Planets of the Solar System

We explain what the planets of the solar system are, their individual and general characteristics. Also, how the solar system formed.

What are the planets in the solar system?

The solar system or planetary system is the set of astronomical objects gravitationally linked to each other, of which the planet Earth is a part along with seven other known planets: Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune.

Broadly speaking, Planets are large spherical masses of compact matter that orbit the Sun in regular periods some of them composed of solid elements, others of gaseous accumulations. The immense force of gravity of the Sun is what keeps them in their respective orbits, whose location allows them to be classified into two subsets: the inner planets and the outer planets.

• The inner planets. This first group is made up of Mercury, Venus, Earth and Mars, distributed between the Sun and the asteroid belt after Mars. They are smaller planets with a solid surface, which move in shorter and faster orbits. Of them, only Mars and Earth have their own satellites.
• The outer planets. This second group is made up of Jupiter, Saturn, Uranus and Neptune, distributed between the asteroid belt and the Kuiper belt, in the farthest portion of the solar system. They are larger planets with a gaseous constitution (they are often known as “gas giants”), for which it is not yet known whether or not they have a surface.

Just as the planets orbit the Sun, other small celestial bodies, known as Natural satellites orbit around some planets, trapped in their gravitational field. In some cases these satellites are tiny and very numerous, as is the case with Saturn, while in other cases they are larger and have their own name, such as the moons of Mars: Deimos and Phobos.

In addition to the planets and their respective satellites, the solar system is made up of other astronomical objects, among which the following stand out:

• The Sun. It is the central star of the system and the brightest object in the Earth's sky. It occupies 99.86% of the mass of the solar system and is a G-type star passing through its main sequence, with a diameter of 149,597,870.7 km.
• The dwarf planets. In addition to the eight known planets, there are five dwarf planets, with similar properties but much smaller in size and little orbital dominance (that is, they can share their orbits with other objects).
• The minor bodies. This is the name given to conglomerates of astronomical objects of various sizes and shapes, which are neither planets nor satellites, and do not always have stable and predictable orbits. These objects are grouped in the asteroid belt that separates the inner solar system from the outer one, as well as in the Kuiper Belt that is beyond the orbit of the last planet, or the Oort cloud that is even further away, almost one light year from the Sun.

Finally, it is important to understand that the solar system is not a static place, but in turn moves within a larger system that is the galaxy, and which in our case we have named the Milky Way. Within this galaxy, our solar system is located in a peripheral region, at the end of one of its spirals.

Formation of the solar system

According to scientific estimates, the solar system It was formed about 4.6 billion years ago, following the gravitational collapse of a molecular cloud large size. Most of the matter accumulated in the center and gave rise to the Sun, while the rest of it flattened out to form a protoplanetary disk, that is, a disk of matter around a young star, from which much emerged. then the planets and asteroids.

This explanation responds to the theory proposed in the 18th century by Emmanuel Swedenborg (1688-1772), Immanuel Kant (1724-1804) and Pierre-Simon Laplace (1749-1827), although in later centuries (especially in the 20th century, with the beginning of space exploration) was refined and redefined to incorporate the most recent discoveries and observations in space matters.

From its earliest moments, the solar system has evolved and transformed intensely, as a result of the expulsion of materials from the Sun, and also the numerous collisions that occurred between objects born from the planetary protodisc, or also coming from outside the solar system. But gradual cooling was necessary (especially of the inner solar system) for unions of very volatile molecules to emerge and rocky planets, like ours, could be formed.

Characteristics of the planets of the solar system

The planets that make up the solar system are of two types: eight law planets and five dwarf planets. The difference between them has to do, according to the International Astronomical Association, with three main features:

• A planet must orbit the Sun (and not other astronomical objects in the solar system);
• A planet must have enough mass to reach hydrostatic equilibrium and acquire a relatively spherical shape;
• A planet must exercise orbital dominance that is, it should not share its orbit with other celestial bodies.

Therefore, the planets of the solar system are eight (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune) and the dwarf planets are five (Pluto, Ceres, Eris, Haumea and Makemake).

The planets, however, are masses in constant motion, which complete their journey around the Sun (that is, their translational movement) at different times: the further away they are from the Sun, the slower and longer their orbit will be. In addition, the planets rotate on their own axis (that is, a rotation movement) at a different rate and in a uniform direction (except for Venus and Uranus, which rotate “in reverse”). The axis and rotation speed of each planet is different and depends in principle on its composition.

A comparison of the properties and characteristics of the known planets yields data such as the following:

Planets of the solar system

1. Mercury

Represented astronomically and astrologically with the symbol ☿, Mercury It is the closest planet to the Sun and the smallest of all the inner planets. Devoid of natural satellites, it is a rocky planet, made up of 70% metallic elements (iron, in particular) and the remaining 30% are various silicates, making it the second densest planet in the entire solar system. after Earth.

Mercury It has a dry surface full of meteorite impact craters and other astronomical objects, many of which are close to 4 billion years old, since the planet has practically no atmosphere to slow down these objects. Being so close to the Sun, Mercury's surface It is hot during the day, hovering around 350 °C ; but at the same time the absence of an atmosphere gives it freezing nights of around -170° C.

