Since time immemorial, people have watched the night sky and noticed that in addition to motionless objects, there are those that change their position relative to the rest. Usually we say that they are stars, but is that so? 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 several 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, for a celestial body to be recognized as a planet, it must meet the following four conditions:
- The object should not be a star.
- Near the object’s orbits, other large celestial bodies should not move.
- The object must be in a shape close to spherical.
- The object should revolve around the star.
Planet and star. What is the difference?
With a question about which celestial bodies are called planets, we figured out, but what is the difference between them and stars? A 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 is a celestial natural body , which is capable of triggering thermonuclear reactions of helium, hydrogen and other gases of which it consists, while radiating into space an incredible amount of energy converted into light, heat and electromagnetic fluxes.
The solar system and its constituent planets
According to modern claims from a science called astronomy, planets in the solar system began to form about 4.5 billion years ago, becoming the result of a powerful explosion of one or more giant supernova stars. The solar system was originally a gas cloud with dust particles that formed in motion and due to their mass a disk 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 revolve around a common, central star 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 small size, being at a closer distance to the Sun than others.
• Planet giants - Neptune, Saturn, Jupiter and Uranus. Large, consisting mainly of the planet’s gas, with rings characteristic only of them, which are formed from many rocky debris and icy dust.
Until August 25, 2006, it was believed that nine planets were in the solar system. But after clarifying the definitions by which it was accepted what can be called a planet in the scientific world, Pluto, previously included in the solar system as the ninth, most distant object, passed into the dwarf category.
What was the reason for making this decision? The thing is that as telescopes and other astronomical equipment improved, scientists 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 exact requirements were introduced on 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 space. So the question will remain for many years about how life nevertheless originated on our planet, in the Solar system and in the whole Universe.