Planets of the solar system
In 2006, the International Astronomical Union presented the latest definition of a planet. Because there is a dispute prevailing to this day regarding the definition of planets, as it must meet three conditions, which are:
- It must orbit a star.
- It must be large enough to have a gravitational pull that forces it to form a sphere.
- It must be large enough for its gravity to remove any objects of the same size that pass near its orbit around the sun. 
There are eight planets in the solar system, and they are divided into:
- The inner rocky planets, which are: Mercury, Venus, Earth, and Mars.
- The outer planets, which include the giant planets, which are the two planets of Jupiter, Saturn, and the icy giants, which are the two planets of Neptune and Uranus.
It should be noted that behind the planet Neptune there are a number of planets called dwarf planets, the most famous of which is Pluto. 
The planet Mercury (in English: Mercury) is one of the five planets that were known in the past, and the ancient called them the Wandering Stars. The Greeks believed that Mercury was an evening star if it was near the place of sunset and called it Hermes, while if it was near a place The sunrise is a morning star and they called it Apollo, and they believed at the time that it differs from one another, and the name Mercury was named after the messenger of the gods to the Romans. 
It should be noted that the planet’s atmosphere is very thin and it can hardly be explored. The following are some of the characteristics of the planet Mercury: 
- The average distance between Mercury and the sun: 57,910,000 km, equivalent to 0.3871 astronomical units, which means that the average distance between Mercury and the sun is 0.387 Double the distance between the sun and the earth.
- Mercury has a diameter: 4,878 km, equivalent to 0.38 times the diameter of Earth.
- Length of a year on Mercury: 88 days. Period of rotation of Mercury: 58.65 days.
- The average speed of the planet’s rotation around the Sun: 48 km / s. Temperature: Daytime 426.6 ° C, and at night -184.4 ° Celsius. Planet tilt of its axis of rotation: 2 °.
- Mercury is dense: Slightly less than Earth’s. Planet mass: About 0.055 times the mass of the Earth.
The name Venus (in English: Venus) after the Roman goddess of love, while it was known to the Teutons as Frig after the wife of their god and religion. 
The size of Venus is approximately the size of the planet Earth, and Venus is located at its closest state 42 million km from the Earth, and its atmosphere is characterized by being thick and composed of carbon dioxide gas, and the atmospheric pressure on the surface of the planet is equivalent to about 90 times the atmospheric pressure on the surface of the earth. Some of the characteristics of Venus: 
- The average distance between Venus and the sun: 108,200,000 km, equivalent to 0.723 astronomical units, meaning that the average distance between Venus and the sun is 0.732 times the distance between the sun and the earth.
- The diameter of Venus: 12,102 km, equivalent to 0.95 times the diameter of planet Earth.
- Length of a year on Venus: 225 days.
- Planet rotation time: 243 days counterclockwise.
- The average speed of the planet’s rotation around the sun: 35 km / s. Temperature: 475 ° C. Tilt of the planet off its axis of rotation: 177.3 °. Planet density: 0.9 times that of planet Earth. Planet Mass: About 0.81 times the mass of the planet Earth.
Planet Earth (in English: Earth) is the only planet that is characterized by the presence of life on it, due to its containment of water in a liquid form, and no spacecraft that reached any planet mentioned the presence of water on it, and life forms on it range from micro-organisms to complex such as humans ,  The following are some of the characteristics related to the planet: 
- Average distance between the Earth and the Sun: 149,600,000 km, which is equal to one astronomical unit.
- Planet Earth’s diameter: 12,765 km.
- The length of a year on planet Earth: 365.26 days. Planet rotation time: 23.93 hours.
- The tilt of the planet off its axis of rotation: 23.45 °.
- The average velocity of the planet around the sun: 29.79 km / s. Average temperature: 15 ° C. The number of its moons: one.
is the fourth closest planet to the sun, and despite its small size, it has captured the imagination and scientific interests of humans for centuries, and although it is not the closest planet to Earth, it is very similar to Earth. The planet has undergone some of the processes associated with the formation of the planets of Mercury, Earth and Venus, such as volcanoes, erosion and other effects of the atmosphere, in addition to that, it is similar to Earth in the growth and retraction of the icy poles cover with the change of seasons during the planet’s rotation around the sun,  Below are some characteristics of a planet Mars: 
- The average distance between Mars and the sun: 228,000,000 km, which is equivalent to 1,524 astronomical units, meaning that the average distance between Mars and the sun is equivalent to 1.52 times the distance between the sun and Earth.
- Mars has a diameter: 6,792 km, which is equivalent to 0.532 times the diameter of Earth.
- The length of the year on Mars: 687 days.
- Rotation time of the planet: 24 hours and 37 minutes. Tilt of the planet off its axis of rotation: 25.2 °.
- Average planet velocity around the Sun: 24.14 km / s.