The first observations of Mercury date back to the earliest antiquity (the 3rd millennium BC), but its current name refers to the Roman god Mercury, a variant of the Greek god Hermes. The latter was the name the Greeks gave it when they observed it at dusk, while in the morning sky they called it Apollo. The first to realize that it was the same star was the philosopher and mathematician Pythagoras of Samos (c. 569 – c.475 BC).

2. Venus

Represented with the sign ♀ in astronomy and astrology, Venus It is an inner planet that lacks satellites and is the second brightest object in the Earth's night (after the Moon). Its name pays tribute to the Roman goddess of passionate love, the same one that the Greeks called Aphrodite.

Like the other inner planets, Venus is a rocky planet, but it is enveloped in a dense atmosphere of carbon dioxide (CO₂), molecular nitrogen (N₂) and hydrogen sulfide (H₂S), which are known greenhouse gases. For this reason, Venus It is the hottest planet in the solar system much more than Mercury, despite the latter being closer to the Sun. Its average temperature is 463.85 °C.

This atmosphere also gives Venus a yellowish-white color, and an atmospheric pressure 90 times greater than Earth's. On the other hand, its rotation movement is particularly slow (and opposite in direction to that of most planets), so that on Venus a day lasts much longer than a year. In general, It is a place incompatible with life, although there is evidence of certain organic compounds on its surface that could suggest bacterial presence.

3. The Earth

Earth, our planet, is quite exceptional compared to the rest of the solar system. Not only because we are in it, the only self-aware living beings of which we have news, but because It is the only planet that has liquid water and a flourishing biosphere for several billion years. There are many theories and explanations for this phenomenon, but the truth is that the planet is at the ideal distance from the Sun, which means that it is neither too hot nor too cold.

It is the densest planet in the entire solar system, and the fifth largest in proportions. The Earth has a core of iron and nickel whose internal movements generate a powerful magnetosphere, and at the same time a not too dense atmosphere, composed of 78% nitrogen, 21% oxygen and the rest are other substances such as argon, carbon dioxide, ozone and water vapor. Thanks to the caloric retention of the atmosphere, the planet has a benign and stable climate otherwise its average temperatures would be around -18 °C.

71% of the Earth's surface is covered by water, particularly salt water from the oceans, and the hydrological cycle is what keeps the atmosphere cool and stable, in addition to the exchange of elements that led to the emergence of life. The Earth It has only one natural satellite, the Moon whose origin is estimated to be a dwarf planet or planetoid that shared orbit with the planet and ended up crashing into it approximately 4.53 billion years ago.

The name of the Earth comes from the Roman voice Terra, equivalent to the Greek Gaia, a primal goddess associated with fertility and femininity, equivalent to Mother Earth in other mythologies and religions. In astronomy and astrology the planet is represented by the symbol ♁.

4. Mars

Mars is the last of the inner planets, named after the Roman god of war, equivalent to Ares of the Greeks, and Also known as the “red planet” because of the abundant iron oxide on its surface. It has two natural satellites, small and irregular in shape, called Phobos (from the Greek phobos“fear”) and Deimos (from the Greek we said“terror”), whose origin is unknown but which could be asteroids captured by the planet's gravity.

It is a smaller planet than Earth, but it shares numerous physical characteristics with it, in addition to a similar rotation period and orbital cycles. Mars has a light atmosphere (100 times less dense than Earth's), composed mostly of carbon dioxide (CO₂), and an arid and sandy surface, full of dunes moved by the Martian winds.

However, this desert planet It has dense accumulations of ice on its polar caps large enough to flood the entire planet under 11 meters of water, if these ices could melt.

Humanity has observed Mars since ancient times, since can be seen with the naked eye on a clear night. Astronomy and astrology represent it with the symbol ♂ and, after the Moon, it is one of the most coveted space destinations for humans in their contemporary career of space exploration.

5. Jupiter

Jupiter is the first of the outer planets, that is, those that are beyond the asteroid belt of the solar system. It is a gigantic gas planet surpassed in volume only by the Sun, since Jupiter holds two and a half times the total mass of the rest of the planets combined. Its volume, for example, is 1,321 times greater than that of the Earth, but at the same time it is much less dense than it.

Unlike the inner planets, Jupiter It does not have a defined surface, but is a ball composed of hydrogen (87%), helium (13%) and other substances such as argon, methane, ammonia and hydrogen sulfide in very small quantities. All these gases are around a rocky core covered by a deep layer of metallic hydrogen in a liquid state. This means that there is no clear separation between the atmosphere and the liquid interior of the planet, but rather it passes from one to the other gradually.

Jupiter has in the tropical region of its southern hemisphere an enormous anticyclone known as the Great Red Spot first observed in 1664 by the English scientist Robert Hooke (1635-1703). It is a huge whirlpool at least three centuries old, on the periphery of which winds of up to 400 kmph have been recorded. Our entire planet could fit twice in this gigantic storm.

