What celestial bodies are called the planets of the solar system?

From time immemorial people watched the night sky and noticed that in addition to fixed objects, there are also those who change their position relative to the others. Usually we say that these are stars, but is it so? What celestial bodies are called planets and what criteria should an object possess so that it can be called a planet? Which of them are part of the solar system?

Planet. Definition and characteristics

Any object that does not emit light, heat, and exceeds several meters in size is considered a planet ("wandering" - in translation from the Greek language). In the second half of the last century more precise definitions were gradually introduced, and to date, so that the celestial body is recognized as a planet, it must correspond to the following four conditions:

• The object should not be a star.

• Near the orbit of the object, other large celestial bodies should not move.

• The object should be a shape that is close to spherical.

• The object should rotate around the star.

Planet and star. What is the difference?

With the question of what heavenly bodies are called planets, we have figured out, but what is the difference between them and the stars? The planet under the power of its own gravity is able to take a rounded shape and has a high density. But this mass is not enough for thermonuclear reactions inside the body to begin. The star is a celestial natural body capable of triggering the thermonuclear reactions of helium, hydrogen and other gases, of which it consists, while radiating into space an incredible amount of energy converted into light, thermal and electromagnetic fluxes.

The solar system and its constituent planets

According to modern science claims, which is called "astronomy," the planets of the solar system began to form about 4.5 billion years ago, becoming the consequence of a powerful explosion of one or several giant supernovae. The solar system was originally a gas cloud with particles of dust forming a disk in motion and at the expense of its mass, in the center of which a new star was born, which we all know as the Sun.

So what celestial bodies are called planets entering the solar system? The answer to this question is very simple: all objects that have their own orbit and which rotate around a common, central star and are called planets of the solar system. They are divided into two small groups, four objects in each:

• Earth group of planets - Mars, Venus, Earth and Mercury. All of them have a rocky surface and a small size, being at a closer distance to the Sun than others.

• Planet giants - Neptune, Saturn, Jupiter and Uranus. Large, consisting mainly of gas of the planet, with rings characteristic for them only, which are formed from a multitude of rocky debris and ice dust.

Up to August 25, 2006, it was believed that the Solar System contains nine planets. But after clarifying the definitions by which it was accepted what it is possible to call a planet in the scientific world, Pluto, which was previously part of the solar system as the ninth, most remote object, went into the category of pygmy.

What was the reason for making such a decision? The thing is that with the improvement of telescopes and other astronomical equipment, scientists have discovered similar in characteristics with Pluto, heavenly objects, the number of which will increase in time. In order to exclude possible confusion in the future and more precise requirements were introduced to which celestial bodies are called planets.

Conclusion

The study of planets and stars will continue for a very long time, and no one can know how many more riddles conceal the cosmic distances in oneself. So for many years the question remains about how life started on our planet, in the solar system and in the whole universe.