There are eight planets in the solar system. They rotate around the Sun in the following order, starting from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. In addition to the full size planets there are 5 dwarf planets that also make their way around the Sun: Ceres, Pluto, Haumea, Makemake, Eris.
All of the major planets orbit the Sun in one direction. The orbits of all major planets are ellipses that are very close to the circles, and the planes of their orbits are inclined to the ecliptic plane at small angles. The masses of all the planets put together, make up only 0.0013 of the Sun. In addition to these planets, a large number of smaller bodies, called minor planets or asteroids, is moving mostly between Mars and Jupiter. The number of known asteroids is constantly growing as a result of new discoveries, and currently there are more than 1,600 different bodies known to the scientists.
Planets in the solar system can be easily divided into two groups based on their size. The first group consists of relatively small planets that are closest to the Sun: Mercury, Venus, Earth and Mars. This group is commonly known as the Earth group. The second group consists of the largest planets in the Solar System: Jupiter, Saturn, Uranus and Neptune. This group is also known as the Jupiter group. The asteroid belt separates these groups. Pluto stands alone in this grouping, as it is insufficiently explored.
Looking at the physical characteristics of the major planets, we can easily divide them into two groups. For example, the average density of planets in the first group is 4.5 g/cm3 and an average density of planets in the Jupiter group is 1.21 g/cm3. Judging by the density, we can say that the planets in the Earth group are solid. The densities of the planets in the Jupiter group, however, are similar to the density of the Sun and cannot be considered solid.
There is quite a simple explanation for the strong differences between the four giant planets, and four terrestrial planets. Pluto is an exception to this scheme; however it looks more like one of the moons of distant planets. Hydrogen-rich planets consist of relatively little altered original substance from which the Solar System was formed. Solid planets of the Earth group on the other hand have lost much of the light gases that form the basis of this substance. Instead of primary atmosphere captured from the gas-dust cloud, from which the Solar System was formed, they have secondary atmospheres that emerged after the planets were formed.
Comparing the rotation periods, we, once again, can notice a big difference between the two groups. Terrestrial planets take more than a day to revolve around their axis – they rotate slowly. Planets of the Jupiter group revolve around their axis much faster – on average the rotation period is less than half a day. Huge Jupiter revolves in only 9h 50m – it is clear that its linear velocity at the equator will be much larger than the corresponding rate of rotation of the Earth (28 times larger). Because of this, planets in the Jupiter group have a higher rate of compression, reaching 0.1 on Saturn, and the planets in the Earth group are almost perfect spheres.
The number of satellites of the planets of both groups is also quite different. The entire first group has only three satellites; the second group has more than 160 moons, and this number is constantly increased by new discoveries.
Thermonuclear reactions started on the Sun around 4.6 billion years ago. Because of this, temperatures of the Solar System planets, especially the interior ones noticeably increased. There were two crucial factors that determined the future look of the planets – their size and distance from the Sun. Small size planets were unable to keep the lightest gases – hydrogen and helium. Planet’s ability to retain hydrogen and helium was the decisive factor during the formation of the Solar System. Another determining factor was the distance from the Sun. The fact is that the closer the planet is to the Sun, the more it is heated, and the more difficult it is to retain light gases on its surface. The combined effect of these factors had a significant influence on the formation of planets in the Solar System and divided them into the two main groups.
[images: www.nasa.org]
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