavy over time as the ice mass increased that it overturned Pluto and brought its axial tilt to its present size (about 120 degrees). According to researchers, the subsurface ocean is the best explanation for this phenomenon. If Pluto has a liquid ocean and enough energy, it could be a haven for life.

Some of the elements that make up Pluto, according to NASA, are as follows:

Composition of the atmosphere: methane and nitrogen. New Horizons observations show that Pluto’s atmosphere extends up to 1,600 km above the surface of this dwarf planet.

Magnetic Field: Scientists still don’t know if Pluto has a magnetic field or not, But the dwarf planet’s small size and slow rotation suggest that such a field is weak or non-existent.

Chemical composition: Pluto is probably composed of a mixture of 70% rock and 30% water ice.

Internal structure: The dwarf planet probably has a rocky core surrounded by a mantle of water ice, and unusual frozen elements such as methane, carbon monoxide, and nitrogen cover its surface.

Pluto’s highly elliptical orbit can take it over 49 times the distance from Earth to the Sun. Because the dwarf planet’s orbit is highly eccentric, or non-circular, Pluto’s distance from the Sun can vary dramatically. The dwarf planet was actually closer to the Sun than Neptune for 20 years of its 248-year orbital period, giving astronomers a rare opportunity to study this small, cold, and distant world.

As a result of such an orbit, after being considered the eighth planet from the Sun for 20 years, Pluto passed the orbit of Neptune in 1999 and became the farthest planet from the Sun until it was finally demoted to a dwarf planet in 2006.

As Pluto moves closer to the Sun, its surface ice melts, temporarily forming a thin atmosphere composed mostly of nitrogen and some methane. The insignificant gravity of Pluto, which is a little more than one-twentieth of the gravity of the Earth, causes this atmosphere to expand to a much higher height compared to the Earth’s atmosphere.

As the dwarf planet moves farther from the Sun, much of its atmosphere appears to freeze and disappear. However, when Pluto has an atmosphere, it can probably experience strong winds. This atmosphere also has changes in brightness, which can be caused by gravity waves or airflow over the mountains.

Although Pluto’s atmosphere is too thin to allow liquids to flow today, liquid elements may have flowed on the dwarf planet’s ancient surface in the past. New Horizons captured an image of a frozen lake in the Tampa area that appeared to have ancient waterways nearby. At one point in its history, Pluto could have had an atmosphere almost 40 times thicker than that of Mars.

In 2016, scientists announced that they may have observed clouds in Pluto’s atmosphere using data from New Horizons. The researchers saw seven bright objects that were located near the boundary between light and dark. This area is usually where clouds form. These possible clouds were all at low altitudes and almost the same size, which indicates that they are separate complications. The composition of the clouds, if they really exist, would probably be acetylene, ethane, and hydrogen cyanide.

• Pluto’s rotation is retrograde compared to other worlds in the solar system; This means that the dwarf planet rotates backwards and from east to west.

• The average distance from the sun: 5,906,380,000 km or 39.4 astronomical units.

• Periphery (shortest distance to the Sun): 4,434,987,000 km or 30.1 AU.

• Apogee (farthest distance from the Sun): 7,304,326,000 km or 48 AU.

• Pluto’s orbital path around the Sun is not in the same plane as the eight planets of the solar system; Rather, it is located at an angle of 17 degrees.

Those who went to school until 17 years ago and before that, had learned in textbooks that Pluto is the ninth planet of the solar system. But in August 2006, the International Astronomical Union (IAU) downgraded Pluto to a “dwarf planet”. This meant that from then on, only the rocky worlds of the inner solar system and the gas giants of the outer reaches of the planet were considered.

The “inner solar system” is a region of space smaller than the radius of Jupiter’s orbit around the Sun. This range includes the asteroid belt as well as rocky planets such as Mercury, Venus, Earth, and Mars. Gas giants including Jupiter, Saturn, Neptune, and Uranus also form the outer limits of the solar system. As a result, now we have eight planets instead of nine.

According to the IAU definition, a “dwarf planet” is a celestial body that orbits directly around the Sun and has enough mass to be controlled by gravitational forces rather than mechanical forces, and as a result, is elliptical in shape; But it doesn’t clear the surrounding area from other objects. The three IAU criteria for a planet are as follows:

• It revolves around the sun.

• It has enough mass to achieve hydrostatic equilibrium (almost spherical shape).

• It has cleared the area around its orbit.

Pluto only meets the above two conditions and does not meet the third criterion, and in all the billions of years it has existed at this point, it has not been able to clear its vicinity. You may ask, what does “clearing the surrounding area of ​​other objects” mean? This condition means that the planet is dominated by gravity and there is no other body of similar size in its vicinity, except for moons or objects that are influenced by its gravity; While Pluto shares its neighborhood with Kuiper belt objects like plutinos.

