What is a planet? Disputes continue
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 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 for the ancients special, different from other celestial objects. The moon and the sun were also classed as "wandering bodies" of a somewhat 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, there is a clear connection between this "magnificent seven" and the days of the week: Monday - the Moon, Tuesday - Mars, Wednesday - Mercury, Thursday - Jupiter, Friday - Venus, Saturday - Saturn, Sunday - Sun .
Uranus was discovered in the 18th century. Then, thanks to the studies of Leverrier and Adams, Neptune (in the 19th century). In 1930, the world learned about the existence of Pluto. This discovery was made by Clyde Tombaugh. Pluto is still called the ninth planet. Is it so? In order to answer this question, it is necessary to understand what a planet is?
This concept was formed historically by us. In the sky, we see five planets with the naked eye. And everything would be much more complicated if the asteroids were brighter. Astronomers have agreed to call a planet an object that is in the orbit of a star, besides, it must be large enough to become a sphere because of gravity. Many comets and asteroids have very strange shapes.
The exact definition of "what is the planet" was not. But still this name was fixed for Pluto. In fact, this is a very strange object, and it is very different from the other major planets. Its orbit is somewhat more elongated. As it turned out, Pluto is very small. Much less than previously believed by astronomers. All the planets of the solar system are located "by growth": 4 terrestrial groups in the beginning, then the giant planets. Pluto here clearly does not fit.
Then they discovered the moon of Pluto - Charon, which is almost the same size. This is also somewhat atypical: all planets have a much smaller satellite.
Kuiper (American astronomer) suggested thatBehind Neptune is the asteroid belt. This was another blow to the "status" of Pluto. Such a belt really is! At present, it is called the Kuiper belt. The bodies in it are somewhat different from asteroids. But very much like Pluto, only smaller size. Now it became clear to everyone that Pluto is one of the largest objects of the Kuiper belt. But while the former name is still preserved. Scientists, in the majority, people are conservative, therefore have decided to leave all as it is.
And yet, what is a planet? Soon this issue will become very relevant. Since it is known that the planets of other stars are already being discovered.
The main parameter is mass. Smaller objects - asteroids, more massive - stars. With the stars, everything is clear. They must have a high temperature so that thermonuclear processes occur. The planet in its orbit should be one, if there are many objects ("belt"), then these are asteroids.