The name of this planet pays tribute to the father god of the Roman pantheon, equivalent to Zeus of the Greeks, and in astronomy and astrology it is represented with the symbol ♃. Throughout history, around 79 natural satellites of different sizes and shapes have been attributed to it, among which the four “Galilean moons” stand out (since Galileo Galilei was the first to observe them): Io, Europa, Ganymede and Callisto.

6. Saturn

Saturn is the sixth planet in the solar system and one of the largest known. Its size and mass are second only to Jupiter, and its ring belt visible from Earth constitutes a very characteristic feature. It was one of the most distant planets observed in ancient times, and was thought to mark the end of the known universe.

Like Jupiter, Saturn It is a gas giant, which has the shape of a flattened sphere at the poles. It is a very low-density planet (it is less dense than water) and with a low relative gravity, composed mainly of hydrogen (96%) and helium (3%), as well as few traces of methane, water vapor and ammonia. It is unknown whether it has a liquid or rocky core of metallic hydrogen beneath the outer 30,000 kilometers of its atmosphere.

Saturn has multiple natural satellites the oldest are Mimas, Enceladus, Thetis, Dione, Rhea, Titan, Hyperion, Iapetus and Phoebe. These moons are located beyond the ring of materials that orbits the planet, made up of millions of smaller particles that rotate at a speed 15 times faster than that of a bullet.

The name Saturn comes from the titan of Roman mythology, father of Jupiter and the Olympian gods, whom the ancient Greeks called Cronus, and is represented in astronomy and astrology with the symbol ♄.

7. Uranus

Uranus is the fourth most massive planet in the solar system and, although it is observable with the naked eye in the Earth's night sky, it was not discovered until 1781, thus becoming the first planet to be discovered through the use of a telescope. Like Neptune, it has a very different composition from the other two gas giants, which is why these last two planets are often called the “ice giants.”

Its atmosphere is the coldest in the solar system with an average temperature of -224 °C. This atmosphere is composed of hydrogen and helium, but also water vapor, ammonia, methane and traces of hydrocarbons. Besides, The interior of the planet is made up of a multi-layered ice mantle and a heart of frozen rock, but even so it is a very low-density and very low-mass planet compared to the others.

A peculiar detail of Uranus has to do with the arrangement of its poles: since its axis of rotation is so inclined, Its poles are at the height of what the equator should be. Another detail points to its particular coldness, so accentuated that even Neptune, a planet more distant from the sun, radiates greater temperature.

Uranus too It has a ring system comparable to that of Saturn composed of materials of very variable size, from micrometers to almost a meter, organized in 13 concentric rings just a few kilometers thick.

Uranus is named after the early Greek deity who personifies the sky, later called by the Romans Caelus. The astronomical and astrological symbol for this planet is ♅.

8. Neptune

The last planet in the solar system is the distant Neptune, an ice giant whose name comes from the Roman god of the sea, equivalent to the Greek god Poseidon. It was the first planet to be discovered thanks to pure mathematical calculations in 1846 and has a composition very similar to that of Uranus, a planet considered its “twin”. In astronomy and astrology it is represented with the symbol ♆, similar to the trident with which the god of the sea was represented.

Neptune It has a small core of rock covered by a frozen crust, all submerged in a thick, dense atmosphere of clouds of hydrogen, helium, water and methane. The atmosphere is so dense that it reaches pressures almost 100,000 times greater than those experienced on Earth, and its average temperature is -218 °C, receiving very little solar radiation, which points to an internal heat source that is still unknown. accurately.

For the rest, Neptune is a much more dynamic planet than it seems, with an atmosphere full of storms and winds of around 2,200 kilometers per hour, separated into bands of clouds and provided with a blue coloration derived from methane.

Also It has a very tenuous ring system different from those of Uranus and Saturn, and made up of ice particles, silicates and very dark organic compounds. To date, three of these outer rings and a very weak sheet of materials that extend towards the surface of the planet are known. There are also 14 satellites known to date.

Is Pluto a planet?

For a long time, Pluto was considered the last and most distant planet in the solar system, which seems to be evidenced by its name, referring to the Roman god of the underworld, a variant of Hades of the Greeks.

However, as the exploration and study of the solar system yielded more information about astronomical objects, the organization in charge of standardizing astronomical criteria, the International Astronomical Union (IAU), understood that Pluto has more features in common with the other dwarf planets than with ordinary planets.

These features include its small size, its orbit outside the ecliptic (that is, contrary to that of the rest of the planets) and the presence of Charon, an orbit companion of the same size and mass discovered in 1978, in addition to other objects. of smaller size that accompany them on their unusual journey through the solar system. Thus, since August 2006, Pluto entered the list of dwarf planets in the solar system, and was no longer considered an ordinary planet.

Continue with: Origin of the universe

References

• “Solar system” in Wikipedia.
• “Formation and evolution of the solar system” on Wikipedia.
• “Physical characteristics of the components of the solar system” in the Biosphere Project of the Spanish Ministry of Education.
• “The planets of the Solar System and the definition of the International Astronomical Union” in BBVA Open Mind.
• “Solar System” in NASA Science.