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I asked this previously in Worldbuilding stack exchange, and they recommended that I take it here.

How far out from a star (lets just say one identical to our sun, for simplicities sake) where can gas giants form? Is there some limit where gas gaints stop forming and only ice giants would form? Or would gas giants theoretically be able to form at any point within the system?

So, for instance, is it theoretically possible for a gas giant to form at 15 AU from a G2V 1M-1R-1L star? Or would the only giants at this point be ice giants?

From what I understand the way these two planet types form is very different from one another. Which suggests that it would even be able to form relatively distant from the star, if our system's planetary formation just went differently, but I want to make sure that this understanding of mine stands up to science.

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    $\begingroup$ It depends on the density of the gas disc and the disc dispersal timescale. $\endgroup$
    – ProfRob
    Commented Dec 5, 2023 at 17:02
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    $\begingroup$ There is migration too. You could have one form close in, and then get flung out by a resonance. $\endgroup$
    – Anders Sandberg
    Commented Dec 5, 2023 at 18:47

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It would not be outrageous to have a gas giant at 15 AU from a Sun-like star. Most of the directly imaged exoplanets are well outside of 15 AU, and these are giant planets (but around various types of stars).

As mentioned by @Anders-Sandberg, planets migrate and drift during formation, so a planet ending up at 15 AU may have actually begun its formation closer to the star.

Giant planets usually form by first accreting a solid core, leading to runaway gas accretion when the core is massive enough. If the gas disk has dispersed before the core reaches that critical mass (as mentioned by @ProfRob), then the planet will not grow to become a gas giant.

You mentioned outcomes being varying if "planetary formation just went differently." Factors like the initial mass of gas and heavy elements in the disk, and the metallicity of the system, influence growth rates and formation locations. The outcomes are very sensitive to the relative timing of planet migration, disk thermal evolution, and gas dissipation.

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  • $\begingroup$ Nice answer, with respect to the gas vs ice giants part of OPs question, do snow lines affect whether the planet formed is a gas giant or an ice giant? $\endgroup$
    – justauser
    Commented Dec 30, 2023 at 8:39

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