Some scientists in recent years have demanded that by changing the definition of a planet, Pluto will return to the group of planets in the solar system. However, if this happens, the number of planets in our cosmic neighborhood may exceed the current number.

Pluto has five moons: Charon, Stokes, Nyx, Cerberus, and Hydra, of which Charon is the closest moon to Pluto and Hydra is the farthest.

In 1978, astronomers discovered that Pluto has a very large moon, almost half the size of the dwarf planet itself. This moon was named Charon or Kharon, inspired by the spirit-carrying creature in Greek mythology that led souls to the underworld.

Because Charon and Pluto are so similar in size, their orbits differ from those of most of the planets and their moons. Much like binary star systems, Pluto and Charon both orbit a point in space that lies between them. For this reason, scientists refer to Pluto and Charon as a binary dwarf planet, binary planet, or binary system.

Pluto and Charon are only 19,640 kilometers apart, or less than the distance between London and Sydney by plane. Charon’s orbit around Pluto takes 6.4 Earth days, and one Pluto revolution (one Pluto day) takes the same amount of time. The reason for this is that Charon hovers over the same point on Pluto’s surface, and the same side of the moon is always seen from the dwarf planet. This phenomenon is called fatal lock.

While Pluto has a reddish hue, Charon appears more grayish. This moon may have contained a subsurface ocean in the early days of its formation; But today, it probably cannot support such a complication. Compared to most of the planets and moons of the solar system, the system of Pluto and Charon is turned sideways with respect to the Sun.

New Horizons observations of Charon revealed the existence of valleys on the moon’s surface. The largest Sharon valley is 9.7 km deep, and a large mass of rocks and depressions stretches 970 km in the middle of the moon. A part of the moon’s surface near one pole is covered with much darker material than the rest.

Much of Charon’s surface is similar to Pluto’s crater-free regions, indicating that the moon is quite young and geologically active. Scientists have observed evidence of landslides on Charon’s surface, the first observation of such phenomena in the Kuiper Belt. The moon may also have its own form of plate tectonics; A phenomenon that causes geological changes on earth.

In 2005, in preparation for NASA’s New Horizons mission, scientists photographed Pluto using the Hubble Space Telescope and found two other small moons of this dwarf planet. These moons, named Nyx and Hydra, are two and three times more distant from the dwarf planet than the distance between Charon and Pluto. Based on New Horizons measurements, the length and width of Nix are estimated to be 42 and 36 kilometers, respectively; While Hydra is 50 km long and 30 km wide. It is likely that the surface of Hydra is mainly covered by water ice.

Using the Hubble telescope, scientists discovered Pluto’s fourth moon, Herbrus, in 2011. This moon has a two-part shape, the big part is about 8 km long and the small part is about 5 km wide. On July 11, 2012, a fifth moon named Stokes (with an estimated diameter of 10 km) was discovered, further fueling the debate over Pluto’s planet status.

The main hypothesis for the formation of Pluto and Charon is that the nascent Pluto had a surface collision with another body of its size. According to this idea, most of the combined material of the two bodies became Pluto and the other remnants formed Charon. The other four moons may have formed from the same collision that created Charon.

The New Horizons spacecraft is the first probe to closely study Pluto, its moons, and other Kuiper Belt worlds. The spacecraft was launched in January 2006 and successfully made its closest approach to the dwarf planet on July 14, 2015. The New Horizons probe is carrying some of the ashes of Pluto discoverer Clyde Tamba.

Limited knowledge of the Pluto system created unprecedented risks for the New Horizons probe. Before the mission launch, scientists knew of only three moons around Pluto. Herbrus and Stokes’ discovery during the spacecraft’s journey fueled the idea that more unseen moons may be orbiting the dwarf planet. Hitting these hidden moons or even small debris could seriously damage the spacecraft. However, the New Horizons team equipped the space probe with tools to protect it during its journey.

In October 2015, New Horizons made history by sending the first close-up images of Pluto and its moons. You can see these amazing pictures at the end of the article.

Currently, no other mission after New Horizons is officially planned to visit Pluto; But at least two conceptual designs are under study. In April 2017, a workshop was held in Houston, Texas to discuss ideas for the next Pluto mission. Possible goals discussed by the team for such a mission include mapping the surface with an accuracy of 9 meters per pixel, observations of Pluto’s smaller moons, how Pluto changes as it rotates on its axis, and topographic mapping of regions darkened by the dwarf planet’s axial tilt. They are long-term.

New Horizons principal investigator Ellen Stern believes that if an orbiter were sent to study Pluto, we could map 100 percent of the dwarf planet, even the surfaces in total shadow. New Horizons data indicated the possible existence of a subsurface ocean on Pluto, and researchers believe that the orbiter mission can also find evidence of such a complex.