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Why is Pluto the smallest planet? Why did pluto drop out of the planets. Orbit and rotation

Pluto was discovered in 1930 by astronomer Clyde Tombaugh at the Lowell Observatory in Arizona. His search was carried out for 15 years, since the existence of a trans-Neptunian planet was predicted by Percival Lowell from disturbances in the movement of Uranus and Neptune. These calculations turned out to be erroneous, but by pure chance, Pluto was discovered not far from the predicted location.

Pluto is the only planet that has never been visited by spacecraft. Therefore, data on the characteristics of this planet are known only approximately: the diameter is about 2200 km, the temperature on the surface is 35-55 K (about -210 ° C). Pluto is made up of a mixture of rocks and ice, while the atmosphere is made up of nitrogen and methane.

The largest of Pluto's moons, Charon, named after the mythological carrier of the dead across the river of the dead - Styx to the gates of Hades, was discovered in 1978 by Jim Chrisley. Charon has a diameter of about 1200 km and revolves with a period of 6.4 days in an orbit around the center of gravity common with Pluto, which lies between them. Pluto and Charon always face each other on the same side. In 2005, the Hubble Space Telescope discovered two more very small satellites (61 and 46 km) around Pluto, which a year later were named Hydra and Nyx. The same letters begin the words in the name of the first interplanetary probe New Horizons - "New Horizons", which in the same year went on a 10-year trip to Pluto.

Since the end of the 20th century, celestial bodies with a diameter of several hundred to several thousand kilometers have been increasingly found beyond the orbit of Neptune, which have come to be called trans-Neptunian objects. Collectively, they are sometimes referred to as the Kuiper belt. As he explored, it became increasingly clear that Pluto is an ordinary trans-Neptunian object. In 2003, an object UB 313, larger than Pluto, was found on the outskirts of the solar system.

As a result, in August 2006, the International Astronomical Union decided to deprive Pluto of the status of a planet and introduce a new category of dwarf planets, which originally included Pluto, UB 313 and the "upgraded" asteroid Ceres from the main asteroid belt between Mars and Jupiter. Thus, Pluto stayed in the status of a planet for 76 years and became the first celestial body that lost this status.

On August 24, 2006 in Prague at the XXVI Assembly of the International Astronomical Union (IAU), 2500 astronomers decided that Pluto is not a planet, as previously thought, but a dwarf planet.

AiF.ru figured out why scientists thought Pluto was a dwarf planet.

What celestial body can be called a planet?

A planet can only be considered a celestial body that revolves around the Sun and has enough gravity to have a shape close to a sphere. In addition, a planet is a body whose orbit does not intersect with anything.

Why didn't Pluto qualify as a "planet"?

According to the IAU definition, a planet must meet three requirements:

1. It must revolve around the Sun (or another star).

2. It must be massive in order to take a spherical shape under the influence of its own gravity.

3. It must clear its own orbit (there should be no other bodies of the same size nearby, except for its own satellites).

Pluto falls under points 1 and 2, but does not meet the third requirement, since he was unable to clear his own orbit. The mass of a dwarf planet is only 0.07 of the mass of all objects in its orbit. For example, the mass of the Earth is 1.7 million times greater than the rest of the bodies in its orbit.

Why was Pluto named that way?

Pluto was discovered in 1930 by the American astronomer Clyde Tombaugh. For a long time, he and his colleagues could not come up with a name for a new object in the solar system. Chance helped them to cope with this task. English 11-year-old learned about the discovery of a new celestial body from the newspapers schoolgirl Venice Burney. The girl decided that it would be nice if Pluto appeared in space - as the ancient Romans called the god of the underworld. She told her grandfather about it. Faulconer Meidan who worked at the Bodleian Library at Oxford University. Meydan passed the proposal of his granddaughter to a friend Professor Herbert Turner who wired it to American astronomers. For some reason, the name Pluto seemed very successful to them, and they chose it. For her contribution to the history of astronomy, Venice Burney received a symbolic award of five pounds sterling.

