If you know what a planet is, then you know more than astronomers. Because they are not sure about the definition of this term. For example, some consider Pluto a planet, others do not.
Originally so called Mars, Venus, Jupiter, Saturn, Mercury. By the way, the names of the planets came to us from Greek and Roman mythology. The earth was special for the ancients, different from other celestial objects. The Moon and the Sun were also ranked as “wandering bodies” of a slightly different nature. Have you ever wondered why we have one day off in seven days? This is not reflected in the Russian language. But in French, Italian, for example, the connection between this “magnificent seven” and the days of the week is clearly traced: Monday - the Moon, Tuesday - Mars, Wednesday - Mercury, Thursday - Jupiter, Friday - Venus, Saturday - Saturn, Sunday - the Sun .
Uranus was discovered in the 18th century. Then, thanks to the studies of Le Verrier and Adams, Neptune (in the 19th century). In 1930, the world learned about the existence of Pluto. This discovery was made by Clyde Tombo. Pluto is still called the ninth planet. Is it so? In order to answer this question, you need to figure out what is a planet?
This concept itself has been formed historically in our country. In the sky we see five planets with the naked eye. And everything would be much more complicated if the asteroids were brighter. Astronomers agreed to call the planet an object that is in the orbit of a star, in addition, it must be large enough to turn into a sphere due to gravity. Many comets and asteroids have very strange shapes.
There was no exact definition of "what is a planet". But still, such a name was attached to Pluto. In fact, this is a very strange object, and it is very different from the rest of the big planets. Its orbit is somewhat more elongated. As it turned out, Pluto is very small. Much less than astronomers had previously believed. All the planets of the solar system are located “in growth”: 4 terrestrial groups at the beginning, then giant planets. Pluto clearly does not fit here.
Then they discovered the satellite of Pluto - Charon, which is almost the same in size. This is also somewhat atypical: all planets have a much smaller satellite .
Kuiper (an American astronomer) suggested that there is
an asteroid belt beyond Neptune
. This was another blow to the "status" of Pluto. There really is such a belt! Currently, it is called the Kuiper belt. The bodies in it are slightly different from asteroids. But they are very much like Pluto, only smaller. Now it became clear to everyone that Pluto is one of the largest objects in the Kuiper belt. But while the former name is retained by him. Most scientists are conservative people, so they decided to leave everything as it is for now.
And yet, what is a planet? Soon this issue will become very relevant. Because it is known that the planets of other stars are already opening.
The main parameter is mass. Smaller objects are asteroids, more massive are stars. With the stars, everything is clear. They must have a high temperature in order for thermonuclear processes to occur. The planet in its orbit should be one, if there are many objects ("belt"), then these are asteroids.