Since many years ago, and using a great variety of telescopes, we have been able to observe hundreds of planets orbiting other stars. By observing young stars in different stages of their lives, and deeply studying the structure of our solar system, we have been able to develop a theory to explain the formation of planets.
A gas disc around the star
Stars come from giant gas clouds (named nebulae) that condense to the point where atom fusions get triggered in their centre, which generates an enormous quantity of energy. By orbiting the star, the existent gas ends up flattening, forming a disc around the star. This disc is called a protoplanetary disc.
Apart from hydrogen and helium (the two main elements that form the star) the protoplanetary disc also contains small quantities of other chemical elements, formed and released by previous generations of stars. As the star grows, it absorbs most of the material, accumulating 99% of the system’s mass, leaving the remaining 1% to the disc that surrounds it. It is from this disc that all the planets of the star will appear.
The formation process of planets starts from minute particles of dust and rock that get together, forming bigger fragments of rock. These fragments then collide against others, resulting in ever bigger rocks. Finally will appear what we name planetesimals (bodies that have a diameter above 10 kilometres) that will compete between them to become planets. This process of growth by the aggregation of many bodies is called accretion.
On the other hand, in the outer parts of the disc appear the gaseous planets, generally much bigger than the rocky ones. Even if we don’t know exactly why this type of planets form far away from the star, a theory states that the solar wind, a continuous flow of highly energetic particles emanating from the star, blasts away the lightest elements (such as hydrogen and helium) to the outer orbits. When they reach this part of the system, the gravity of the forming planets is capable of attracting all this gas towards them. The final result is the formation of gaseous giants, such as Jupiter and Saturn in our solar system.
This video made by NASA illustrates this process of planet formation:
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