From time immemorial, people have watched the night sky and noticed that in addition to stationary objects, there are those that change their position relative to the rest. Usually we say that these are stars, but is it really? What celestial bodies are called planets and what criteria must an object have in order for it to 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 a few meters in size is considered a planet ("wandering" - translated from Greek). In the second half of the last century, more precise definitions were gradually introduced, and today, in order for a celestial body to be recognized as a planet, it must meet the following four conditions:
Object must not be a star
No other objects should move near the object's orbitlarge celestial bodies
The object must be nearly spherical
The object must revolve around the star
Planet and star. What's the difference?
We figured out what celestial bodies are called planets, but what is the difference between them and stars? The planet under the force of its own gravity is able to take a rounded shape and has a high density. But this mass is not enough to start thermonuclear reactions inside the body. A star, on the other hand, is a celestial natural body that is capable of launching thermonuclear reactions of helium, hydrogen and other gases of which it consists, while radiating an incredible amount of energy into space, converted into light, heat and electromagnetic flows.
The solar system and its constituent planets
According to modern statements of science called "astronomy", the planets of the solar system began to form approximately 4.5 billion years ago, becoming the result of a powerful explosion of one or more giant supernovae. The solar system was originally a gas cloud with dust particles that formed a disk due to their movement and due to their 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 in the solar system? The answer to this question is very simple: all objects that have their own orbit and that revolve around a common, central star are called planets of the solar system. They are divided into two small groups,four objects each:
• Terrestrial group of planets - Mars, Venus, Earth and Mercury. All of them have a rocky surface and are small in size, being closer to the Sun than others.
• The giant planets are Neptune, Saturn, Jupiter and Uranus. Large, gas-dominated planets with their characteristic rings, which are formed from many rocky debris and icy dust.
Up until August 25, 2006, it was believed that there were nine planets in the solar system. But after clarifying the definitions according to which it was accepted that in the scientific world it was possible to call a planet, Pluto, previously included in the solar system as the ninth, most distant object, moved into the category of dwarf objects.
What was the reason for making such a decision? The thing is that as telescopes and other astronomical equipment improve, scientists have discovered celestial objects similar in characteristics to Pluto, the number of which will increase over time. In order to eliminate possible confusion in the future, more precise requirements were introduced for 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 mysteries are hidden in the cosmic distances. So it will remain for many years the question of how life nevertheless originated on our planet, inthe solar system and in general throughout the entire universe.