Not so long ago, Pluto was excluded from the list of planets in the solar system and classified as a dwarf planet. Let's figure it out why is pluto not a planet.

1. History, or everything is fine

Pluto was first discovered in 1930 by Clyde Tombaugh at the Lowell Observatory in Arizona. Astronomers had long predicted that there was a ninth planet in the solar system, which they called Planet X. Tombo was given the laborious task of comparing many photographic plates with images of areas of the sky taken two weeks apart. Any moving object, such as an asteroid, comet or planet, had to change its position in different photographs.

Pluto is smaller than the moon. Its mass is too small to clear space in its orbit from other similar objects.

After a year of observations, Tombaugh finally found an object with a suitable orbit and claimed that he had finally found Planet X. Since the discovery was made at the Lowell Observatory, the observatory team got the right to give the planet a name. The choice was made in favor of the name Pluto, which was suggested by an 11-year-old schoolgirl from Oxford, England (after the Roman god of the underworld).

The solar system acquired the 9th planet.

Astronomers could not determine the mass of Pluto until the discovery of its largest moon, Charon, in 1978. Then, having determined the mass of Pluto (0.0021 Earth masses), they were able to more accurately estimate its size. According to the latest data, the diameter of Pluto is 2400 km. Pluto is just tiny, but then it was believed that there was nothing bigger than this dwarf planet beyond the orbit of Neptune.

2. Something went wrong, or the root of the problem

However, over the past few decades, new powerful ground-based and space-based observatories have completely changed previous ideas about the outer regions of the solar system. Rather than being the only planet in its region, like all the other planets in the solar system, Pluto and its moons are now known to be an example of a large number of objects united under the name Kuiper belt. This region extends from the orbit of Neptune to a distance of 55 astronomical units (the boundary of the belt is 55 times farther from the Sun than the Earth).


Kuiper belt. Source: Nature

According to recent estimates, there are at least 70,000 icy objects in the Kuiper belt that are 100 km or more in diameter and have the same composition as Pluto. According to the new rules for identifying planets, the fact that Pluto's orbit is inhabited by such objects is the main reason why Pluto is not a planet. Pluto is just one of many Kuiper belt objects.

That's the whole problem. Since the discovery of Pluto, astronomers have been discovering larger and larger objects in the Kuiper Belt. The dwarf planet 2005 FY9 (Makemake), discovered by Caltech astronomer Mike Brown and his team, is only slightly smaller than Pluto. Later, several other similar objects were discovered (for example, 2003 EL61 Haumea, Sedna, Orc, etc.).

Astronomers have realized that the discovery of an object larger than Pluto in the Kuiper belt is only a matter of time.


Some currently known dwarf planets compared to Earth.

And in 2005, Mike Brown and his team broke the astonishing news. They found an object beyond the orbit of Pluto that was probably the same size, maybe even larger. Officially named 2003 UB313, the facility was later renamed Eridu. Astronomers later determined that Eris had a diameter of about 2600 km, plus it had a mass about 25% greater than that of Pluto.

With Eris, more massive than Pluto, made up of the same mixture of ice and rock, astronomers have been forced to rethink the concept that the solar system has nine planets. What is Eris - a planet or a Kuiper belt object? What is Pluto? The final decision was to be taken at the XXVI General Assembly of the International Astronomical Union, which was held from 14 to 25 August 2006 in Prague, Czech Republic.

3. Pluto is no longer a planet, or a difficult decision

The astronomers of the association were given the opportunity to vote for various options for determining the planet. One of these options would increase the number of planets to 12: Pluto would continue to be considered a planet, Eris and even Ceres, which was previously considered as the largest asteroid, would be added to the number of planets. Various proposals supported the idea of ​​9 planets, and one of the options for determining the planet led to the deletion of Pluto from the list of the planetary club. But then how to classify Pluto? Do not consider it an asteroid.

