A planet usually refers to a celestial body that surrounds a star without giving off light. Its revolution direction is often the same as the rotation direction of the surrounding stars. Generally speaking, the planet needs to have a certain mass, and the mass of the planet should be large enough to approximate a sphere, so that nuclear fusion reaction can occur like a star.
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After thousands of years of exploration, it was not until Copernicus established the Heliocentrism in16th century that people generally realized that the earth is one of the planets revolving around the sun, and the eight planets including the earth constitute the main members of the planetary system revolving around the sun-the solar system.
Planets themselves generally do not emit light, but emit light by reflecting the light of stars to their surfaces. Starting from the fact that planets originate from different forms of matter, the eight planets can be divided into three categories: terrestrial planets (including water, gold, earth and fire), giant planets (wood and earth) and distant planets (heavenly kings and Neptune).
The motion of the planet around the star is called revolution, and the orbit of the planet revolution has three characteristics: coplanar, isotropic and approximately circular. Coplanarity means that the orbital planes of the eight planets are almost on the same plane; Isotropy means that they revolve around the star in the same direction; Nearly circular means that their orbits are very close to circular.
Around some planets, there are rings of matter that revolve around them. They are made up of a large number of small objects (such as rocks and ice cubes), which emit light by reflecting sunlight and are called planetary rings. Before the 1970s, people always thought that only Saturn had rings, and later it was discovered that Uranus and Jupiter also had rings, which provided new information for studying the origin and evolution of the solar system.
References:
Baidu Encyclopedia-Eight Planets