giant planet formation

There are two main classes of models of giant planet formation:

• core accretion model - that the planet formation begins like that of sizable rocky planets, then gas is accreted to form the very deep atmosphere.
• gravitational instability model aka disk instability model - that the gas of the protoplanetary disk becomes non-uniform to the point that at some place, the Jeans criterion is met, and gas collapses into a gas giant (gravitational collapse), with any intermixed solids falling into the center.

Note that models of various astrophysical phenomena incorporate gravitational instability, such as star formation, galaxy formation, formation of large scale structure, and it may even be a factor in triggering the formation of (solid) planet cores, but the term gravitational instability model is currently often used for the model of the formation of giant planets: the alternate term disk instability model is perhaps intended to reduce the ambiguity.

The instabilities of the gravitational instability model would result from turbulence, a Rossby wave instability, disk resonances, from electromagnetism effects (magnetohydrodynamics) and/or possibly from the disturbance of passing massive objects.

The gravitational instability model would form giant planets much quicker, possibly merely thousands of years rather than ten million plus for the core accretion model. The latter might be too slow to work, given current estimates of the lifetime of protoplanetary disks. The core accretion model would allow time for cooling during the lengthy accretion phase, thus is referred to as cold start, versus hot start for the gravitational instability model.

With current and developing methods of extra-solar planet observation, there may be ways to determine which, or if both of these mechanisms take place. Planet demographics provide evidence, once methods are developed without the biases inherent in current methods/capabilities, e.g., radial velocity method, transit method, both of which disfavor long orbital periods and are presumed to miss many planets. It may become possible to measure temperature or other characteristics that provide evidence for a hot start versus a cold start.