Is there a reason the Gas Giants are further away from the sun that most terrestrial planets or is it a coincidence?
In: Planetary Science
Early in the solar system heat from the sun, combined with a strong solar wind from the sun’s formation drove most of the easily-vaporized material out of the inner solar system, leaving behind mostly rock and metal. Beyond a point called the frost line, between Mars and Jupiter, these materials remained in place so we got gas giants.
Heat. At distances relatively close to the sun, gases expand too much to “group up” enough to create the gravitational forces to form a planet. Heat makes things expand. Solids like metals and rocks can remain solid at higher temperatures, so the gases closer to the sun were too heated to condense into gas giant planets. But rocks could.
Further away from the sun is colder, so gases are denser.
As others have said, there are reasons why gas giants are found further away from their stars. With that being said, we actually have observed gas giants that are very close to their stars, referred to as [“hot Jupiters”](https://en.wikipedia.org/wiki/Hot_Jupiter). It’s not clear exactly how they form.
Not a coincidence! The sun generates something called the solar wind which is a stream of charged particles. This solar wind can be relatively forceful (but we can’t feel it at all) when interacting with very small and light things like gas molecules.
The early solar system was a lumpy disk of gases with a gravitational center that would eventually grow large and dense enough to self-ignite. Once that occurred, the solar wind began pushing the lightest elements of the gas cloud such as hydrogen and helium outward. Heavier elements like carbon, silicon and iron remained behind and aggregated into the rocky inner planets.
The mass of gases pushed outward by the solar wind eventually slowed as the force of wind dissipated and enabled the formation the gas giants. Compositionally, Jupiter is quite similar to the sun with predominance of hydrogen and helium – it’s just too small to become a star on its own.
Hope this helps!
While there are many theories about how the solar system formed, the most commonly accepted ones these days all agree that the planets most likely did not form exactly where they are. They disagree on exactly when they moved, or where exactly they came from, but some form of “migration” is the only way to explain data we’ve collected about the general composition of the planets.
Many of these models involve the gas giants forming much closer towards the sun. As their orbits adjusted, they ended up in what’s called an orbital “resonance,” meaning several large planets ended up repeatedly close to each other in their orbits. This may have effectively slingshot-launched some of them outwards into much further orbits, and may have actually ejected some planets from the solar system entirely.
This kind of movement is not far-fetched: we now believe that at some point, a Mars-sized planet was flung into Earth, and the impact debris later formed the moon. And many planets show a later period of sudden, intense crater formation and collisions all around the same time, a period known as the Late Heavy Bombardment. Circumstantial evidence at best, but still suspicious.
It was generally believed until relatively recently that gas giants cannot form closer due to the solar wind — a claim that many comments here are repeating. This was a tidy explanation, until we got more data about planet composition and started looking at other star systems in the past few decades. This has since been proven false. Looking out across the universe, we find that gas giants that orbit much closer to their stars are actually quite common, and rocky planets often orbit much further out.
Again, whether this is just a product of the many different ways that planets can form initially, or whether some kind of later “migration” is common throughout the universe, is still unknown.
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