How did the Solar System form?

Billions of years ago, gravity caused a large cloud of gas and dust to collapse into its own structure. The result of this collapse is the Solar System we know today, made up of our star, eight planets, many moons, and a multitude of objects that roam the neighborhood we call home in the Milky Way. But, after all, how exactly did the Solar System form?

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In addition to the other seven planets that orbit the Sun, our “space backyard ” also has dozens of moons, several dwarf planets, millions of asteroids, comets and meteoroids. So the Solar System goes far beyond our neighboring planets—for example, beyond Neptune’s orbit is the Kuiper Belt, a donut-shaped region that houses frozen objects, and even Pluto, the most famous dwarf planet in this mysterious location.

Beyond the Kuiper Belt is the Oort Cloud, a large cloud of icy debris that surrounds the Solar System (Image: Reproduction/M. Kornmesser/ESO)

Amidst several questions about the origins of the Solar System and what is in it, is its formation. Thanks to radiometric dating, the method that determines the rate of radioactive decay of elements, astronomers have been able to determine that the Earth and the rest of our system are approximately 4.6 billion years old. This number does not come from the rocks of our planet, but from meteorites, fragments of ancient objects from the early period of the Solar System.

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Although many ideas in astronomy have gone through radical changes over time, the explanation of how our neighborhood formed has had relatively little change over the past two centuries—and it came up with a Swedish scientist, in the century 18. In 1734, scientist and philosopher Emanuel Swedenborg proposed that the Solar System came from a large cloud of gas and dust. This idea was reused by the German philosopher Immanuel Kant, who presented a theory that a large nebula of matter, called the “solar nebula”, was part of an even larger cloud of gas and dust.

This cloud would have collapsed onto its own structure and started a rotational movement. Then it was the turn of French mathematician and astronomer Pierre-Simon Laplace to come up with his ideas, which suggested that the Sun had a large, hot atmosphere that stretched across the Solar System. This “protostellar cloud” cooled and contracted, initiating a rotating motion that expelled material that gave rise to the planets we know. Today, the understanding of this process has already expanded, but the main one remains.

How the Solar System Formed

Currently, astronomers consider that when the molecular cloud began to collapse, it measured about 50 astronomical units (each unit is equivalent to the average distance between the Sun and Earth, approximately 149 million kilometers), and which was up to three times the mass of the Sun. It’s still unclear whether a nearby supernova explosion or some other event triggered this collapse, but, however, as it “collapsed” into its structure, there were other processes going on that helped to accelerate the cloud’s collapse. As a result, her temperature rose and she began a rotating movement.

Representation of the Solar System during the beginning of its formation, with particles that gave rise to planetesimals and, eventually, to planets (Image: Reproduction/NASA/JPL-Caltech )

This movement distributed the material on a relatively flat disc, and the gravitational potential energy was turning into heat. With that, the density increased and the disk started to spin even faster while decreasing in size; as more and more pockets of gas and dust collided with each other and held together, a protoplanetary disk formed. But it was at the center of it that all the action took place: there, the “baby” protostar that became the Sun began to accumulate matter. It took almost 50 millions of years for our star to get enough matter to start the Nuclear fusion.

There, near the newly formed Sun, the temperature was high enough to allow the formation of minerals and metals; on the other side of the disk and further away from the solar heat, less volatile compounds formed. As the disk cooled, the solids gathered and formed large blocks of mass, which little by little picked up all the dust that was left. This is how rocky planets with metallic cores were born, which come from the hot rocky material near the center of the system. In the colder areas, the gas giants and icy were born.

After the sun “lit”, it released a strong solar wind that helped to carry smaller and smaller debris away from the disk. dust particles. It was at this stage that the gas giant planets stopped “growing” into larger bodies. Because they are massive, they helped “clean up” the disk, helping to assemble fragments in other regions, such as the Oort Cloud and the Asteroid Belt between Mars and Jupiter.

The Asteroid Belt has thousands of irregular objects (Image: Playback/Daniel Roberts/Pixabay)

The rocks that managed to escape are scattered throughout the Solar System, and they travel aimlessly like asteroids that, as they preserved the conditions of that period, they are excellent objects of study for astronomers to better understand the formation of our neighborhood. In the end, the rocky planets went through a violent period, in which countless objects bombarded the surface of these worlds – today, we believe that this process gave rise to the Moon and several other natural satellites of other planets, such as those of Mars.

Questions still unanswered

The solar nebula theory is widely accepted among the scientific community, but even so it has some questions that astronomers have yet to answer. For example, all the planets around a star should have the same inclination with respect to the ecliptic, but we know that the inner and outer planets of the Solar System have quite different axes of inclination. In addition, it works well to describe the formation of rocky worlds, but it leaves doubts regarding the gaseous ones.

It’s just that these massive planets take a few million years to form, and that time is greater than the amount of light gases available in the early Solar System — not to mention the migration of our neighbors who, when they were “babies”, should have spiraled towards the Sun. A relatively new model called “disk instability” , suggests that pockets of gas and dust came together in the early Solar System, and would form giant planets in about a thousand years. By retaining more gases and reaching a certain mass, they would have achieved greater orbital stability.

For a long time, scientists considered that planets formed where they are today, but that changed with the discovery of exoplanets, which showed that even massive objects can migrate (Image: Reproduction/NASA/FUSE/Lynette Cook)

To top it off, scientists have noticed some irregularities in the theory when studying exoplanets — and some of them are related to the existence of so-called “hot Jupiters”, nickname given to gas giant planets that orbit their stars in periods of just a few days. Thus, astronomers have been making small adjustments to the theory to thus solve some of these problems, even with many unanswered questions.

Finally, the Earth also continues to keep secrets: the evolution of the Solar System did not end after the planets were formed, and the Earth stands out compared to other rocky planets due to the large amount of water that exists around here. However, our planet is in a location where the temperature would be too high to collect water in the early Solar System, which suggests that the water may have come here in some other way—suddenly brought in by colliding comets, for example. .

Source: Universe Today,, Astronomy, NASA, NHM

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