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.
Initially they 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 among the "wandering bodies" of a slightly different nature. Have you ever wondered why we have one day off every seven days? This is not reflected in Russian. 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 - the Sun.
Uranus was discovered in the 18th century. Then, thanks to the research 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 Tombaugh. Pluto is still called the ninth planet. Is it so? In order to answer this question, you need to figure out what a planet is?
This concept itself has been formed historically. We see five planets in the sky with the naked eye. And everything would be much more complicated if the asteroids were brighter. Astronomers have agreed to call an object orbiting a star a planet, plus it must be large enough to become 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 assigned to Pluto. In fact, this is a very strange object, and it is very different from the rest of the large planets. Its orbit is somewhat more elongated. As it turned out, Pluto is very small. Much less than astronomers previously thought. All the planets of the solar system are located "by growth": 4 earth groups at the beginning, then the giant planets. Pluto clearly doesn't fit in here.
Then they discovered Pluto's satellite - Charon, which is almost the same in size. This is also somewhat atypical: all planets have a much smaller satellite.
Kuiper (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! It is currently known as the Kuiper belt. The bodies in it are somewhat different from asteroids. But they are very similar to Pluto, only smaller in size. Now it has become clear to everyone that Pluto is one of the large objects of the Kuiper belt. But for now, it retains its original name. Scientists are mostly humanconservative, so we decided to leave everything as it is for now.
And yet, what is a planet? Soon this question will become very relevant. Because it is known that planets of other stars are already being discovered.
The main parameter is mass. Smaller objects are asteroids, more massive are stars. Everything is clear with the stars. They must have a high temperature for thermonuclear processes to occur. The planet in its orbit should be one, if there are many objects (“belt”), then these are asteroids.