What is a planet according to the new definition? Is Pluto a planet? Does it pass the classification? For a solar system object to be considered a planet, it must meet four requirements defined by the IAU:

  • The object must orbit around the Sun - And Pluto passes.
  • It must be massive enough to form a spherical shape with its force of gravity - And here everything seems to be in order with Pluto.
  • It must not be a satellite of another object. Pluto itself has 5 moons.
  • It should be able to clear the space around its orbit from other objects - Aha! This rule breaks Pluto, it is the main reason why Pluto is not a planet.

What does it mean to "clear the space around your orbit from other objects"? At a time when the planet is just being formed, it becomes the dominant gravitational body in a given orbit. When it interacts with other, smaller objects, it either absorbs them or pushes them away with its gravity. Pluto is only 0.07 of the mass of all objects in its orbit. Compare with the Earth - its mass is 1.7 million times the mass of all other objects in its orbit combined.

Any object that does not meet the fourth criterion is considered a dwarf planet. Therefore, Pluto is a dwarf planet. In the solar system, there are a lot of objects with similar sizes and masses that move in approximately the same orbit. And until Pluto collides with them and takes their mass to its hands, it will remain a dwarf planet. The same is true of Eris.

Satellites of Pluto.

Characteristics:

  • Distance from the Sun: 5,900 million km
  • Planet Diameter: 2,390 km*
  • Days on the planet: 6 days 8 hours**
  • Year on the planet: 247.7 years***
  • t° on the surface: -230°C
  • Atmosphere: Composed of nitrogen and methane
  • Satellites: Charon

* diameter at the equator of the planet
** period of rotation around its own axis (in Earth days)
*** orbital period around the Sun (in Earth days)

Pluto is one of the farthest small objects in the solar system (since 2006, the status of the planet has been replaced by the status of a dwarf planet). This small dwarf planet is located 5900 million km from the Sun and makes one revolution around the celestial body in 247.7 years.

Presentation: planet Pluto

* Correction to presentation video: New Horizons spacecraft has already explored Pluto

The diameter of Pluto is relatively small, it is 2390 km. The approximate density of this celestial body is 1.5 - 2.0 g / cm³. In terms of its mass, Pluto is inferior to other planets, this figure is only 0.002 of the mass of our Earth. Astronomers have also found that one day on Pluto is equal to 6.9 Earth days.

Internal structure

Since Pluto remains a little-studied planet due to its considerable distance from the Earth, scientists and astronauts can only speculate about its internal structure. Officially, it is believed that this planet consists entirely of frozen gases, in particular methane and nitrogen. Such an assumption was put forward on the basis of spectral analysis data carried out in the late 1980s. However, there is reason to believe that Pluto has a core, possibly with ice content, an icy mantle and crust. The main constituents of Pluto are water and methane.

atmosphere and surface

Pluto, which occupies the ninth place in size among the planets of the solar system, has its own atmosphere, unsuitable for any living organisms to live on it. The atmosphere consists of carbon monoxide, methane gas, which is very light and poorly soluble in water, and a large amount of nitrogen. Pluto is a very cold planet (about -220 °C), and its approach to the sun, which occurs no more than once every 247 years, contributes to the transformation of part of the ice covering its surface into gas and lowering the temperature by another 10 °C. At the same time, the temperature of the atmosphere of a celestial body fluctuates within - 180 ° C.

The surface of Pluto is covered with a thick layer of ice, the main component of which is nitrogen. It is also known that it has flat terrain and rocks made of hard rocks with an admixture of the same ice. The south and north poles of Pluto are covered with eternal snow.

Moons of the planet Pluto

For a long time it was known about one natural satellite of Pluto, its name is Charon, and it was discovered in 1978, but it turned out to be not the only satellite of a distant planet in the solar system. In a re-study of Hubble telescope images in 2005, two more satellites of Pluto, S/2005 P1 and S/2005 P2, were discovered, which soon received the names Hydra and Nix. To date, in 2013, 5 satellites of Pluto are known, the fourth discovered was the satellite with the temporary designation P4 in June 2011, and the fifth P5 in July 2012.

As for the main large satellite by Pluto's standards, Charon, its size is 1200 km in diameter, which is only half the size of Pluto itself. Their strong differences in composition lead scientists to the hypothesis that the entire Pluto-Charon system was formed as a result of a powerful collision of the future planet with its future satellite during the stage of their independent formation from the proto-cloud.

