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The gas planets of our solar system are thought to have formed through a process called accretion, in which tiny dust sized grains slowly began to clump together over millions of years until they reached the size of planets. The same process is thought to have happened to the inner planets, but with a key difference. The gas giants of the outer solar system formed beyond the "frost line," where, beyond the orbit of Mars, it is cold enough for compounds such as water, ammonia, and methane to freeze into grains of ice. This greatly increased the number of planetary building blocks farther from the Sun, which caused the outer planets to grow large enough to collect vast amounts of light hydrogen and helium gasses and resulted in their large sizes today.

This theory however has recently been challenged by the discovery of several large gas exoplanets which are extremely close to their parent stars, in some cases closer than even Mercury is to our Sun. This does not fall in line with current models and has led to a new theory that gas giants generally have a high tendency for their orbits to change, being thrown through large gravitational interactions into very tight trajectories around their stars after having formed further out. The fact that our solar system has the gas giants near where they are thought to have formed may in fact be a lucky coincidence, the exception rather then the rule.

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13y ago

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