why pluto is not a planet
When and who discovered Pluto
Pluto was discovered by an American astronomer Clyde Tombaugh in 1930, who calculated mathematically that beyond the orbit of Uranus there should have been some other celestial body that made small “adjustments” to its movement in orbit. Then everything was a matter of technique - having a model of the motion of Uranus, taking into account the gravity of the other planets and the Sun, and comparing it with the observed orbit, it was possible to estimate in which orbit the perturbing body moves and what mass it has. However, these estimates were very approximate.
The orbit of Pluto - as can be seen from the figure, it is significantly inclined relative to the plane of the solar system, and in the far regions "runs" far into the Kuiper Belt
When Pluto was finally found, its approximate size was estimated to be about the size of Earth. No need to laugh at such a gross mistake in the calculations, it is worth recalling that astronomers of that time still did not have computers at their disposal, and Pluto is 39 times farther from the Sun than the Earth.
It was possible to understand the error and clarify the size of Pluto only in 1978, with the discovery of its first satellite - Charon, only twice as large as Pluto itself. By studying the interaction of Pluto and Charon, astronomers have found that the mass of Pluto is extremely small and is only about 0.2 of the mass of the Earth.
So, suddenly and completely unexpected for science, Pluto from a large celestial body suddenly “shrank” strongly and decreased in size. However, even being very small in size, Pluto was still considered a full-fledged planet.
Discovery of Eris and other dwarf planets beyond Neptune
With the advent of the 1990s. a new era has begun in space exploration, which could be safely called the "Hubble era" after the Hubble space telescope, designed to observe distant space objects.
It soon became clear that Pluto and Charon were not the only objects beyond the orbit of Neptune. In a space that previously seemed only “emptiness”, new objects began to appear one after another, mostly icy in composition, orbiting the Sun at a great distance, some of which were quite impressive in size. These were definitely not asteroids - their sizes are too large, but at the same time they “did not reach full-fledged planets”.
When in 2005 a group of astronomers led by Mike Brown from California Institute of Technology opened Eridu, a space object twice as far from the Sun as Pluto, but at the same time almost as large as it, scientists faced a difficult task. The fact that Eris and Pluto are in many ways similar celestial bodies, there was no doubt. But, is Eris another planet in the solar system?
In a word, the existing astronomical ideas required revision.
Pluto is no longer a planet in the solar system
In 2006 International Astronomical Union adopted an official definition of the term “ planet“.
A planet is a body that orbits the Sun and is not a satellite of another body, large enough to take on a spherical shape under the influence of its own gravitational forces, able to "cleanse nearby areas" of other similar bodies, but not large enough to start a thermonuclear reaction.
Since Pluto has not cleared its area and peacefully coexists in it with other objects, it has ceased to be a planet. Instead, the Union decided to name Pluto and Eris dwarf planets- space objects that do not fully comply with the definition of a “full-fledged” planet, but do not fully correspond to it either.
Whether this decision was right or not quite right - sometimes they argue to this day. Nevertheless, questions are raised rather by the “vague” wording of the definition of the concept of “planet”, the very fact of the existence of dwarf planets is accepted by everyone.
At the moment, the list of “official” “dwarfs” of the solar system includes the long-known, fairly well-studied Pluto, and a bunch of “newcomers” from the outskirts of the system: Eris, Haumea, Makemake, Sedna (possibly). However, it is already clear now that there are no fewer dwarf planets, but most likely much more than “large” ones.
New discoveries await us in the very near future.
Not so long ago, Pluto was excluded from the list of planets in the solar system and classified as a dwarf planet. Let's see why Pluto is 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.
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).
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, powerful new 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).
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.
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, and 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 the Sun - And Pluto passes.
It has to be massive enough to be spherical by its force of gravity - And here Pluto seems to be all right.
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. It's the same with Eris...
Specifications:
- 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.
In 2006, the International Astronomical Union (IAU) excluded Pluto from the planets of the solar system, classifying it as a dwarf planet. Before we find out why this happened, and what should now be understood by the term "planet", let's remember how mankind discovered stellar bodies.
Our solar system is the central star and all the cosmic bodies that revolve around it, including the Earth. Despite its huge size (according to modern estimates, the length of the solar system is at least 60 billion kilometers), it is just one of hundreds of billions of systems that make up the Milky Way galaxy. In turn, many billions of galaxies, larger or smaller than the Milky Way, make up the entire Universe.
Most of the stars and planets in the solar system formed in outer space about 4.5 billion years ago. First, the central and largest object, the Sun star, formed from a giant gas and dust cloud. Then various solid bodies of much smaller size and mass began to form around the Sun from the remaining building material: asteroids, comets and planets. All of them appeared due to the successive “capture” of smaller objects. For example, asteroids are made of rock and metals, comets are made of ice. It was not for nothing that we called them small, against the background of the Sun, even the largest planets look very modest: we calculated that 99.86% of the entire mass of the solar system falls on our central star.
