SOLAR SYSTEM: PLANETS, CHARACTERISTICS, ORIGIN, EVOLUTION - ARTE - 2025

The solar system is a set of planets and astronomical objects linked by the gravitational attraction produced by the single central star: the Sun. Within this planetary system there are a multitude of smaller bodies such as moons, dwarf planets, asteroids, meteoroids, centaurs, comets or cosmic dust.

The solar system is 4568 million years old and located in the Milky Way. If you start counting from the orbit of Pluto, it is calculated that it measures 5,913,520,000 km, the equivalent of 39.5 AU.

Figure 1. The members of the solar system. Source: Wikimedia Commons.

The closest known planetary system is Alpha Centauri, located about 4.37 light years (41.3 billion kilometers) from our Sun. In turn, the closest star would be Proxima Centauri (probably of the Alpha Centauri system), located about 4.22 light years away.

Sun

The Sun is the most massive and largest object in the entire solar system, weighing no less than 2 x 10 ³⁰ kg and a diameter of 1.4 x 10 ⁶ km. A million Earths fit comfortably inside.

Analysis of sunlight shows that this huge sphere is made up mostly of hydrogen and helium, plus 2% of other heavier elements.

Inside it is a fusion reactor, which constantly transforms hydrogen into helium, producing the light and heat that it radiates.

The Sun and the other members of the solar system probably originated at the same time, by the condensation of an original nebula of matter, at least 4.6 billion years ago. The matter in this nebula could well have come from the explosion of one or more supernovae.

Although the Sun is not the largest or most luminous star, it is the most important star for the planet and the solar system. It is a medium-sized star, quite stable and still young, located in one of the spiral arms of the Milky Way. Rather ordinary on the whole, but lucky for life on Earth.

Figure 2. Structure of the Sun. Kelvinsong

With its powerful gravitational force, the Sun makes possible the surprising variety of scenarios in each of the planets of the solar system, since it is the source of its energy through which it maintains the cohesion of its members.

What planets make up the solar system?

Illustration of the solar system; shows the Sun, inner planets, asteroid belt, outer planets, Pluto, and a comet. This image is not to scale.

There are 8 planets in the solar system, classified into inner planets and outer planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Inner planets

The inner planets are Mercury, Venus, Earth, and Mars. They are small, rocky planets, while outer planets like Jupiter are gas giants. This difference in density has its origin in the way the matter in the original nebula condensed. The farther from the Sun, the temperature decreases and, therefore, matter could form different compounds.

In the vicinity of the Sun, where the temperature was higher, only heavy elements and compounds such as metals and silicates were able to slowly condense and form solid particles. Thus arose the dense planets: Mercury, Venus, Earth and Mars.

Outer planets

The outer planets are Jupiter, Saturn, Uranus, and Neptune. They formed in more remote regions, in which matter quickly condensed into ice. The rapid growth of these ice accumulations resulted in objects of enormous size. However, inside these gigantic planets are not frozen, in fact they still radiate a large amount of heat into space.

The border between the inner and outer planets is the Asteroid Belt, remnants of a planet that failed to form because of the massive gravitational pull of Jupiter, which dispersed them.

Is Pluto a planet in the solar system?

For a long time Pluto was considered a planet until 2006, when astronomers designated it as a dwarf planet because it lacks orbital dominance, one of the characteristics that a celestial body must have to be considered a planet.

This means that other bodies of a similar size and with similar gravity should not exist in its environment. This is not the case with Pluto, whose size is similar to that of its moon Charon and very close to each other.

Main characteristics of the planets

The planets orbit the Sun following elliptical orbits, according to Kepler's laws. These orbits are all approximately in the same plane, which is the plane of the ecliptic, on which the movement of the Earth around the Sun passes.

Figure 3. Orbit of the planets of the solar system

In fact, almost all the objects of the solar system are in this plane, with small differences, except Pluto, whose orbital plane is inclined 17º with respect to the ecliptic.