- The average density of Mars: 3.95 g / cm3, equivalent to 0.7 times the density of Earth. Planet mass: 0.108 times the mass of the Earth. The number of its moons: Qumran.
has the advantage of Jupiter (in English: Jupiter) that it is the largest planet in the solar system, and it is a huge ball of gas reaching 300 times more than the size of the earth, and it is 5 times farther from the sun than the earth from it, so the year is on Jupiter Twelve times longer than the earthly year, while the day on Jupiter is about 10 hours only, and dozens of moons revolve around Jupiter, some of them hot, some volcanic and the other icy, and like the rest of the giant planets, the planet has rings surrounding it. Concerning Jupiter: 
- The average distance between Jupiter and the sun: 778,340,821 km, or about 5.2 astronomical units.
-  The diameter of Jupiter: 139,822 km. Length of a year on Jupiter: approximately 12 Earth years.  Period of rotation of Jupiter on its axis: 9 hours 55 minutes 30 seconds 
- The inclination of the planet from its axis of rotation: 3.1 °.
- The average speed of Jupiter around the sun: 47,002 km / h. Density of Jupiter: 1.326 g / cm3. Jupiter’s mass: 1,898,130 x 2,110 kg.
- The temperature of Jupiter: ranges between (-108 °) – (161-°) degrees Celsius. 
Saturn is the sixth planet farthest from the sun, and can be seen with the naked eye, and it is the second largest planet in the solar system, thanks to its discovery to the world Galileo (English: Galileo) in 1610 AD, and is characterized by its seven rings surrounding it, and in the past Astronomers used to think that these rings were moons, but Christian Huygens was able to understand the structure of these rings, and Saturn was named after the Roman god of agriculture, as well as the Greek god Cronos, . Some of the characteristics related to Saturn include: 
- The average distance between Saturn and the sun: 1,427,000,000 km, which is equivalent to 9,539 astronomical units, and this means that the average distance between Saturn and the sun is equivalent to 9.5 times the distance between the sun and the earth.
- Saturn has a diameter: 120,536 km, which is equivalent to 9.4 times the diameter of Earth.
- Length of a year on Saturn: 29.46 Earth years. Rotation time of the planet: 10.66 hours.
- Tilt of the planet off its axis of rotation: 26.73 °.
- The average speed of rotation of the planet around the Sun: 9.64 km / s.
- The number of its moons: more than 50. Mass of Saturn: 95 times that of Earth. Planet density: 0.13 times that of Earth.
Uranus is the seventh planet farthest from the sun. In addition, it is the third largest planet in the solar system in terms of diameter, and the fourth largest planet in terms of mass,  and was spotted by Mr. William Herschel (English: Sir William Herschel) in 1781 AD, and it was explored by the Voyager 2 spacecraft (in English: Voyager) in 1986 AD.  The planet Uranus is distinguished by its chemical composition of Neptune, but different from the chemical composition of Saturn and Jupiter, and for this reason scientists classify them on They are planets of ice giants to distinguish them from the gas giants, in addition to that, the composition of the atmosphere of the planet is similar to the planets Saturn and Jupiter in terms of their containment of elements of hydrogen and helium, except that it contains more ice such as: water, ammonia, methane and a small amount of other hydrocarbons, as well. It also contains layers of clouds with a complex structure, as water forms the bottom layer of clouds, while methane forms the top layer of it, and for the inner part of the planet it consists of ice and rocks, It should be noted that the temperatures on its surface reach -24 ° Celsius, so this planet is considered the coldest planet in the solar system,  and the following are some of the characteristics related to Uranus: 
- The average distance between Uranus and the sun: 2.871 billion km, which is equivalent to 19.19 astronomical units, which means that the average distance between Uranus and the sun is equivalent to 19.2 times the distance between the sun and the earth.
- Planet Uranus has a diameter: 51,118 km, which is equivalent to 4 times the diameter of planet Earth.
- Length of the year on Uranus: 84.01 Earth years. Rotation time of the planet: 17.24 hours.
- Tilt of the planet off its axis of rotation: 97.92 °. The average speed of the planet’s rotation around the Sun: 6.81 km / s.
- The number of its moons: more than 20. Mass of Uranus: 14.5 times that of planet Earth. Planet density: 0.22 times that of planet Earth.
The planet Neptune (in English: Neptune) is the eighth planet farther from the sun, and it is a giant, icy, cold and dark planet, and its wind speed reaches faster than the speed of sound,  and due to its distance from the sun, temperatures reach -255 ° C, It is distinguished by its blue and green color. Because methane is present in its atmosphere, and its atmosphere consists mainly of hydrogen, helium, and methane, and it was discovered by scientists: Galle, Challis, Adams, and Le Verrier in 1846, In 1989, after the Voyager 2 spacecraft explored Neptune, it was able to correct the belief at the time that the rings surrounding Neptune were arcs, as it proved that the rings completely encircle the planet, but the thickness of each ring varies according to its length, and with some characteristics related to Neptune: 2]
- Average distance between Neptune and the sun: 4.497 billion km, equivalent to 30.06 astronomical units; This means that the average distance between Neptune and the sun is equivalent to 30.06 times the distance between the sun and the earth.
- Planet Neptune’s diameter: 49,528 km, equivalent to 3,883 times the diameter of planet Earth. Length of the year on Neptune: 165 Earth years. Planet rotation time: 16.11 hours. The tilt of the planet off its axis of rotation: 29.6 °.