Giants and dwarfs
There are 8 largest celestial bodies or planets in the solar system, including the Earth. In addition to it, 7 more planets make their journey in space around the Sun: Mercury, Venus, Jupiter, Saturn, Uranus and Neptune. The last two can only be observed through a telescope, the rest are visible to the naked eye.
The planets closest to the Sun: Mercury, Venus, Mars and the Earth are usually called "terrestrial planets". They got this name because they have a number of features,.
Astronomers call the second group of planets in the solar system "gas giants", this includes Jupiter, Saturn, Uranus and Neptune. They are conditionally called giants due to the fact that they are much larger than the terrestrial planets.
Criteria for distinguishing planets:
| EARTH TYPE | GAS GIANTS |
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From antiquity to modern times
The discovery of the planets of the solar system did not happen immediately, but step by step, based on victories and mistakes.
Mars, Venus, Jupiter and Saturn are very bright objects that are clearly visible in the night sky, so they have all been known to mankind since ancient times.
It was more difficult to detect the existence of a small Mercury (its diameter is about 4900 km), moving in the closest orbit to the Sun. However, it was also successfully identified by ancient Greek astronomers.
Terrestrial scientists were able to replenish the planetary list only at the end of the 17th century thanks to the most important technical invention, the optical telescope.
The next in a row was Uranus, discovered by a British astronomer William Herschel March 13, 1781 Herschel's colleagues - European astronomers and mathematicians jointly calculated the approximate size and orbit of Uranus, and after the first calculations it became clear that the new planet is about twice as far from the Sun as Saturn. Thanks to the discovery of Uranus, the boundaries of our solar system have been significantly expanded.
The planet Neptune was generally discovered by scientists “in absentia”. More precisely, scientists learned about its existence and alleged location even before it was actually discovered. In the early 40s. 19th century Englishman John Couch Adams and French Urbain Le Verrier independently of each other, they established that the orbit of Uranus calculated earlier by theorists slightly differs from the experimentally recorded trajectory of its movement. Adams and Le Verrier stated that the reason for this must be the gravitational pull of Uranus from another planet, as yet unknown to science. They managed to quite accurately determine the orbit of this "stranger", its mass and current location in the sky.
Le Verrier sent a letter to an employee of the Berlin Observatory, Johann Galle, with a request to check these calculations, and on the night of September 23-24, 1846, Galle, using a telescope, discovered the eighth planet from the Sun at a distance of less than 1 ° from the point predicted by Le Verrier.
It was nine, now it's eight
Soon after the discovery of Neptune, scientists suggested that a ninth planet must be at an even greater distance from the Sun. After many decades of unsuccessful searches in 1930, the long-awaited success came to the American Clyde Tombo who finally managed to make out a tiny, dim dot in the starry sky, called Pluto. Encouraged by this discovery, astronomers immediately officially recognized him as a new full member of the planetary club. However, subsequent observations of Pluto have shown that it is not like the other eight planets. So, in size (Pluto's diameter is only 2390 km), it is inferior to the seven satellites of the giant planets of the solar system, its very elongated orbit periodically intersects with the orbit of Neptune, and sometimes it even becomes closer to the Sun than Neptune. In addition, in 1978, a “satellite” Charon was discovered near Pluto, half the size of its “master” (its diameter is just over 1200 km). Moreover, scientists then found that in fact Charon is rather not a satellite of Pluto, but its neighbor: Pluto and Charon make one complete revolution around a common center of mass in more than six Earth days.
For more than six decades, Pluto seemed to scientists an incomprehensible anomaly. The situation began to change dramatically in the early 1990s. XX century, when, thanks to significant progress made in telescopic technology, astronomers were finally able to experimentally detect new distant space objects with similar properties in the vicinity of Pluto.
All these very cold and hard objects belonged to the donut-like region of our solar system, located beyond the orbit of Neptune, the existence of which was predicted in 1951 by the Dutch astronomer Gerard Kuiper.
In honor of the Dutchman, this region was named the Kuiper belt, but it was not until 1992 that astronomers discovered its first inhabitant. And a few years later, scientists realized that the Kuiper belt is indeed inhabited by a large number of different celestial bodies, the dimensions of many of which, in theory, should be quite comparable to Pluto's.
For the time being, the International Astronomical Union (IAU), the main world organization responsible for putting things in order in the scientific terms and concepts used, remained silent about the emerging great embarrassment with the understanding of what will now be denoted by such a familiar word “planet”.
