Most of us learned at school that we have nine planets of the solar system and that the last planet is called Pluto. But that has changed one day, and now Pluto is not considered a planet. Why did the professional community take such a step and what is the history of the discovery of this dwarf planet?
The journey to the discovery of the dwarf planet Pluto was very interesting and took many decades. As early as the end of the 19th century, astronomers noticed that the orbit of Uranus was not only affected by the hitherto unknown Neptune, but that it must be affected by yet another object in the solar system. Later, astronomers called this object Planet X and continued their search diligently. Pluto was not discovered until 1930 by American astronomer Clyde Tombaugh. However, the first photograph of Pluto dates back to 1909. It is worth mentioning that Pluto was not really the Planet X, it was only an error in calculating the mass of Neptune.
Pluto is now officially considered a dwarf planet. This category arose with the consensus that Pluto is not one of the main planets. It differs from other planets in the solar system in many ways, such as a strongly deformed orbit or a large deviation from the axis of the ecliptic. Its mass reaches only 0.2 % of the Earth's mass, and the seven moons of the main planets are even larger than Pluto. After the discovery of other large bodies in the so-called Kuiper belt, to which Pluto belongs, there was a growing response that it should not be included among the planets.
Many debates and disputes have arisen around the categorization of Pluto. The final speech was given at the 26th General Assembly of the International Astronomical Union in August 2006 in Prague. To this day, however, there are proponents of Pluto as a planet in the professional community, and the widespread adoption of its new classification is likely to take a long time.