- Mercury

Figure 5. Mercury. Source: NASA.

It is a small planet, barely larger than a third of the Earth and the closest to the Sun. On its surface there are rock formations similar to those of the Moon, as seen in the images. Typical are lobed escarpments, which astronomers say are an indication that Mercury is shrinking.

It also has other characteristics in common with our satellite, for example the chemical composition, the presence of ice at the poles and a large number of impact craters.

Figure 4. Caloris Plain, one of the largest impact surfaces in the Solar System. In the antipodes there is a mountain range that was probably formed by the shock waves of the impact. Source: NASA via solarsystem.nasa.

Mercury is occasionally visible from Earth, very low above the horizon, just at sunset or very early, before sunrise.

This small planet has coupled its rotational and translational motion around the Sun, thanks to the so-called tidal forces. These forces tend to decrease the speed of rotation of the planet around its axis, until they equal the speed of translation.

Such couplings are not uncommon between objects in the solar system. For example, the Moon has a similar movement and always shows the same face to the Earth, like Pluto and its satellite Charon.

Tidal coupling is responsible for Mercury's extreme temperatures, along with the planet's thin atmosphere.

The face of Mercury exposed to the Sun has scorching temperatures, but it is not the hottest planet in the solar system, even if it is the closest to the sun king. That distinction is for Venus, whose surface is covered in a dense blanket of clouds that traps heat inside.

Table 1. Mercury: characteristics and movement

- Venus

Figure 6. Venus. Source: Wikimedia Commons.

In size, mass, and chemical composition, Venus is very similar to Earth, yet its dense atmosphere prevents heat from escaping. This is the famous greenhouse effect, which is responsible for the surface temperature of Venus reaching 400 ºC, close to the melting point of lead.

The Venusian atmosphere is composed mainly of carbon dioxide and traces of other gases such as oxygen. The atmospheric pressure is about 100 times greater than the terrestrial one and the distribution of the fast winds is extremely complex.

Another detail of the remarkable atmosphere of Venus is its rotation around the planet, which takes about 4 Earth days. Note that the rotation of the planet itself is extremely slow: a Venusian day lasts 243 Earth days.

Deuterium is abundant on Venus, an isotope of hydrogen that is due to the lack of a protective ozone layer against ultraviolet rays from the Sun. There is no evidence of water at present, however, so much deuterium indicates that Venus could have it in the past.

As for the surface as such, radar maps show landforms such as mountains, plains and craters, in which basalt is abundant.

Volcanism is characteristic on Venus, as is slow retrograde rotation. Only Venus and Uranus rotate in the opposite direction to the other planets.

The hypothesis is that it is due to a past collision with another celestial object, but another possibility is that the atmospheric tides caused by the Sun slowly modify the rotation. Possibly both causes have contributed equally to the movement that the planet now has.

Table 2. Venus: characteristics and movement

- The earth

Figure 7. Earth seen from space.

The third planet closest to the Sun is the only one that harbors life, at least as far as we know.

The Earth is at an ideal distance for life to proliferate and also has a protective ozone layer, abundant liquid water (up to 75% of the surface is covered by this element) and an intense magnetic field of its own. Its rotation is also the fastest of the four rocky planets.

Earth's atmosphere is made up of nitrogen and oxygen, with traces of other gases. It is stratified, but its limits are not defined: it progressively thins until it disappears.

Another important characteristic of the Earth is that it has plate tectonics, so its surface undergoes continuous changes (in geological times of course). Hence, the evidence of craters that abound in the other planets of the solar system have already been erased.

This provides the Earth with a wide variety of environmental settings: mountains, plains and deserts, along with the abundance of water, both in the vast oceans and in fresh water on the surface and underground.

Together with the Moon, its natural satellite, it forms a remarkable duo. The size of our satellite is relatively large compared to that of the Earth and exerts a remarkable influence on it.

