Abstract
I discuss the role that disc fragmentation plays in the formation of gas giant and terrestrial planets, and how this relates to the formation of brown dwarfs and low-mass stars, and ultimately to the process of star formation. Protostellar discs may fragment, if they are massive enough and can cool fast enough, but most of the objects that form by fragmentation are brown dwarfs. It may be possible that planets also form, if the mass growth of a proto-fragment is stopped (e.g. if this fragment is ejected from the disc), or suppressed and even reversed (e.g by tidal stripping). I will discuss if it is possible to distinguish whether a planet has formed by disc fragmentation or core accretion, and mention of a few examples of observed exoplanets that are suggestive of formation by disc fragmentation.
Highlights
There are two primary mechanisms for the formation of planets: (i) core accretion [1,2,3,4,5], and (ii) gravitational fragmentation of gas, i.e. in the same way that stars form
It is possible that protostellar discs may fragment and produce low-mass mass objects; these may be planets, brown dwarfs, and/or low-mass hydrogen-burning stars
The proto-fragments formed by fragmentation survive but their growth is suppressed or even reversed and end up as gas giant or rocky planets [12, 63,64,65,66], on close or wide orbits depending on the effects of migration
Summary
There are two primary mechanisms for the formation of planets: (i) core accretion [1,2,3,4,5], and (ii) gravitational fragmentation of gas, i.e. in the same way that stars form. In the core accretion model planets form by coagulation of dust particles in circumstellar discs. Some of these cores subsequently accrete envelopes of gas from the disc to become gas giants, whereas the rest end up as rocky planets. In the disc fragmentation model gas giants form by direct gravitational fragmentation of protostellar gas in discs within a few thousand years.
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