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Planet Formation Through Gravitational Instabilities  

Ken Rice

It is now widely accepted that planets form in discs around young stars, with the most widely accepted planet formation scenario being a bottom-up process typically referred to as “core accretion.” The basic process involves a core growing through the accumulation of solids and, if it gets massive enough while there is still gas present in the disc, undergoing a runaway gas accretion phase to form a Jupiter-like gas giant. However, early models of this process suggested that the formation timescale for a Jupiter-like gas giant exceeded the lifetime of the gas disc, suggesting that massive, gas giant planets form via some alternative process. One possibility is that they form via direct gravitational collapse. During the earliest stages of star formation, the disc around a young star can have a mass that is comparable to that of the central protostar and can be susceptible to the growth of a gravitational instability. One outcome of such an instability is that the disc fragments into bound objects that can then contract to become gas giant planets. This would happen very early in the star formation process and is very rapid, overcoming the timescale problem. Subsequent work has, however, both illustrated that core accretion may operate on timescales shorter than disc lifetimes and that disc fragmentation is very unlikely to operate in the inner parts of planet-forming discs. Hence, it is very unlikely that disc fragmentation plays a role in the direct formation of close-in exoplanets. However, disc fragmentation may operate at large orbital radii and is expected to preferentially form either massive gas giant planets or brown dwarfs. Therefore, it is intriguing that exactly such objects are starting to be directly imaged at orbital radii where disc fragmentation may operate. Additionally, even if a self-gravitating phase doesn’t play a direct role in the formation of gas giant planets, it may play an indirect role in the planet formation process. The spiral density waves that develop due to the gravitational instability can act to enhance the local density of solids, potentially accelerating their collisional growth or leading to the direct gravitational collapse of the solid component of the disc. This could then provide some of the building blocks for planets that later form via core accretion.