To begin with, the Moon is responsible for the tides, which exert a powerful influence on life on earth. The Moon is in synchronous rotation with our planet: its periods of rotation and translation around the Earth are the same, that is why it always shows us the same face.

Table 3. The Earth: characteristics and movement

- Mars

Figure 8. The red planet. Source: Wikimedia Commons.

Mars is slightly smaller than Earth and Venus, but larger than Mercury. Its surface density is also somewhat lower. Very similar to Earth, the curious always believed they saw signs of intelligent life in the reddish star.

For example, since the mid-nineteenth century many observers claimed to have seen "channels", straight lines that crossed the Martian surface and that they attributed to the presence of intelligent life. Maps of these alleged channels were even created.

However, the images from the Mariner probe demonstrated in the mid-sixties of the 20th century that the Martian surface is desert and that the channels were non-existent.

The reddish color of Mars is due to the abundance of iron oxides on the surface. As for its atmosphere, it is thin and consists of 95% carbon dioxide, with traces of other elements such as argon. There is no water vapor or oxygen. The latter is found forming compounds in rocks.

Unlike Earth, Mars does not have its own magnetic field, so particles from the solar wind strike directly on the surface little protected by the thin atmosphere.

As for the orography, it is varied and there are indications that the planet once had liquid water. One of the most notable features is Mount Olympus, the largest known volcano in the Solar System so far.

Mount Olympus far surpasses the largest volcanoes on Earth: it is three times the height of Mount Everest and 100 times the volume of Mauna Loa, the largest volcano on Earth. Without tectonic activity and with low gravity, the lava could accumulate to give rise to such a colossal structure.

Table 4. Mars: characteristics and movement

- Jupiter

Figure 9. Jupiter and the Galilean moons.

It is undoubtedly the king of the planets due to its large size: its diameter is 11 times greater than that of Earth and its conditions are much more extreme.

It has a rich atmosphere crossed by fast winds. Jupiter's well-known Great Red Spot is a long-standing storm, with winds of up to 600 km / h.

Jupiter is gaseous, therefore there is no solid ground below the atmosphere. What happens is that the atmosphere becomes denser as depth increases, until it reaches a point where the gas is liquefied. Hence it is quite flattened at the poles, due to rotation.

Despite the fact that most of the matter that makes up Jupiter is hydrogen and helium -like the Sun-, in its interior it has a nucleus of heavy elements at a high temperature. In fact, the gas giant is a source of infrared radiation, which is why astronomers know that the inside is much hotter than the outside.

Jupiter also has its own magnetic field, 14 times stronger than Earth's. A notable feature of that planet is the large number of natural satellites it has.

Due to its enormous size, it is natural that its gravity could have captured many rocky bodies that happened to pass through its surroundings. But it also has large moons, the most notable of which are the four Galilean moons: Io, Europa, Callisto and Ganymede, the latter being the largest of the moons in the solar system.

These large moons probably originated at the same time as Jupiter. In their own right, they are fascinating worlds, as there is the presence of water, volcanism, extreme weather, and magnetism, among other characteristics.

Table 5. Jupiter: characteristics and movement

- Saturn

Figure 10. Image of Saturn

Undoubtedly, what most draws the attention of Saturn is its complex ring system, discovered by Galileo in 1609. It should also be noted that Christian Huygens was the first to notice the annular structure, a few years later, in 1659. Surely Galileo's telescope did not have sufficient resolution.

Millions of ice particles make up Saturn's rings, perhaps remnants of ancient moons and comets that impacted the planet - Saturn has nearly as many as Jupiter.

Some satellites of Saturn, called shepherd satellites, are in charge of keeping the orbit free and confine the rings in well-defined regions of the planetary equatorial plane. The planet's equator is quite pronounced, being a very flattened spheroid due to its low density and rotational movement.

