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Gas Giant Formation

Artist’s rendition of planet forming.

The formation of Gas Giants begins, along with the formation of terrestrial planets and stars, with the collapse of a dense molecular cloud. Once a dense molecular cloud reaches the Jean’s Mass it will collapse under its own gravity, forming a protoplanetary disk orbiting a protostar. This protostar will turn into a main sequence star given enough mass and time (greater than .08 solar masses and ~1 million years). During this stage of protoplanetary disk evolution gas giants can begin to form as well.

Two main theories are prevailing over others in regards to gas giant formation, core accretion and gravitational instability. While both theories are plausible, neither is shown to be incorrect or correct, evidence for and against both theories are available.

Core accretion of gas giants formation generally states that their formation follows a similar path to that of terrestrial planet formation (the core) and continuing afterwards accreting all the gas present within the protoplanetary disk within the planet’s Hill sphere radius.

Gravitational instability of gas giant formation generally states that their formation follows a similar path to that of star formation, only without accreting mass to start fusion within them. In essence creating brown dwarfs.

Gravitational Instability

Gravitational Instabilities can form within a protoplanetary disk within a set of parameters determined by the equation

$Q={\frac {c_{s}k}{\pi G\Sigma }}$

Protoplanetary disk simulation.

where c_s is the sound speed, in a uniform Keplerian disk k can also be represented as Ω the angular speed, G is the gravitational constant, and Σ is the mean surface density.

Generally disks are stable while Q>1, and unstable while Q<1. The gravitational stability of protoplanetary disks can change depending on their shape and time. For a disk that is turning into a spiral, gravitational instabilities will occur when Q<1.5.

Once a gravitational instability forms within a disk, it can start to form a gas giant. Many things can stop this process including but not limited to, fracturing, cooling, or entering a self-regulating phase. If the gravitational instability lives past these issues, it will form a gas giant.

While evidence of protoplanetary disks shows that they can exist and be massive and cool enough to cause gravitational instability, the primary issue with the theory has to do with the composition of the disk. Generally in high metallicity systems more gas giants are present. However, composition of the system does not encourage nor discourage gravitational instabilities directly.

Core Accretion

Core Accretion states that gas giant formation is similar in nature to terrestrial planet formation. After the core of the planet is accreted however, it will continue to coalesce gas around itself, forming a planet. This theory holds true with the correlation between higher metallicity systems and higher concentration of planets as the ‘dust’ phase is easier to get past with higher density materials.

Artist's illustration of exoplanets PDS 70 b and c (2019-26-4488).

The primary issue with core accretion is its characteristic timescale. A protostar will turn into a star after around 10 million years. Once this happens the star will photo evaporate the rest of the protoplanetary disk and the gas surrounding it. The problem is that a gas coalescing to a core can take around the same timescale. This leaves little room for error and only creates planets with masses of around 0.75 Jupiter Masses.

Alternate Theories

Some alternate theories include a mix of the two, with gravitational instability aiding core accretion to speed up the timescale. Or through a process known as pebble accretion.