Solar system, definition and study. A-Z Index of Cognitio.

The solar system is the planetary system consisting of a variety of celestial bodies kept in orbit by the gravity of the Sun;

Sun, definition and study. A-Z index of Cognitio.


Earth also belongs to you. It is composed not only of the Sun, but also of eight planets (four internal rock squares and four outdoor gigantic giants), their natural satellites, five dwarf planets and billions of smaller bodies. The latter category includes asteroids, largely split between two asteroid belts (the main band and the Kuiper band), comets (predominantly located in the Oort cloud hypothetical), meteorids and interplanetary dust. The size of the solar system is difficult to define;

Asteroid, definition and study. A-Z index of Cognitio.


Approximately it is considered to be an irregular sphere.

Heavenly sphere, definition and study. A-Z index of Cognitio.

Heavenly sphere

Within the solar system the space between a celestial body and another is not empty: it is permeated by the so-called interplanetary medium, including dust, gas, and elemental particles. The Sun has a particular importance, both because it is the only star, and therefore the only source of thermal energy of the system, and because it represents the gravitational center with great approximation. Solar mass, in fact, constitutes about 99.9% of the material present in the system. It is thought that the Sun and the planets formed from a contraction of interstellar gas nebula, about 4.6 billion years ago. The Sun is a glowing gas sphere that continuously produces energy due to hydrogen fusion in the nucleus. Its core has very high temperatures and densities, so the hydrogen nuclei blend together forming helium. These reactions release a tremendous energy, which crosses the radiation and convection zone, reaches the surface (photosphere) beyond the solar atmosphere (chromosphere) and frees itself in space. The main celestial body of the solar system is the Sun, a star of the main spectral spectrum (yellow dwarf) sequence, containing 99.86% of all known mass in the solar system. Jupiter and Saturn, the two most massive planets orbiting the Sun are more than 90% of the remaining mass. The planets are very different from each other by composition, size, temperature and other characteristics. According to physical chemical characteristics, planets can be distinguished in two groups: Earth-like planets (Mercury, Venus, Earth and Mars), that is, similar to Earth, and young planets (Jupiter, Saturn, Uranus, Neptune) that is, similar to Jupiter.

Uranus, definition and study. A-Z Index of Cognitio.


Between Mars and Jupiter there is the so-called main artery of asteroids, made up of millions of rocky objects characterized by more or less variable orbits. Among them, Ceres is currently considered to be the only one to present a hydro-static equilibrium (or a spheroid form) and to deserve the status of a dwarf planet. Beyond Neptune there is another band of asteroids, the Kuiper band, whose actual density is unknown. Among them are Pluto and Eris, which since 2006 are recognized as dwarf planets by the International Astronomical Union. Previously Pluto was considered the ninth planet. Even more externally, between 20 000 and 100 000 Years of Light away from the Sun, it is assumed that the cloud of Oort is believed to be the source of the comets.

Solar system, definition and study. A-Z Index of Cognitio.



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Solar system, definition and study. A-Z Index of Cognitio.

Solar system