How Was the Solar System Formed? The Origin

Imagine that space is a huge workshop where a talented sculptor, Gravity, creates his majestic masterpieces. From cosmic gas and dust, as if from magic clay, it moulds stars and planets, giving birth to order from chaos, and light from darkness. How was the Sun made? How was the Solar System formed? Today we will look into this unique workshop to reveal the secrets of the mastery of the Great Architect of everything that exists.

Nebular Hypothesis: Unravelling What Was Before the Solar System

How did the Solar System form? Before answering this question, it would be logical to find out what was before it. And here the nebular hypothesis comes to the rescue, which dates back to Immanuel Kant and Pierre-Simon Laplace, and which is adhered to by most scientists today. 

So, before our Sun lit up and the planets took their orbits around it, there was a giant interstellar molecular cloud in their place. In simple words, a huge nebula of gas and dust drifting between the stars in the galaxy. It consisted mainly of hydrogen and helium left over from the Big Bang, as well as heavier elements that were ejected into space by supernova explosions – carbon, oxygen, iron, and microscopic particles of dust and ice. The cloud was cold and rarefied, remaining in a state of relative peace until gravitational collapse occurred 4.6 billion years ago, which caused the formation and evolution of the Solar System.

What role did gravity play in the formation of the Solar System? 

Gravity was the chief engineer and architect in the Solar System formation. It not only gave the first push but also continued to tirelessly “sculpt” the appearance of the entire system.

Under the influence of gravity, possibly from the shock wave of a nearby supernova explosion, the interstellar molecular cloud began to collapse, gradually forming into the Sun and planets surrounding it.

How Did the Sun Form?

During the compression process, most of the interstellar substance gathered in the centre, forming a clot — a protostar. The protostar continued to grow and heat up, attracting more and more dust and gas. When the temperature in the core of the protostar reached an incredible 10 million degrees Celsius, a thermonuclear reaction of hydrogen conversion into helium was launched. Hydrogen atoms began to merge, turning into helium, and releasing a huge amount of energy. This energy created a powerful pressure that balanced the gravitational compression of the protostar. Thus, from the “embryo” a full-fledged star was born – our Sun.

How Did Earth Get in the Solar System?

Our Earth was born within a protoplanetary disk, the remaining part of the nebula that surrounded the young Sun. In this disk, gas and dust collided and stuck together, gradually forming ever larger objects called planetesimals.

 Planetesimals continued to collide and combine into protoplanets, eventually forming the planets. The inner part of the disk, closer to the Sun, was hotter, so planets made mostly of rock and metal formed there — Mercury, Venus, Mars, and our Earth. Then, in the outer, cooler part of the disk, planets formed that were able to attract large amounts of gas — the gas giants.

Problems of Nebular Hypothesis

Despite all its harmony, the nebular hypothesis cannot explain several oddities of our solar system. Imagine that this is a story with some puzzle pieces missing.

For example, angular momentum. The sun should spin very fast, like a top, since it collected 99% of all the substance from the original cloud. But in reality, it rotates very slowly (the rotation period is about 25 days at the equator). But the planets, especially the large ones (Jupiter and Saturn), having less than 1% of the mass, carry more than 99% of the solar system’s angular momentum, as if they had absorbed all the rotational energy.

There is one more inconsistency — how tiny dust particles were turned into large rocks, and then planets. The theory says that they stuck together and gained mass (accreted) under the influence of gravity, but scientists are still not sure about this. After all, when dust particles collided with each other, they should have repelled rather than stuck together.

The gas giants, Jupiter and Saturn, are also a mystery. To become so huge, they had to accumulate a huge amount of gas very quickly. However, the young Sun actively emitted a solar wind, which scattered the remaining gas from the protoplanetary disk. It turns out that the gas giants had very little time to fill up with gas before the Sun scattered it. But how did they manage to do it?

And finally, some planets behave strangely. Instead of rotating around its axis with the Sun, Venus rotates in the opposite direction, and Uranus “lying on its side.” This says that after the formation of the system, something greatly disturbed them, perhaps collisions with other large celestial bodies.

All these oddities are like black holes in the nebular hypothesis. They show that the story of the birth of our planetary system is not yet complete, and scientists continue searching for the answers.

Alternative Theories of How the Solar System Was Formed 

Planetesimal Hypothesis was proposed by Forrest R. Moulton and Thomas C. Chamberlin in 1900. The theory was widely accepted for 35 years before being debunked. Credit: Getty Images

Over the past 125 years, many other theories have appeared to explain the peculiarities of Solar System forming. Looking ahead, we will say that almost all of them were rejected as less valid than the nebular hypothesis.

Planetesimal Model of Chamberlin-Moulton

This model of Solar System forming, developed by the American geologist Thomas Chamberlin and astronomer Forest Moulton in 1900, became one of the first serious alternatives to the nebular hypothesis. It assumed that another star passed by the single Sun at a close distance. Its gravity caused powerful tidal forces, which led to the ejection of solar substances in the form of giant prominences. Cooling, this substance condensed into many small solid particles — planetesimals. Then, due to collisions and gravitational attraction, planetesimals gradually accreted, forming increasingly larger bodies, up to planets.

However, the Chamberlin-Moulton theory turned out to be untenable for several reasons. First, the probability of such a close passage of the star is minuscule. Second, calculations showed that tidal forces could not have ejected enough substance from the Sun to form all the planets, especially the giants. Third, the ejected hot substance would have dispersed into space rather than condensed into planetesimals. Finally, the accretion mechanism based on random collisions turned out to be insufficiently effective — collisions would more often lead to fragmentation than to merging.

