Essentially, a planet differs from a star in its much smaller amount of mass. Because of this deficit, the planets do not develop thermonuclear fusion processes and cannot emit their own light; limited to reflect that of the star around which they rotate.
Historically, nine planets have been distinguished in the Solar System: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto; However, there are other planetary bodies that, due to their large dimensions, could also be considered as planets. This is the case of Ceres, with a diameter greater than 1,000 km, however, classified as an asteroid.
All planets travel their orbits around the Sun counterclockwise, a phenomenon known as direct translation. The Planets have virtually circular orbits, according to Kepler's laws are ellipses or flat circles. The deviation from the circular shape is quantified by the eccentricity value.
The average Earth-Sun distance is used as a unit of length and is called Astronomical Unit (AU). The average distances between the Sun and the Planets increase in geometric progression from Mercury to Pluto.
Each Planet makes a complete revolution around the Sun at a time called Sidereal Period. This period increases geometrically with the distance to the Sun according to Kepler's third law. The sidereal periods range from 88 days of Mercury to 248 years of Pluto. The orbital velocities of the planets decrease with distance (from 45 km / s for Mercury to 5 km / s for Neptune), but they are all in the same direction.
The Planets have a movement of rotation around their own axis and in the same sense as that of their translation around the Sun. The periods of rotation range from 243 days of Venus to 10h that it takes Jupiter to turn around . The axes of rotation of the planets show various inclinations with respect to the ecliptic. Most of the Planets possess numerous satellites, which generally orbit in the equatorial plane of the planet and in the same direction of its rotation. The orbits of the different satellites of a planet in turn follow the law of Titus-Bode.
Light or giant planets are located on the outside of the Solar System. They have small densities, which reflect their small amount of silicates. They are planets consisting basically of hydrogen and helium, a reflection of the composition of the primitive solar nebula. They have important meteorological activities and gravitational processes in which the planet is compacted, with a small nucleus and a large mass of gas in permanent convection. Another common feature is to have rings formed by small particles in orbits closer than those of their satellites. To this type belong Jupiter, Saturn, Uranus and Neptune.
The great planets, Jupiter and Saturn, have satellite systems, which in a way, are miniature models of the Solar System. Although they do not have thermonuclear energy sources, they continue to release gravitational energy in an amount greater than the solar radiation they receive.
The dense or terrestrial planets are located in the inner part of the Solar System, an area that comprises from the orbit of Mercury to the asteroid belt. They have densities between three and five grams per cubic centimeter. There has been a very high selection of matter, giving rise to products such as uranium, thorium, and potassium, with unstable nuclei that accompany radioactive fission phenomena. These elements have developed enough heat to generate volcanism and important tectonic processes. Some are still active and have erased the features of their original surface. Examples are Earth, Io, and Venus.
However, there are other planetary bodies that have suffered intense craterization of their surface (Moon, Mars, Phobos, Demos, Venus, in part, Mercury and even asteroids). The presence of craters on planetary surfaces indicates how the abundance of objects in interplanetary space has varied throughout its evolution, providing a key to understanding the history of each of the inner planets.
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