It was only in July 2005 that the leaders of the IAU were stirred up, after a group of American astronomers announced that they had discovered an object in the Kuiper belt that was possibly even larger than Pluto. And the IAU hastily created a special commission to develop an updated definition of the term "planet". After a long and bitter debate, at the end of August 2006, at the 26th Assembly of the IAU in Prague, a historic decision was made that shocked many.
Instead of recognizing the new object, later named Eris in honor of the ancient Greek goddess of discord, as the next tenth planet in the solar system, the IAU decided that not only Eris, but also Pluto should not be considered normal planets and again reduced the official list of planets to eight.
In order to move Pluto, Eris and other potential candidates from the Kuiper belt somewhere, the IAU came up with a new special category, “dwarf planets”.
According to various estimates of scientists, from several dozen to several hundred new objects that have the characteristics of “dwarf planets” can be found in the Kuiper belt or even further from it.
What are the new criteria of the IAU, which divided the planets into two categories - "normal" and "dwarf"?
Their two common characteristics are:
1) circulation around the Sun, and not around any other celestial bodies;
2) the presence of sufficient mass in order to have a spherical shape under the influence of gravitational forces.
The fundamental difference between them, according to the IAU, is that dwarf planets, unlike “normal” ones, “cannot clear the surrounding outer space from other neighboring objects”, that is, they do not absorb other objects in their orbit, and do not push them out with their gravity.
This criterion, of course, can be understood in different ways, and many scientists today express strong dissatisfaction with such a strange decision of the IAU. A number of astronomers insist that the artificial division of the planets into two subcategories is generally wrong and the IAU should still recognize Pluto, Eris and other relatively large inhabitants of the Kuiper belt as full-fledged planets.
Johns Hopkins University scientist Kirby Runyon is set to present a paper at the Lunar and Planetary Science Conference in Texas in which he intends to prove that Pluto meets all the properties of the planets. Runyon and six other scientists define planets as "sub-stellar massive bodies that have never undergone nuclear fusion." But if this definition is accepted, it will not only return planetary status to Pluto, but also add about 110 planets to the solar system.
In addition to our solar system, over the past twenty-odd years, scientists have been able to detect hundreds of planets far beyond its borders (such objects are called). Therefore, our hopes that sooner or later signs of intelligent life will be found on some of them have greatly increased today.
You have no idea how many people were upset when the decision was made to stop considering Pluto a planet in the solar system. Kids whose favorite cartoon dog, Pluto, is suddenly named after who knows what. Recall that in ancient Greek mythology, this is one of the names of the god of death. Chemists and nuclear physicists were saddened, who called this name plutonium - a radioactive element capable of destroying all of humanity. And what about astrologers? The unfortunate charlatans have fooled people for decades, describing how strong this degraded object has on their fate and character, and it’s good if indignant clients do not present material claims to them.
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 the point of view of logic, which is what science always calls us to do.
The demolition of Pluto took place at the 26th General Assembly of the International Astronomical Society, held in Prague, and this decision caused a lot of controversy and objections. Some scientists wanted to keep it as a planet, but the only argument they could make to justify their desire was that "it would break tradition." The fact is that there is not, and never has been, any scientific reason 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 them there, these objects. And all of them are blocks of stone and ice, like, in fact, Pluto. It's just the first one we've seen.
It is, of course, very large compared to most of its neighbors, but it is not the largest object in the Kuiper Belt. Such is Eris, which, if inferior to Pluto in size, is quite a bit, so small that the 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, and Makemane, and Ceres, which is located in the asteroid belt between Mars and Jupiter. According to scientists, in total we may have about a hundred such strong men. Waiting to be noticed.
There is no fantasy here. No animators, no chemists. Astrologers should have enough, but few serious people care about their interests. This is exactly the main reason why we stopped considering Pluto a planet. Because, together with him, we, in theory, should raise so many celestial bodies to this rank that the very word “planet” 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 enough gravity to bring themselves into a more or less spherical shape, and almost completely clear their orbit of other objects. Pluto cut off at the last point. Its mass is only 0.07% of the mass of everything that is on its circular trajectory. To give you an idea of how insignificant this is, let's say that the mass of the Earth is 1,700,000 times the mass of other matter in its orbit.
Earth, Moon, Pluto for comparison
I must say that the International Astronomical Society turned out to be not entirely heartless. It came up with a new category for celestial bodies, satisfying only the first two criteria. Now they are dwarf planets. And in deference to the place that Pluto once occupied in our worldview and in our culture, it was decided to call the dwarf planets that are further than Neptune "plutoids." Which, of course, is pretty sweet.
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 is to visit this celestial body. As of this moment in time, this interplanetary station has completed its task by 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 online.
Let's hope that humanity's interest in Pluto does 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.
Offended Pluto