Saturn is so light, it could float in a hypothetical ocean big enough to contain it. Another reason for the deformation of the planet is that the rotation is not constant, but dependent on latitude and other interactions with its satellites.

Regarding its internal structure, the data collected by the Voyager, Cassini and Ulysses missions assure that it is quite similar to that of Jupiter, that is, a gaseous mantle and a nucleus of very hot heavy elements.

Temperature and pressure conditions make it possible for metallic liquid hydrogen to form, which is why the planet has its own magnetic field.

Towards the surface, the weather is extreme: storms abound, although not as persistent as those of neighboring Jupiter.

Table 6. Saturn: characteristics and movement

- Uranus

Figure 11. View of the frozen planet Uranus. Source: Pixabay.com

It was discovered by William Herschel in 1781, who described it as a small greenish-blue dot on his telescope. At first he thought it was a comet, but soon after he and other astronomers realized it was a planet, just like Saturn and Jupiter.

The movement of Uranus is quite peculiar, being e retrograde rotation, like Venus. Furthermore, the axis of rotation is very inclined with respect to the plane of the orbit: 97.9º, so it practically rotates sideways.

So the planet's seasons - revealed through Voyager images - are quite extreme, with winters lasting 21 years.

The blue-green color of Uranus is due to the methane content of its atmosphere, much cooler than that of Saturn or Jupiter. But about its internal structure, little is known. Both Uranus and Neptune are considered ice worlds, or rather gaseous or quasi-liquid.

Although Uranus does not produce metallic hydrogen due to its lower mass and pressure inside, it does have an intense magnetic field, more or less comparable to Earth's.

Uranus has its own ring system, although not as magnificent as Saturn's. They are very faint and therefore are not easily seen from Earth. They were discovered in 1977, thanks to the temporary occultation of the planet by a star, which allowed astronomers to see its structure for the first time.

Like all outer planets, Uranus has many moons. The main ones are Oberon, Titania, Umbriel, Ariel and Miranda, names taken from the works of Alexander Pope and William Shakespeare. Frozen water has been detected on these moons.

Table 7. Uranus: characteristics and movement

- Neptune

Figure 12. Image of Neptune taken by the Voyager 2 probe. Source: Wikimedia Commons.

At the edge of the solar system is Neptune, the planet furthest from the Sun. It was discovered due to unexplained gravitational disturbances, which suggested the existence of a large yet undiscovered object.

The calculations of the French astronomer Urbain Jean Leverrier finally led to the discovery of Neptune in 1846, although Galileo had already sighted it with his telescope, believing it a star.

Seen from Earth, Neptune is a small greenish-blue dot and until not long ago, very little was known about its structure. The Voyager mission provided new data in the late 1980s.

The images showed a surface with evidence of strong storms and fast winds, including a large Jupiter-like patch: the Great Dark Spot.

Neptune has an atmosphere rich in methane, as well as a faint ring system, similar to those of Uranus. Its internal structure is composed of a crust of ice that covers the metallic nucleus and has its own magnetism.

As for the moons, about 15 have been discovered to date, but there could be a few others, since the planet is very distant and is the least studied yet. Triton and Nereid are the main ones, with Triton in retrograde orbit and possessing a tenuous nitrogen atmosphere.

Table 8. Neptune: characteristics and movement

Other astronomical objects

The Sun and the large planets are the largest members of the solar system, but there are other objects, smaller but equally fascinating.

We talk about dwarf planets, moons or satellites of the major planets, comets, asteroids and meteoroids. Each one has extremely interesting peculiarities.

Tiny planets

Figure 13. Pluto. Source: Pixabay.com

In the asteroid belt that is between Mars and Jupiter, and beyond the orbit of Neptune, in the Kuiper belt, there are many objects that according to astronomical criteria, do not fall into the category of planets.

The most prominent are:

- Ceres, in the asteroid belt.

- Pluto, which was previously considered the ninth largest planet.

