Until now, among people who vigilantly follow events in the scientific world, the discussion of the question “Is Pluto a planet or not?” has not yet subsided. Heated debate began in 2006, when at the next meeting of the IAU (International Astronomical Union) the main classes of celestial bodies were finally determined. Pluto and several other objects in the solar system are included in the list of dwarf planets. The public's outrage knew no bounds.

Many refused to accept the fact that in our piece of the Galaxy there are now not nine, but eight planets. However, scientists, having clearly substantiated their position, are not going to revise the accepted definitions again in the near future. Today the question is “Is Pluto a planet or not?” no longer evokes as many emotions, but remains relevant. A brief excursion into history will help to understand the reasons for the loss of status of this cosmic body.

Predicted

The discovery of the planets Neptune and Pluto are similar in many ways. These objects are so far from the Sun and Earth that they are impossible to observe with the naked eye. And not every telescope can distinguish such a distant body from a dim star. Therefore, the planets Neptune and Pluto were observed some time before their official discovery, but were mistakenly classified as luminaries.

Both objects were initially discovered theoretically and only then seen through a telescope. The discovery of the planets Neptune and Pluto was a consequence of the development of knowledge and technology. The existence of the first of them was the most logical explanation for the changes in the movement of Uranus, which did not coincide with the calculations of astronomers. Two scientists, Urbain LaVerrier and John Cooch Adams, independently determined the location of the proposed planet and calculated its orbit with varying degrees of accuracy. The date of discovery of Neptune is considered to be September 23, 1846.

Even further from the Sun

However, the new planet did not solve the problem of changing the orbit of Uranus. The gravitational influence of Neptune could not explain all the discrepancies with theoretical constructions. Then the idea arose about a planet even more distant from the Sun. A new proposed trans-Neptunian object was also initially calculated and only then discovered in the sky. The discovery of the planet Pluto occurred in 1930, its author was Clyde Tombaugh, an American astronomer. As in the case of Neptune, a study of photographs from previous years showed that the object had been observed many times before, but was mistakenly attributed to dim stars.

Options

Immediately after the discovery and for a long time, no one thought: Is Pluto a planet or not? It was assumed that it was similar in size to Mars. After observing the passage of Pluto across the disk of the star in 1965, its diameter was clarified: no more than 5.5 thousand kilometers, which is somewhat less than previously thought. The mass of the planet could not be accurately estimated until 1978. Then the scientific world was delighted with the new discovery. Astronomer J. Christie discovered a satellite of the planet with a diameter of about 500 kilometers in photographs of Pluto.

The new object was named Charon. It made it possible to determine the mass of Pluto with great accuracy. It turned out to be equal to 1/500 of the same parameter of the Earth. The diameter has also been clarified - only 2,600 kilometers. Pluto, thus, turned out to be a cosmic body inferior in size even to Mercury.

Dual system

Studies have shown that Charon's mass is approximately 11.65% of that of Pluto. The satellite and the planet always face the same side to each other. It is believed that this relative position of the two objects is an illustration of the future of the Earth and the Moon. Now the satellite of our planet is visible only from one side, and after some time the Earth will always be facing it in the same way.

The center of mass around which Pluto and Charon revolve is located outside the planet. In this regard, today in the scientific world these objects are considered parts of a dual system, and practically equal. The satellite and the planet stand out in it only conditionally and, rather, out of habit.

First doubts

It was from the moment new data appeared on the dimensions of the trans-Neptunian object that the question arose for the first time: “Is Pluto a planet or not?” Doubts about the status were caused by its small size. However, this issue was not seriously considered until 1992. The turning point was the discovery of Kuiper Belt Objects. All of them were cosmic bodies consisting of a mixture of ice and rocks, that is, they were very similar to Pluto. Its main differences are its impressive size compared to the objects in the belt and the high brightness created by the ice on the surface.

Like the giant planets, Pluto is mostly composed of volatile substances that exist here in a frozen state due to constant low temperatures. This also makes it similar to Kuiper belt objects. The discovery of many similar bodies led to the need to clarify the concept of “planet”. Scientists were faced with a task: either to give this status to all such objects, or to allocate them to a new class.

Final decision

The issue was closed in 2006. The IAU has clearly defined the criteria for a planet:

• this is a body orbiting the Sun;
• it has such a mass that it is capable of maintaining hydrostatic equilibrium, that is, it has the shape of an almost perfect ball;
• the orbit of the body must be free from other objects.

It is the latter criterion that Pluto does not meet. The concept of “dwarf planet” was introduced for him. Ceres, previously considered a Main Belt asteroid, was also classified as this type of object.

The discovery of the planet Pluto did not become less valuable for science after 2006. The classification of this trans-Neptunian object into one category or another does not affect its existence in any way, and therefore public emotions will soon completely subside. But the study of the Charon-Pluto system, remarkable in many ways, will continue, which means new discoveries are ahead.

Pluto was discovered in 1930 by astronomer Clyde Tombaugh at the Lowell Observatory in Arizona. The search for it lasted 15 years, since the existence of a trans-Neptunian planet was predicted by Percival Lowell based on 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, the characteristics of this planet are known only approximately: diameter is about 2200 km, surface temperature is 35-55 K (about -210 ° C). Pluto is made of a mixture of rocks and ice, and the atmosphere is made of nitrogen and methane.

