The Solar system is the set of celestial bodies that orbit the Sun: in addition to the planetary system with the 8 planets recognized by astronomers (divided into rocky and giant planets, including gas giants and icy giants) we also find dwarf planets such as Pluto, comets, asteroids and satellites, kept in orbit by the gravitational force of the Sun The planetary part of the Solar System extends for approximately 4.5 billion km from the Sun (i.e. about 30 astronomical units), while the most external offshoots can even touch the 100,000 astronomical units from our star. It was formed approximately 4.6 billion years ago and is located in one of the minor arms of the Milky Way, called arm of Orionapproximately 26,000 light years distance of the galactic center, around which it rotates in approx 230 million years.
The Planets
THE planetsFirst of all, they are celestial bodies incapable of producing energy from nuclear fusion. The Earth’s core, for example, contains partly molten and partly solid material, but nuclear reactions do not occur inside it, as happens in the Sun.
Furthermore, according to the IAU (International Astronomical Union), there are three main characteristics that allow us to define a planet:
- the celestial body must orbit around a star (therefore the Sun);
- it must have sufficient mass such that the force of gravity gives it an approximately sub-spherical shape;
- the gravitational force must have freed the region surrounding the planet’s orbit, i.e. there must be no other celestial bodies of equal or larger size around the planet.
The planets of the Solar System can be mainly divided into two categories: rocky planets And giant planets. The first category includes Mercury, Venus, Earth and Mars and are smaller planets with a solid surface; in the second category we find the two gas giants Jupiter and Saturn and the two frozen giants Uranus and Neptune, which are overall larger planets, colder (due to the greater distance from the Sun) and made up of lighter elements.
Let’s now see the various planets in order, starting from those closest to the Sun up to the furthest ones.
Mercury
Mercurywith its radius of 2440 km, is the smallest planet of the solar system. Its rotation period lasts approximately 58 days and, in this period, the sunny side reaches 430°C while the shaded side drops to -170°C. It is no coincidence that it is the planet with the greater temperature range of the entire Solar System!
This temperature difference is due to the fact that theThe planet’s atmosphere is very thin and, consequently, unable to retain the heat received from the Sun. The reduced thickness of the atmosphere is also the cause of all the craters visible on its surface: not having an atmosphere to slow them down, the asteroids impacted the planet at very high speeds for millions of years.
Venus
When we look at the starry sky, the brightest object we can see after the Moon is precisely Venus. This planet, the only one together with Uranus a rotate clockwisethat’s it warmer of the Solar System. This peculiarity is linked to its atmosphere, made up mainly of carbon dioxide (96.5%), which causes an enormous greenhouse effect. THEThe average temperature of the planet’s surface is approximately 460°C, its rotation period is 243 days while its revolution period is approximately 225 days.
Earth
With its 6378 km radius (at the equator), it is the largest of the four terrestrial planets. Like the others, it has an atmosphere and its internal structure is divided into “layers”: crust, mantle, outer and inner core. Among the planets in the Solar System, Earth is the only one to have such high quantities of liquid water and its atmosphere is the only one that allows it life development as we know it.
Mars
Mars, the red planet, which has always been a dream of space exploration, took this nickname due to the abundant quantity of iron oxides present in the rocks on its surface. The Martian day lasts only 40 minutes longer than the Earth’s and its atmosphere, thinner than ours, retains less heat: the average temperature is -55°C. On this planet there is Mount Olympus which, with its 25 km of height, is the largest volcano in the Solar System. Know that we have dedicated an entire video series to Mars. If you are a fan, we advise you not to miss it: Yes, We Mars!.
Jupiter
TO Jupiter belongs to the crown of largest planetso much so that it has a radius about 11 times larger than that of the Earth. The planet is made up mainly of low-temperature gases, so much so that one is present on its surface layer of partially liquid hydrogen and helium and, only in its nucleus, heavier elements. Jupiter’s revolution period is 12 Earth years while its rotation lasts only 10 hours due to a particularly rapid movement.
Saturn
Just like Jupiter, Saturn also welcomes on its surface a “lake” of liquid hydrogen and helium. Compared to the giant next to him, however, he has a lower density, la lowest in the entire Solar System: 0.68 g/cm3that is, it is less dense than water! Around the planet there are 280,000 kilometers of rings made up of dust, rock fragments of various sizes and gas. The planet has a revolution period of 29.46 years and a rotation period of approximately 10 hours.
