Pluto is one of the least explored objects in the solar system. Due to its great distance from the Earth, it is difficult to observe with telescopes. Its appearance is more like 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? Consider 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, while observing Uranus, scientists discovered a strong influence of foreign bodies on its orbit. 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,made within two weeks. Observations and confirmation of the existence of an object beyond the known limits of the galaxy of another planet took more than a year. Clyde Tombaugh, a young astronomer at the Lowell Observatory that initiated the research, announced the discovery to the world in March 1930. So, the 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% of the mass of the Earth, and its diameter is less than 2400 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 characteristics, not inherent in ordinary planets of the solar system. The orbit, its small satellites and atmosphere are unique in themselves.
Unusual orbit
Orbits habitual for eight planets of the solar system are almost round, having 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 we imagine a model of the solar system, then eight planets will rotate uniformly 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 the planet was another reason why Pluto was excluded from the list of planets.
Satellites
Today there are five known moons of Pluto: Charon, Nikta, Hydra, Kerberos and Styx. All of them, except for Charon, are very small, and their orbits are too close to the planet. This is another one of the differences from the officially recognized planets.
In addition, Charon, discovered in 1978, is half the size of Pluto itself. But for a satellite it is too big. Interestingly, the center of gravity is outside of Pluto, and therefore it seems to swing from side to side. For these reasons, some scientists consider this object 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 that is almost inaccessible. It is assumed that Pluto consists of rocks and ice. The atmosphere on it was discovered in 1985. It consists mainly of nitrogen, methane and carbon monoxide. Its presence was able to determine when studying the planet, when it closed the star. Objects without an atmosphere cover the stars abruptly, while objects with an atmosphere close gradually.
Due to the very low temperature and elliptical orbit, ice melt produces anti-greenhouseeffect, which leads to an even greater decrease in the temperature on the planet. After research conducted in 2015, scientists concluded that atmospheric pressure depends on the approach of the planet to the Sun.
Latest technology
The creation of new powerful telescopes marked the beginning of further discoveries beyond the known planets. So, over time, space objects were discovered that are within the orbit of Pluto. In the middle of the last century, this ring was called the Kuiper belt. To date, hundreds of bodies are known with a diameter of at least 100 km and a composition similar to Pluto. The found belt was the main reason why Pluto was excluded from the planets.
The creation of the Hubble Space Telescope made it possible to study outer space in more detail, and especially distant galactic objects. As a result, an object called Eris was discovered, which turned out to be farther than Pluto, and over time, two more celestial bodies that were similar in diameter and mass.
The AMS New Horizons spacecraft sent to explore Pluto in 2006 confirmed many scientific data. Scientists have a question about what to do with open objects. Are they classified 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.
Status Review
When was Pluto removed from the list of planets? 25-th of AugustIn 2006, the participants of 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 in the solar system. This meant that many textbooks, as well as star charts and scientific papers in this field, had to be revised and rewritten.
Why was this decision made? Scientists have had to rethink the criteria by which planets are classified. A long debate led to the conclusion that the planet must meet all the parameters.
First, the object must revolve around the Sun in its orbit. Pluto suits this parameter. Although its orbit is highly elongated, it revolves around the Sun.
Secondly, it must not be a satellite of another planet. This point also corresponds to Pluto. At one time it was believed that he was a satellite of Neptune, but this assumption was discarded with the advent of new discoveries, and especially his own satellites.
Third point - 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 gravitational field in order to clear your orbit from other cosmic bodies. On this one point, Pluto does not fit 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 a path for itself in orbit.
Now I understand why Plutoremoved from the list of planets. But where do we list such objects? For such bodies, the definition of "dwarf planets" was introduced. They began to include all objects that do not correspond to the last paragraph. So Pluto is still a planet, albeit a dwarf one.