It turns out that Charon was formed from the ejected fragments of the planet, and with it other much smaller small satellites of Pluto.

Pluto is considered a separate dwarf planet in the solar system, although some astronomers are willing to argue with this. This celestial body is located in the so-called Kuiper belt, which consists mainly of massive asteroids and dwarfs (minor planets), which include some volatile substances (for example, water) and certain rocks. Therefore, a number of scientists believe that it would be very appropriate to call Pluto not a planet, as everyone is used to, but an asteroid. Since 2006, Pluto has been classified as a dwarf planet.

Exploring the planet

Pluto was discovered by astronomers relatively recently (in 1930), its satellite Charon in 1978, and other satellites - Hydra, Nikta, P4 and P5 - even later, just a few years ago. Initially, the assumption of the existence of such a celestial object in the Kuiper belt was made by the American astronomer Percival Lovell back in 1906. However, the instruments used to observe the planets at the beginning of the 20th century did not allow us to determine its exact location. For the first time in the pictures, Pluto was captured in 1915, but its image was so subtle that scientists did not attach any importance to it.

Today, the discovery of the ninth planet is associated with the name of Clyde Tombaugh, an American who has been studying asteroids for many years. This astronomer was the first to take a high-quality image of Pluto, for which he received an award from the Astronomical Society of England.

For a long time, much less attention was paid to the study of Pluto than to other planets, although some attempts to send a spacecraft to a celestial body so far from the Sun (almost 40 times farther than from the Earth) were made. This planet is not of particular interest to scientists, since their attention is focused primarily on those celestial bodies on which the probability of the existence of any life is several times higher. One such object is Mars.

Nevertheless, on January 19, 2006, NASA launched the New Frontiers interplanetary automatic station to Pluto, which on June 14, 2015 made a flyby at the closest possible distance to Pluto (~ 12500 km) and within 9 days transmitted a lot of important for scientific mission images and data (~ 50GB of information).

(An image of the surface of Pluto taken by New Horizons at very close range. The picture clearly shows the plains and mountains.)

This is one of the longest space travels, the New Horizons mission is designed for 15 - 17 years. By the way, the New Frontiers spacecraft has the highest of all other automatic stations. Also, during its long flight, the spacecraft studied Jupiter, transmitting a lot of new images and successfully crossed the orbit of Uranus, and after studying the dwarf planet Pluto, continued on its way towards distant Kuiper belt objects.

Against the backdrop of media hype caused by the American spacecraft "New Horizons", we invite you to recall the history of Pluto, as well as to understand the reasons why it was excluded from the list of planets.

History of Pluto

At the end of XIX - beginning of XX century. astronomers from all over the world hunted for the planet, which was conventionally called "Planet X". She, judging by the studies, was further than Neptune and had a significant impact on its orbit. In 1930, Clyde Tombaugh, an explorer at the Lowell Observatory in Arizona, claimed that he had finally found this planet. The discovery was made based on images of the night sky taken at intervals of two weeks, which made it possible to track changes in the location of objects. The right to name the new celestial body belonged to the Lowell Observatory, and the choice fell on the option proposed by an 11-year-old schoolgirl from England. Venice Burney, that was the name of the girl, suggested naming the planet " Pluto”, in honor of the Roman god of the underworld. In her opinion, such a name very well suited such a distant, dark and cold planet.

Pluto diameter, according to the latest data, is 2370 km, and the mass is 1022 kg. By cosmic standards, this is a tiny planet: pluto volume 3 times smaller than the volume of the moon, and weight and does 5 times inferior to the moon. Wherein pluto area is 16.647.940 km2, which is approximately equal to the area of ​​Russia (17.125.407 km2).

