Planets are approximately round cosmic bodies whose mass is less than 80 MJ. They travel through space alone or orbit a star. It must also be true for the planet that it has cleaned the surroundings of its orbit from other bodies and produces very little or no energy through the thermonuclear reaction. It does not emit its own light all over the area and shines only with the reflected light of the star. Our planetary system (Solar System) contains 8 planets, 4 with a gaseous surface (jovally) – Jupiter, Saturn, Uranus, Neptune, 4 with a solid surface (terrestrial).
Planets can have their own natural orbits. For larger bodies (on the order of at least a kilometer difference), they are called moons. A large number of smaller bodies that orbit in close orbits in one plane and from a distance may appear as a compact hoop encircling the planet is called a ring.
In the nineties of the twentieth century, the first planets were discovered, which do not orbit the Sun, but other stars. Such planets are called extrasolar (exoplanets). Exoplanets are currently known to be more than 4,200.
The planets are thought to have formed from the remnants of the nebula from which the parent star of the planet formed. They formed by the accumulation of gas and dust orbiting the star in a dense star disk before nuclear reactions ignited in the star’s core and the stellar wind blew the remaining material away. The initial stage of the planet is considered to be the protoplanetary disk from which the planetaryzimals are formed. By joining and colliding with planetesimals, protoplanes are formed. Further connection and collision of protoplanets form the planets themselves. Large gaseous planets, so-called jovial planets, probably formed by packing gas on rocky cores. Another theory assumes that they formed by shrinking the mass of the protoplanetary disk in a manner similar to the way stars form from a nebula.
Physical properties are limit weight, size and shape and energy.
The planets of the solar system orbit the Sun in ellipses slightly different from circles. The speed of their movement decreases with distance from the Sun, so their orbital periods are very different – from 88 days for Mercury to 165 years for Neptune. However, the orbits of exoplanets are often very eccentric, so their distance from the parent stars varies greatly.
The planet consists of:
Kernel – the area around its center. It is the densest and hottest part of the planet.
Sheath – the layer around the core, the temperature and density in it decrease.
Crust – on solid planets or atmosphere on gaseous planets. Solid planets have an atmosphere above the crust.
Most planets in the solar system generate a global magnetic field. Thanks to it, it is also covered by the magnetosphere.
There are also so-called Dwarf planets – a cosmic body that orbits the Sun has enough mass for its gravity to stabilize its shape corresponding to hydrostatic equilibrium (approximately spherical), but not enough to clear the surroundings of its orbit. It must not even be a satellite of the planet.