Tidal Hypothesis of Jeans-Jeffreys

In 1917, British astrophysicist James Jeans proposed his tidal theory, which, although similar to the Chamberlin-Moulton theory, had an important difference. Jeans also proceeded from the assumption of a close passage of a star past a single Sun. But, in his opinion, the star’s gravity did not cause individual emissions, but pulled a single, cigar-shaped “sleeve” of hot substance from the Sun. This “sleeve”, being gravitationally unstable, disintegrated into separate clots — protoplanets, which then became planets.

Jeans’ tidal theory, despite the refinements made to it by another British scientist Harold Jeffreys in 1929, unfortunately, also encountered insurmountable difficulties. As in the case of the planetesimal theory, the probability of such a close passage of a star is negligibly small. Calculations showed that the mass of the elongated substance would not be enough to form giant planets. In addition, the hot “sleeve” would have a tendency to disperse, rather than to compress into clumps. And finally, the mechanism by which the “sleeve” could split into separate, gravitationally bound protoplanets remained unclear.

Littleton scenario

In 1936, British astronomer and mathematician Raymond Arthur Lyttleton proposed a modification of the tidal theory, attempting to solve the problem of the lack of mass that the Jeans and Chamberlain-Moulton theories faced. His scenario introduced the idea of ​​a binary system, where the Sun had a companion star. It was assumed that a passing star collided with this companion, resulting in the formation of a debris cloud, from which the planets then formed.

A controversial point of this hypothesis was the stability of the planetary orbits after such a collision. The orbits should have been highly elliptical and unstable, which contradicts observations. Finally, the chemical composition of the planets, and in particular the isotopic composition of the substance, indicates that the planets formed from a single cloud, which is inconsistent with the Lyttleton scenario. If they formed from material of a collision, it is difficult to explain the differences in the compositions of the terrestrial planets and the giant planets.

Theory of capture

In contrast to the “catastrophic” scenarios, this theory of planets origin suggested that the Sun captured already formed planets from a passing star. However, this poorly explaines how captured planets could end up in almost circular, coplanar orbits. With random capture, the orbits of the planets would be much more chaotic and varied in inclination.

Nice model

The Nice Model, named after the place where it was originally developed at the French Côte d’Azur Observatory (Nice, 2005), describes the evolution of the Solar System from the time when the giant planets were located on the orbits, closer to the Sun. Beyond the orbits of the giants was a vast disk of planetesimals — icy and rocky bodies. The gravitational interaction of the giant planets with these planetesimals led to their gradual, slow migration. The key moment was the entry of Jupiter and Saturn into a 2:1 orbital resonance. This caused a sharp destabilization of the system: the orbits of the giant planets underwent significant changes, Neptune was thrown to the outer edge of the Solar System, and many planetesimals rushed into the inner region, causing an intense bombardment of the terrestrial planets — the so-called Late Heavy Bombardment. After this stage of dynamic instability, our system acquired its current configuration. Some later versions of the model suggest that the system initially contained a fifth giant planet, which was ejected from it as a result of gravitational interactions.

Overall, the Nice model is a working model that is constantly evolving and being refined. It is not without inaccuracies, but today it is the most complete and consistent explanation of many features of the Solar System. Research continues, and perhaps in the future new models will appear that will describe the evolution of our cosmic home even better.

How Do We Know How the Solar System Was Formed?

Now, when you know all the main versions of the Solar System formation, it’s time to find out how scientists managed to look so far into the past? Don’t worry, there is no deception, they have a whole arsenal of methods at their disposal that allow them to recreate the picture of our cosmic home birth.

Observation of young stars

Modern space telescopes James Webb, Hubble, Spitzer, as well as ground-based observatories, allow astronomers to observe the processes occurring around young stars. Looking at the protoplanetary disks surrounding these stars, you can see how planets are gradually formed from clouds of gas and dust. It’s like watching the construction of a new house — you can see how the entire structure is assembled from separate materials.

Study of meteorites

Meteorites are fragments of asteroids and comets that have reached the Earth. They have preserved the substance from which the planets were formed in the earliest Solar System. Chemical composition analysis of meteorites is the key to finding the answer to the question of Solar System formation, namely, what substances were prevalent at that time and how they were distributed. It is similar to studying ancient fossils — they give an idea of ​​life form, that existed in the distant past.

Space missions

Automatic interplanetary stations such as Voyager, Cassini, Rosetta and others collect a huge amount of data from planets and satellites. They take photos of the surface, measure magnetic fields, analyse the composition of the atmosphere and soil. Each mission is like an expedition to uncharted lands, which brings new knowledge about how other worlds are arranged.

Computer modelling

Using powerful computers, scientists create models that imitate the processes that occurred in the early Solar System. These models allow testing different hypotheses and see how various factors could have influenced the formation of planets. It is like creating a virtual experiment — you can change the parameters and see how this affects the result.

An eternal masterpiece of perfection

So, our tour of the Gravity workshop has come to an end. We hope it was not in vain for you. From the chaos of the Universe, the Great Sculptor created an eternal masterpiece of perfection and we, woven from the same dust as the stars, are an integral part of it. But the workshop is huge, and secrets remain. New tools and methods will certainly shed light on the secrets of Solar System formation and reveal even more wonders. In the meantime, let us once again look back into eternity and enjoy the grandeur of the process.

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