Kuiper Belt

When scientists discovered Pluto, they believed that there was nothing else beyond the orbit of Neptune. However, a few decades later, the researchers completely changed their minds. Thanks to powerful new telescopes, scientists have discovered that unlike other planets in our solar system, Pluto is surrounded by many other objects along its entire orbit, each with a diameter of more than 100 km, and similar in composition to Pluto itself. The accumulation of these objects began to be called Kuiper Belt. This region extends from the orbit of Neptune to a distance of 55 AU. (astronomical units) from the Sun (1 AU is equal to the distance from the Earth to the Sun).

Why Pluto is not a planet in the solar system

The Kuiper belt wasn't a problem until scientists began to discover larger and larger objects in it that were comparable in size to Pluto itself.

2005 was rich in discoveries. In January 2005 scientists discovered Eridu. This planet not only had its own satellite, but until July 2015 was considered bigger than Pluto. In the same year, scientists discovered 2 more planets - Makemake And Haumea, whose dimensions are also comparable to Pluto.

Thus, with 3 new planets (one of which was considered larger than Pluto), scientists had to make a serious decision: either increase the number of planets in the solar system to 12, or revise the criteria for classifying planets. As a result, on August 24, 2006, the participants of the XXVI General Assembly of the International Astronomical Union decided to change definition of the term "planet". Now, for an object in the solar system to be officially called a planet, it must meet all of the following conditions:

Orbit around the sun;
not be a satellite of another planet;
have sufficient mass to take a shape close to a ball under the influence of their own gravitational forces (in other words, to be round);
the force of gravity to clear the neighborhood of its orbit from other objects.

Neither Pluto nor Eris meet the latter condition, and therefore are not considered planets. But what does it mean to "clear the orbit of other objects?".

Everything is very simple. Each of the 8 planets of the solar system is the dominant gravitational body in its orbit. This means that when interacting with other, smaller objects, the planet either absorbs them or pushes them away with its gravity.

If we consider the situation on the example of our planet, then the mass of the Earth is 1.7 million times larger than all other bodies in its orbit. For comparison, the mass of Pluto is only 0.07 of the mass of all objects in its orbit, and this is absolutely not enough to clear the planet's vicinity from asteroids and other bodies.

For planets that cannot clear an orbit, scientists have introduced a new definition - "dwarf planets". Pluto, Eris, Makemake and many other relatively large objects of our solar system fall under this classification.

Pluto exploration. Results from New Horizons.

Due to its remoteness and small mass, Pluto has long been one of the least explored planets in our solar system. In January 2006, NASA launched an automatic interplanetary vehicle into space. "New Horizons", whose main mission was to study Pluto and its moon Charon.

The surface of the "heart of Pluto"

In July 2015, after 9 and a half years "New Horizons" reached the orbit of Pluto and began to transmit the first data. Thanks to the clear images taken by the station, scientists were able to make several important discoveries:

  1. Pluto is bigger than we thought. The diameter of Pluto is 2.370 km, which means that it is still larger than Eris, whose diameter is 2.325 km. Despite this, the mass of Eris is still considered to be 27% more than the mass of Pluto.
  2. Pluto reddish brown. This color is due to the interaction of methane molecules in Pluto's atmosphere and a specific type of ultraviolet light emitted by both the Sun and distant galaxies.
  3. Pluto has a heart and ice mountains. Flying over the planet, New Horizons photographed a huge bright area in the form of a heart. As the more detailed pictures show, "Heart of Pluto", later called the Tombo region, is an area covered with ice mountains that reach a height of 3,400 m.
  4. Snow may fall on Pluto. According to research, the glaciers on the planet are composed of methane and nitrogen, changing greatly throughout the year. Pluto makes one revolution around the Sun in 248 Earth years, significantly changing its distance from the star. During the summer periods, as scientists suggest, glaciers melt and evaporate into the atmosphere, falling back in the form of snow in winter.
  5. Pluto has an atmosphere made entirely of nitrogen. Studies show that Pluto's nitrogen atmosphere is rapidly escaping into space. Interestingly, this process is in many ways similar to what happened on Earth billions of years ago. Ridding the earth's atmosphere of nitrogen eventually led to the appearance of hydrogen and carbon dioxide, thanks to which life was born on our planet.