Since ancient times, mankind has observed the planets of the solar system with marked curiosity. Celestial bodies of undeniable fascination, and for years have been associated with an unknown and unexplored world. Only with the advancement of technology and with more and more powerful means at our disposal, we have been able to take a closer look at some of the planets that surround us, with future programs that would like to colonize some of them.
To date, thanks to numerous expeditions into space and the use of increasingly sophisticated probes, we have brought to light many of the mysteries that surround the cosmos, and we can admire very close images of everything that is beyond the Sun and the Moon. In a reality increasingly "hungry" for discoveries, it is then important for everyone to know what are the planets that make up our solar system and what are their distinctive features.
All the planets of the solar system
Before analyzing together all the planets of the solar system and their distinctive features, however, let's understand together what is the solar system and how it goes to compose. With this term, in astronomical literature generally means a planetary system consisting of a variety of celestial bodies kept in orbit by the gravitational force of the Sun, to which belongs also our Earth. With a diameter of about 120-130 UA, UA being the astronomical unit, or about the distance between Earth-Sun, it is located in Orion's Arm in the Milky Way, and scientists estimate that it takes about 230 million years to complete one revolution around the galactic center.
The system consists of the Sun, which alone covers 99.86% of its entire extent, eight planets, including four inner rocky planets and four outer gas giants, their respective natural satellites, and five dwarf planets, including Pluto, previously classified as the ninth planet. There is also a set of many smaller celestial bodies, comets and asteroids, whose number is unknown. Planets, asteroids and comets all revolve around the Sun, each along a different orbit: an ellipse, of which the Sun occupies one of the foci. This is why we can distinguish the solar system in internal, which includes the rocky planets and asteroids, and external, which includes the gas giants. They all follow and obey Kepler's three laws.
Specifically the subject of our analysis, the eight planets in order of distance from the Sun are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune; while the five dwarf planets are Ceres, located in the main asteroid belt, Pluto, Haumea, Makemake, and Eris. Among the other prominent elements, we also have the solar wind, which can be defined as a flow of plasma generated by the continuous expansion of the solar corona that permeates the entire solar system, creating at the same time a bubble in the interstellar medium known as the heliosphere, which also extends beyond half of the diffuse disk. Generally speaking, by planets we mean very large rocky or fluid bodies, tending to be spherical in shape.
The outer surface of these celestial bodies is cold, while we know that their size is much smaller than that of the stars and that they plan motion of revolution around the Sun. Thanks to this motion we can see them change position by raising our eyes to the sky, unlike fixed stars. In addition to the motion of revolution, the planets of the solar system have a motion of rotation, which can be classified as a movement in which the planet rotates on itself around an axis of rotation.
The inner solar system
As already mentioned, the planets of the inner solar system are those closest to the Sun, in a region of space that is recognized by the presence of rocky planets and asteroids. We are faced with celestial bodies composed mainly of silicates and metals, with minimal amounts of gas. This is why they are also called terrestrial-type planets, while their density is five times greater than that of water. Not only that, they have few or no satellites, and have no planetary rings, with a rather rarefied atmosphere, and have impact craters and tectonic plates, as amply demonstrated by the presence of rifts and volcanoes. Among the inner planets, we find in order: Mercury, the closest to the Sun, Venus, Earth and Mars.
Of the four terrestrial planets, recognizable by their distinct density, Mercury is the one least distant from the Sun. At the same time, it is the smallest planet in the solar system, at 0.055 Earth masses. Mercury has no natural satellites, and has a higher density than the other planets. Characterized by a period of rotation of 59 Earth days and revolution of 88.97 Earth days, the small planet is without water and atmosphere, except for thin traces of gas probably the result of the interaction of the solar wind with its surface. The observations made to date on the planet Mercury assume a similarity with the Moon, for a temperature that during insolation can reach 420 degrees centigrade, dropping to minus 180 degrees on the shadow side.
Venus shows dimensions very similar to those of the Earth and, just like our planet, has a mantle composed of silicates around a ferrous core, as well as an atmosphere, albeit ninety times denser than ours. It is often mistaken for a star, as it is very visible to the naked eye, just before sunrise and immediately after sunset, and has no natural satellites. It is the hottest planet in the solar system, with surface temperatures above 450 degrees. Experts believe that the extraordinary heat is most likely due to the amount of gas that causes a real greenhouse effect in the atmosphere. The high temperatures, high atmospheric pressure and type of air composition have led to the conclusion that Venus, just like Mercury, is a planet unsuitable for any living form.
Earth is the largest and densest of the inner planets, and most likely the only planet in the solar system where life forms are present. Earth's atmosphere is extremely different from that of the other planets, as it has been altered by the presence of life and is composed of 21% oxygen. It also has a single natural satellite, the Moon, while its liquid hydrosphere is unique among the celestial bodies of the inner solar system, as unique are its characteristic tectonic plates.
Also called the Red Planet, Mars is smaller than Venus and Earth, being able to count on "only" 0.107 Earth masses. Its atmosphere is rather tenuous and rarefied, consisting of high percentages of carbon dioxide and small amounts of steam. It is often involved in large dust storms in which very strong winds, with speeds up to 100 kilometers per hour, erode the surface of the planet to the point of changing it. The raised dust remains suspended in the air, giving the planet its typical reddish color along with the high amounts of iron found in the Martian soil.
The Outer Solar System
We can think of the outer solar system as the "home" of gas giants and their satellites, some of them as large as planets. Sometimes called Jovian planets, the planets of the outer solar system go on to make up a higher proportion of volatile elements , such as water, ammonia, and methane, than their more rocky and dense "cousins" in the inner solar system. Collectively, these four massive celestial bodies make up 99% of the known mass orbiting the Sun, and they are Jupiter, Saturn, Uranus, and Neptune.
The largest planet in the Solar System is Jupiter, characterized by a mass 2.5 times greater than that of all the other planets combined. We know that it is composed of an outer gaseous layer, consisting mainly of hydrogen and helium, and to a lesser extent methane, ammonia and steam. The inner layer is instead more compact due to the presence of metallic hydrogen at a pressure of 3 million atmospheres. Jupiter has 79 known natural satellites, and the four largest, Ganymede, Callisto, Io and Europa, show similarities to terrestrial planets.
Distinguished by its system of rings, Saturn is a solar system planet characterized by a rather large mass, 95 times that of Earth, with a very large surface area and a very low density, the lowest of all the planets in the solar system. There are 82 known satellites, one of which, Titan, is larger than Mercury and is the only satellite in the solar system to boast a dense atmosphere consisting of nitrogen and methane. Although it has a fairly high rotational speed, it needs nearly 30 years to complete an entire revolution around the Sun.
A unique Jovian planet with an atmosphere rich in hydrogen, helium and methane, Uranus is the least massive outer planet. Compared to other gas giants, its core is very cold and consequently does not radiate much heat into space. Uranus is then made up of three layers: a more superficial one in which there is molecular hydrogen, an intermediate layer containing water and metals in liquid phase, and an inner one characterized by a rocky core. Uranus has 27 known satellites, the largest of which are Titania, Oberon, Umbriel, Ariel, and Miranda.
Neptune differs from other planets by its atmosphere, rich in particular percentages of hydrogen, helium, and methane, and by a banded structure and a system of rings perched around the planet itself. Neptune has 13 known satellites, with the largest, Triton, still geologically active. It is a solar system planet that is still partly unknown due to its remoteness, which does not allow for too clear a view. It is certainly smaller but denser than Uranus, and radiates more internal heat.