The formation of the Solar System
Clouds of gas and dust as source material
Clouds of gas and dust are evident between the stars in interstellar space. Astronomers call them nebulae. They consist mainly of hydrogen and helium and represent the starting material for new stars and planets. The disintegration of a star also leads to the formation of a nebula. You can read more about this under Stellar evolution. The nearest nebula from Earth is the Helix Nebula. It is 700 light years away from us and was formed when a star disintegrated. The remnant of a white dwarf can be seen in its interior.
A protoplanetary disk is formed
In some places, the dust and gas cloud is compressed by turbulence and collapses under its own gravity. Stars, such as our Sun, are formed from the contracted matter.
If the cloud rotated even minimally at the beginning, this rotation would be accelerated more and more during the contraction. The reason for this is a physical law that states that the angular momentum of a system is conserved. We are familiar with this effect from figure skaters. When performing a pirouette, they initially rotate slowly with their arms outstretched. However, if you bring your arms closer to your body, the rotation accelerates. The greater the difference between the initial and final spans, the greater the effect.
The same principle means that the rotation of a gas cloud accelerates when it contracts. The angular momentum of the huge cloud is transferred to the comparatively small star. A newly formed sun rotates so quickly that part of its mass is pushed outwards by centrifugal force, forming a disc perpendicular to the sun's axis of rotation. The ejection of mass finally slows down the rapidly rotating sun. A disk of gas known as a protoplanetary disk has formed around the still young star.
From a speck of dust to a planet
The material of the protoplanetary disk cools down over time. Individual elements condense slowly, the most heat-resistant elements near the Sun, the more volatile elements only at a great distance from the Sun. This changes the chemical composition of the disk from the inside to the outside.
More and more condensed dust particles are gradually populating the protoplanetary disk. The dust grains clump together during this to form larger dust agglomerates (a process that also takes place behind the sofa in many people's homes).
These dust clusters eventually form bodies with diameters of several kilometres called planetesimals. Our understanding of the planetary blueprint is still incomplete here. It is not yet known exactly how these planetesimals can join together.
Collisions with other chunks cause the planetesimals to grow, as their gravitational force holds the collision partners together. Solid objects the size of planets gradually develop. As the protoplanetary disk becomes thicker towards the outside, planets are formed in the outer regions that are larger than those in the vicinity of the Sun. Once an object has reached the critical weight of around 10 Earth masses, it also begins to accrete the surrounding gas of the disk. A gas giant eventually forms.
This scenario provides us with a rough picture of our Solar System today, with four small terrestrial planets in the inner region and four large gas giants in the outer region orbiting the Sun in almost circular orbits and perpendicular to the axis of rotation of their central star.