The largest of Pluto's moons, Charon, which received its name from 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 orbits with a period of 6.4 days around a common center of gravity with Pluto, which lies between them. Pluto and Charon always face the same side towards each other. In 2005, the Hubble Space Telescope discovered two more very small satellites (61 and 46 km) near Pluto, which a year later were named Hydra and Nix. The words in the name of the first interplanetary probe, New Horizons, begin with the same letters, which in the same year set off on a 10-year journey to Pluto.

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

As a result, in August 2006, the International Astronomical Union decided to deprive Pluto of its planet status and introduce a new category of dwarf planets, which initially included Pluto, UB 313 and the “promoted” asteroid Ceres from the main asteroid belt between Mars and Jupiter. Thus, Pluto remained in the status of a planet for 76 years and became the first celestial body to lose this status.

You can't imagine how many people were upset when the decision was made to stop considering Pluto a planet in the solar system. Children whose beloved cartoon dog Pluto suddenly began to be named after who knows what. Let us remember that in ancient Greek mythology this is one of the names of the god of death. Chemists and nuclear physicists were saddened because they gave this name to plutonium, a radioactive element capable of destroying all of humanity. What about astrologers? Unhappy charlatans have been fooling people for decades, describing how much influence this demoted object has on their fate and character, and it’s good if indignant clients do not make claims of a material nature.

When did Pluto stop being considered a planet?

Be that as it may, Pluto ceased to be considered a planet in 2006. We must come to terms with this and live with the awareness of this fact. Does not work? Okay, then let's forget about feelings and try to look at the situation from a logical point of view, which is what science always calls us to do.

The demotion of Pluto took place at the 26th General Assembly of the International Astronomical Society, held in Prague, and this decision caused many disputes and objections. Some scientists wanted to keep it as a planet, but the only argument they could give to justify their desire was that “it would break tradition.” The fact is that there is not, and never has been, any scientific basis to consider Pluto a planet. This is just one of the objects of the Kuiper belt - a huge cluster of heterogeneous celestial bodies located beyond the orbit of Neptune. There are about a trillion of these objects there. And they are all blocks of stone and ice, just like Pluto. He is simply the first of those that we managed to see.

It is certainly very large compared to most of its neighbors, but it is not the largest object in the Kuiper Belt. This is Eris, which, even if it is inferior to Pluto in size, is very little, so small that debate about which of them is larger continues to this day. But it is a quarter heavier. This object is located twice as far from the Sun as Pluto. There are many other similar celestial bodies in the Solar System. These are Haumea, Makemane, and Ceres, which is located in the asteroid belt between Mars and Jupiter. According to scientists, we may have about a hundred of these strong creatures in total. Waiting to be noticed.

No imagination is enough here. Neither animators nor chemists. Astrologers should have enough, but few serious people care about their interests. This is precisely the main reason why we stopped considering Pluto a planet. Because together with it, we, in theory, should elevate so many celestial bodies to this rank that the word “planet” itself will lose its current meaning. In this regard, in the same 2006, astronomers defined clear criteria for objects claiming this status.

What are the criteria for a “planet”?

They must orbit the Sun, have sufficient gravity to bring themselves into a more or less spherical shape, and almost completely clear their orbit of other objects. Pluto was cut off at the last point. Its mass is equal to only 0.07% of the mass of everything that is on its circular path. To give you an idea of ​​how insignificant this is, the mass of the Earth is 1,700,000 times greater than the mass of other matter in its orbit.

It must be said that the International Astronomical Society was not entirely heartless. It came up with a new category for celestial bodies that satisfy only the first two criteria. Now these are dwarf planets. And as a sign of respect for the place that Pluto once occupied in our worldview and in our culture, it was decided to call the dwarf planets located further than Neptune “plutoids.” Which is, of course, quite nice.

And in the same year that astronomers decided that Pluto could no longer be called a planet, NASA launched the New Horizons spacecraft, whose mission included visiting this celestial body. As of this moment in time, this interplanetary station has completed its task, transmitting to Earth a lot of valuable data about Pluto, as well as picturesque photographs of this dwarf planet. Don't be lazy, find them on the Internet.
Let's hope that humanity's interest in Pluto will not end there. It is, after all, on our way to other stars and galaxies. We're not going to sit in our solar system forever.

Pluto is one of the least studied objects in the solar system. Due to its great distance from Earth, it is difficult to observe with telescopes. Its appearance is more reminiscent of a small star than a planet. But until 2006, it was he who was considered the ninth planet of the solar system known to us. Why was Pluto excluded from the list of planets, what led to this? Let's look at everything in order.

Unknown to science "Planet X"

At the end of the 19th century, astronomers suggested that there must be another planet in our solar system. The assumptions were based on scientific data. The fact is that, observing Uranus, scientists discovered a strong influence on its orbit of foreign bodies. So, after some time Neptune was discovered, but the influence was much stronger, and the search for another planet began. It was called "Planet X". The search continued until 1930 and was successful - Pluto was discovered.