- Eris, discovered in 2003 and larger than Pluto and further from the Sun than it is.

- Makemake, in the Kuiper belt and about half the size of Pluto.

- Haumea, also in the Kuiper belt. It is markedly ellipsoidal in shape and has rings.

The criteria to distinguish them from the larger planets is both their size and the gravitational attraction they possess, linked to their mass. To be considered a planet, an object must rotate around the Sun, in addition to being more or less spherical.

And its gravity has to be high enough to absorb the other smaller bodies around it, either as satellites or as part of the planet.

Since at least the gravitational criterion is not fulfilled for Ceres, Pluto and Eris, this new category was created for them, to which Pluto ended up in 2006. In the distant Kuiper belt it is possible that there are more dwarf planets like these, not yet detected.

Moons

As we have seen, the major planets, and even Pluto, have satellites that orbit around them. There are more than a hundred belonging to the major planets, almost all of them distributed in the outer planets and three belonging to the inner planets: the Moon from Earth, and Phobos and Deimos from Mars.

Figure 14. Earth's moon. Source: Pixabay.com

There may still be more moons to discover, especially on planets farthest from the Sun, such as Neptune and other icy giants.

Their shapes are varied, some are spheroidal and others quite irregular. The largest ones probably formed next to the parent planet, but others could be captured by gravity. There are even temporary moons, which for some reason are captured by the planet but are eventually released.

Other bodies, in addition to the major planets, also have moons. It is estimated that so far there are about 400 natural satellites of all kinds.

Kites

Figure 15. Halley's Comet.

Comets are debris from the cloud of matter that gave rise to the solar system. They are made up of ice, rocks and dust and are currently found on the outskirts of the solar system, although they do come close to the Sun from time to time.

There are three regions that are very far from the Sun, but still belong to the solar system. Astronomers believe that all comets dwell there: the Kuiper belt, the Oort cloud, and the scattered disk.

Asteroids, centaurs and meteoroids

Asteroids are rocky bodies smaller than a dwarf planet or satellite. Almost all are found in the asteroid belt that marks the boundary on rocky and gaseous planets.

For their part, centaurs receive this name because they share characteristics of asteroids and comets, like the mythological beings of the same name: half humans and half horses.

Discovered in 1977, they have not been properly photographed yet, but they are known to be abundant between the orbits of Jupiter and Neptune.

Finally, a meteoroid is a fragment of a larger object, such as those described so far. They can be as tiny as a wisp of matter - without being as small as a grain of dust - about 100 microns or as large as 50 km in diameter.

Summary of the main characteristics of the solar system

- Estimated age: 4.6 billion years.

- Shape: disk

- Location: Orion's arm in the Milky Way.

- Extension: it is relative, it can be considered to be about 10,000 astronomical units *, up to the center of the Oort cloud.

- Types of planets: terrestrial (rocky) and Jovian (gaseous and icy)

- Other objects: satellites, dwarf planets, asteroids.

* An astronomical unit equals 150 million kilometers.

Figure 16. Scale of the solar system in astronomical units. Source: NASA.

Origin and evolution

Currently, most scientists believe that the origin of the solar system is in the remains of one or more supernovae, from which a gigantic nebula of cosmic gas and dust was formed.

Gravity was in charge of agglomerating and collapsing this matter, which in this way began to rotate faster and faster and to form a disk, in the center of which the Sun was formed. This process is called accretion.

Around the Sun remained the disk of remaining matter, from which over time the planets and other members of the solar system emerged.

From the observation of forming star systems in our own Milky Way galaxy and from computer simulations, scientists have evidence that such processes are relatively common. Newly formed stars often have these disks of matter around them.

This theory quite well explains most of the findings made about our solar system, being a single central star system. However, it would not fully explain the formation of planets in binary systems. And there are, since it is estimated that 50% of exoplanets belong to systems with two stars, being very common in the galaxy.

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