From Jupiter to Saturn, to Uranus, and finally Neptune, our outer Solar System comprises four fascinatingly very different yet very similar planets: the gas giants, also known as the Jovian planets. But why Jovian? As stated, all four planets are very similar in composition. Therefore, the term Jovian referring to the God Jupiter was intended to indicate that all four planets were very similar to Jupiter. Furthermore, as indicated by their name, these four planets are made of gas, unlike the inner planets of the Solar System known as “rocky planets.” However, how and why did these four gas giants form in the outer Solar System? These will be the guiding questions of this article.

Ever Wondered Why Do Gas Giants Occupy The Outer Solar System? 1

How did the gas giants form?

The formation of the gas giants is said to lie around one main principle: the core accretion mechanism. This states that large planets gradually pull in gas from their atmosphere to form huge gas environments eventually. Indeed, planets larger than the size of the Moon can capture an atmosphere. The process can be compared to a vicious circle for a while, capturing part of its atmosphere, the said planet becomes larger and is, therefore, able to capture more of its atmosphere. Once the planet becomes big enough, the absorbed atmosphere grows rapidly and becomes larger than the solid part of the planet. That’s why the gaseous planets still have rocky cores that are significantly smaller than their gaseous “crust.” 

But more specifically, why did the core accretion mechanism lead them to form in the outer areas of the Solar System? The answer is that these gaseous planets all began just like our own planet: big rocky masses, which later turned into gas giants (after the early stages of formation of the Solar System).

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Firstly, the outer area is much colder than the inner Solar System. About 5 AU from the Sun (5 times the distance between the Earth and the Sun), lies an imaginary boundary called the frost line. Beyond this point, the Sun’s energy is weak enough that ices can form from substances like ammonia and methane. Therefore, the water accumulated by the four outer planets could freeze much faster, forming much bigger rocky and solid cores than the inner planets. 

Ever Wondered Why Do Gas Giants Occupy The Outer Solar System? 2
The concept of frost line | Image: David Butler

Then came the accumulation of gas which explains itself through the process in which the Sun formed: a nebula, or cloud of gas, collapsed under its own gravity. However, while our Sun was still a cloud of gas, the outer planets with large solid cores and, therefore, strong gravitational pull attracted hydrogen and helium from the nebula before the sun actually formed. Thereby, the large amounts of gas they attracted now counts for most of the surface of their planets. 

The very specific order of our gas giants

Although all four gas giants followed roughly the same phenomenon, they do not all look alike because of their location and arrangement in the outer Solar System. Indeed, Jupiter and Saturn, closer to the Sun, accumulated more gases (and look more alike). On the other hand, Uranus and Neptune, the last two planets, had smaller cores and were also further away from the Sun. All in all, this meant that it was more difficult for them to attract the gas from our Sun’s nebula as efficiently, explaining why they are smaller but also why their atmosphere consists of a higher percentage of heavier metals, unlike Jupiter’s and Saturn’s which are purer in hydrogen and helium.

Finally, although the location and formation of our Solar System’s gaseous planets seem to be coherent, other gaseous exoplanets have formed in orbits extremely close to their star, putting in doubt our own model. Therefore, scientists speculate that such large planets as our gas giants might move back and forwards in their orbits during their lifetime. However, the reasons why and how are still openly discussed.

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