Pluto's movement was noticed on photographic plates taken over a two-week period. Observations and confirmation of the existence of an object beyond the known boundaries of the galaxy of another planet took more than a year. Clyde Tombaugh, a young astronomer at the Lowell Observatory that initiated the research, reported the discovery to the world in March 1930. Thus, a ninth planet appeared in our solar system for 76 years. Why was Pluto excluded from the solar system? What was wrong with this mysterious planet?

New discoveries

At one time, Pluto, classified as a planet, was considered the last of the objects in the solar system. According to preliminary data, its mass was considered equal to the mass of our Earth. But the development of astronomy constantly changed this indicator. Today, Pluto's mass is less than 0.24% and its diameter is less than 2,400 km. These indicators were one of the reasons why Pluto was excluded from the list of planets. It is more suitable for a dwarf than for a full-fledged planet in the solar system.

It also has many of its own features that are not typical for ordinary planets in the solar system. The orbit, its small satellites and atmosphere are unique in themselves.

Unusual orbit

The orbits familiar to the eight planets of the Solar System are almost circular, with a slight inclination along the ecliptic. But Pluto's orbit is a highly elongated ellipse and has an inclination angle of more than 17 degrees. If you imagine, eight planets will rotate evenly around the Sun, and Pluto will cross the orbit of Neptune due to its angle of inclination.

Due to this orbit, it completes a revolution around the Sun in 248 Earth years. And the temperature on the planet does not rise above minus 240 degrees. Interestingly, Pluto rotates in the opposite direction from our Earth, like Venus and Uranus. This unusual orbit for a planet was another reason why Pluto was excluded from the list of planets.

Satellites

Today there are five known: Charon, Nyx, Hydra, Kerberos and Styx. All of them, except Charon, are very small, and their orbits are too close to the planet. This is another difference from the officially recognized planets.

In addition, Charon, discovered in 1978, is half the size of Pluto itself. But it's too big for a satellite. Interestingly, the center of gravity is outside of Pluto, and therefore it seems to sway from side to side. For these reasons, some scientists consider this object to be a double planet. And this also serves as an answer to the question of why Pluto was excluded from the list of planets.

Atmosphere

It is very difficult to study an object located at an almost inaccessible distance. Pluto is believed to be composed of rock and ice. The atmosphere on it was discovered in 1985. It consists mainly of nitrogen, methane and carbon monoxide. Its presence was determined by studying the planet when it covered the star. Objects without an atmosphere cover the stars abruptly, while those with an atmosphere cover them gradually.

Due to the very low temperature and elliptical orbit, melting ice produces an anti-greenhouse effect, causing the planet's temperature to drop even further. After research conducted in 2015, scientists came to the conclusion that atmospheric pressure depends on the planet’s approach to the Sun.

Newest technologies

The creation of new powerful telescopes marked the beginning of further discoveries beyond the known planets. So, over time, those within the orbit of Pluto were discovered. In the middle of the last century, this ring was called the Kuiper belt. Today, hundreds of bodies are known with a diameter of at least 100 km and a composition similar to Pluto. The found belt turned out to be the main reason why Pluto was excluded from the planets.

The creation of the Hubble Space Telescope made it possible to study outer space, and especially distant galactic objects, in more detail. As a result, an object was discovered, called Eris, which turned out to be further than Pluto, and over time, two more celestial bodies that were similar in diameter and mass to it.

The New Horizons spacecraft, sent to explore Pluto in 2006, confirmed many scientific data. Scientists have a question about what to do with open objects. Should we classify them as planets? And then there will be not 9, but 12 planets in the solar system, or the exclusion of Pluto from the list of planets will solve this issue.

Review of status

When was Pluto removed from the list of planets? On August 25, 2006, participants in the congress of the International Astronomical Union, consisting of 2.5 thousand people, made a sensational decision - to exclude Pluto from the list of planets of the solar system. This meant that many textbooks had to be revised and rewritten, as well as star charts and scientific papers in the field.

Why was this decision made? Scientists have had to reconsider the criteria by which planets are classified. Long debates led to the conclusion that the planet must meet all parameters.

First, the object must be rotating around the Sun in its orbit. Pluto fits this parameter. Although its orbit is highly elongated, it revolves around the Sun.

Secondly, it should not be a satellite of another planet. This point also corresponds to Pluto. At one time it was believed that he appeared, but this assumption was discarded with the advent of new discoveries, and especially of his own satellites.

The third point is to have enough mass to acquire a spherical shape. Pluto, although small in mass, is round, and this is confirmed by photographs.

And finally, the fourth requirement is to have a strong one in order to clear your orbit from others. For this one point, Pluto is not suitable for the role of a planet. It is located in the Kuiper belt and is not the largest object in it. Its mass is not enough to clear its way in orbit.

Now it’s clear why Pluto was excluded from the list of planets. But where should such objects be classified? For such bodies, the definition of “dwarf planets” was introduced. They began to include all objects that do not meet the last point. So Pluto is still a planet, albeit a dwarf one.