- The average speed of the planet’s rotation around the Sun: 5.43 km / s.
- The number of its moons: 14. Mass of Neptune: 17.14 times that of Earth. Planet density: 0.31 times that of planet Earth
Dwarf planets are defined as celestial bodies located in orbit around a star, but they are not moons, and their mass is sufficient to form a spherical shape, and they are unable to remove objects close to their orbit around the sun.  It is estimated that there are 200 A dwarf planet that can be explored in the Kuiper Belt, and it should be noted the importance of classifying and studying dwarf planets. They are as complex as planets, and they may reveal the origin of Neptune’s orbit, and they can also give a glimpse of the early solar system, and they may help scientists discover the ninth planet in the solar system, in addition to that, scientists hope that the dwarf planet Ceres will help study and understand moons with icy oceans. ,  According to the International Astronomical Union, five dwarf planets have been identified, namely: 
is the largest dwarf planet, located within the asteroid belt between Jupiter and Mars, and it was discovered in 1801 AD, and it is the first dwarf planet visited by spacecraft within NASA’s Dawn Mission.  Some information about Ceres: 
- Its radius is about 476 km.
- Its average distance from the Sun is about 413 million km.
- It takes 9 hours to spin around itself.
- It needs 4.6 years to complete its orbit around the sun.
- Its structure is similar to that of terrestrial planets, but is much less dense.
- Its surface has many craters with a diameter of no more than 280 km. He does not have any moons.
- It is characterized by having a very thin layer of the atmosphere consisting mainly of water vapor.
Pluto (in English: Pluto) is one of the most famous dwarf planets, and it was explored in 1980 AD, but it was classified at the time as the ninth planet in the solar system, but it and its moons orbiting around it are located within the Kuiper Belt in the area of ice debris behind the planet Neptune. ,  Here is some information about Pluto: 
- Its radius is about 1,151 km.
- Its average distance from the Sun is about 5.9 billion km. It takes 153 hours to spin around itself.
- It takes 248 years to complete its orbit around the sun. Pluto rotates in reverse east to west like Venus and Uranus.
- It consists of a rocky core and a curtain made of water ice.
- Its surface is characterized by multiple terrain, such as craters, mountains, valleys, and plains.
- It has a thin atmosphere that is mainly composed of molecular nitrogen, plus quantities of methane and carbon monoxide.
- Pluto has 5 moons: Charon, Nix, Hydra, Kerberos and Styx.
has the advantage of the dwarf planet Eris (in English: Eris) that its size is equal to the size of Pluto, and it was discovered in 2003 AD.  The following is some information about Eris: 
- Its radius is about 1,163 km. Its average distance from the Sun is about 10.25 billion km.
- It takes 25.9 hours to spin around itself.
- It takes 557 years to complete its orbit around the sun.
- Little is known about the composition of the interior of Iris, but it does have a surface that is rocky in nature like Pluto.
- Its atmosphere is characterized by its freezing and falling to the surface of the planet when it moves away from the sun, and it melts again when it approaches it.
- Iris has a small moon called Dysnomia.
(English: Haumea) is one of the objects of the Kepler belt, and is characterized by its very high speed of movement, which made its external shape appear closer to that of American football, and it was discovered in 2003 AD, and below is some information about this dwarf planet: 
- It is half Its diameter is about 620 km.
- Its average distance from the Sun is about 6.452 billion km.
- It takes 4 hours to spin around itself. It takes 285 years to complete its orbit around the sun.
- Scientists believe it consists of a rocky core surrounded by ice.
- It has two moons: Namaka and Hi’iaka. It is the first Kepler belt objects known to have rings. Its ring was discovered in 2017 AD while it was passing in front of a star.
(in English: Makemake) is one of the Kepler belt objects that was discovered in 2005 AD. It is the second Kepler belt object in terms of intensity of illumination preceded only by Pluto, and below is some information about Makemake: 
- It has a radius of 715 km.
- Its average distance from the sun is about 6,847 billion km.
- It takes 22.48 hours to spin around itself.
- It takes 305.34 years to complete its orbit around the sun. Little is known about the composition of Makemake, but it appears to the observer in a red-brown color as is the case with Pluto, and scientists were able to discover the presence of methane and ethane in the frozen state on its surface.
- It is expected to possess a thin atmosphere consisting of nitrogen. He owns one satellite, (S / 2015 (136472) 1), and is nicknamed (MK2).
Video about the planets of the solar system
The Solar System
defines the Solar System (in English: Solar System) as a system consisting of a grouping of an average star in the Milky Way, which is the Sun, and around which the various bodies revolve, namely: the eight planets, and their affiliate satellites, which number about 170 moons, as well as an innumerable number of asteroids and some Its moons, comets, and icy bodies, in addition to the presence of vast stretches of dust and low-density gas, known as the Interplantery Medium. 
Although astronomers and stargazers have observed these objects in the solar system since the dawn of time, they were only able to directly explore them in the mid-twentieth century, thanks to the use of spacecraft.