Uranus
Uranus is the penultimate planet of the System, discovered in 1781 by William Herschel. Compared to the others, it has a sub-vertical rotation axis. In other words, the side exposed to the Sun is, periodically, one of the two poles. The temperature is always below -200°C and, like Saturn, it has rings of dust and gas surrounding it. Being now very far from the Sun, the planet’s revolution period is quite long and equal to approximately 84 years, while the rotation period is approximately 17 hours.
Neptune
Neptune is the eighth planet in the Solar System, if you count from the sun, and it is the coldest of all. Its atmosphere is mainly made up of hydrogen, helium and methane, while on its surface there are fluids composed mainly of methane, ammonia and water. Even in this case the average surface temperature is always below -200°C and the winds can exceed 17,000 km/h, i.e. one and a half times the speed of sound! The planet’s revolution period is approximately 165 years while its rotation period is approximately 16 hours.
Pluto and the dwarf planets
Previously we talked about the definition of planet proposed by the IAU. For i dwarf planets the third consideration is not valid – that is, the one relating to the orbit free from celestial bodies of equal or larger dimensions – but, on the contrary, the statement “It doesn’t have to be a satellite”, that is, it must not revolve around another planet. Five dwarf planets have currently been recognized: Pluto, downgraded on August 24, 2006, Ceres, Haumea, Makemake and Eris.
Other celestial bodies
In addition to planets and dwarf planets, the Solar System also hosts other types of celestial bodies. Which? Let’s go!
Asteroids
During the formation of the planets, as we have seen, fragments of rock, dust and gas came together to give life to the 8 main celestial bodies of the Solar System. However, some planets failed to form and what remains of them is classified as asteroid. Most of it is included in the so-called asteroid beltbetween Mars and Jupiter, and more than a million have been identified.
Comets
The comets they are celestial bodies made up of a core of ice and dustcovered by a layer of dark dust. When the Sun heats these bodies, the ice core transforms into gas: this is the phenomenon that gives life to the trails we see in the sky! Among the most famous comets we remember the Halley’s comet which passes by our planet approximately every 76 years to say hello 🙂
Warning: sometimes these bodies are called “comet stars”, but this is wrong because they are not stars at all.
Meteoroids, meteors and meteorites
When small rocky bodies enter the earth’s atmosphere – the so-called meteoroids – they become incandescent and emit a characteristic brilliant trail. When this happens they are called meteors or, vulgarly “shooting stars”. Just as in the case of “comet stars”, talking about “shooting stars” is also wrong because in neither case are we dealing with stars.
If a meteor fragment touches the ground, that rock is called meteorite: it is precisely a meteorite that – according to some theories – contributed to the extinction of the dinosaurs. Here we leave you one of our videos dedicated to meteoroids, meteors and meteorites.
Meteorite sample (credit: NASA).
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Satellites
Satellites are celestial bodies that do not orbit stars but other bodies, such as planets. Also defined moonsin our system there are about 150 of them and, in our small way, the Earth also has one: the Moon. However, most orbit around large gas giants such as Jupiter and Saturn.
The Sun
Mother star of our system, the Sun has approx 4.57 billion years. In astronomy it is defined yellow dwarfthat is, a type of star with a rather long life (it can reach 10 billion years) with a surface temperature of around five thousand degrees. To give you an idea of how big it is, there would be 1.3 million Earth planets inside the Sun!
The star is powered by hydrogen, That, through the process of nuclear fusion, it comes transformed into helium, bringing the temperature of its interior to approximately 15 million degrees! It takes about 1 million years for heat to travel from the star’s core to its surface and from here it is emitted into space in the form of light, heat and other radiation.
How was it formed?
Before talking about “how it’s done“, it is necessary to say a few words about “how the solar system was formed”. The most accredited theory hypothesizes the presence of one primordial nebulathat is, a mass of gas and dust which, about 5 billion years ago, began to thicken in its central portion and rotate slowly. This union of particles gradually acquired larger and larger dimensions and increased its temperature, until it generated the star that we now call Sun. Although our star captured most of the material present in the primordial nebula, the remaining part aggregated to form all the planets and other bodies that have since revolved around it.
The Solar System is therefore formed by a center, that is, the Sunwith all the other celestial bodies that rotate around it (including the planets Mercury, Venus, Mars, Earth, Jupiter, Saturn, Uranus and Neptune) following very specific orbits. Overall there are eight planets and an unknown but very high number of satellites, asteroids, comets and other minor bodies. The Solar System, however, on a larger scale, is part of the arm of Orion from the Milky Waya galaxy that has around 300 billion stars inside.
