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The rapid growth via pebble accretion allows the cores to grow large enough to accrete massive gas envelopes forming gas giants while avoiding migrating very close to the star. In simulations, cold gas giants like Jupiter and Saturn can form via pebble accretion if their initial embryos began growing beyond 20 AU. This distant formation offers ...
Pebble accretion may accelerate the formation of planets by a factor of 1000 compared to the accretion of planetesimals, allowing giant planets to form before the dissipation of the gas disk. [28] [29] However, core growth via pebble accretion appears incompatible with the final masses and compositions of Uranus and Neptune. [30]
The formation of giant planets is an outstanding problem in the planetary sciences. [22] In the framework of the solar nebular model two theories for their formation exist. The first one is the disk instability model, where giant planets form in the massive protoplanetary disks as a result of its gravitational fragmentation (see above). [58]
[46] [47] If most of the growth of planetesimals and embryos into terrestrial planets was due to pebble accretion, a small Mars could be the result this process having been less efficient with increasing distances from the Sun. [48] [49] Convergent migration of planetary embryos in the gas disk toward 1 AU would result in the formation of ...
Hydrodynamic models indicate that the smallest pressure gradients occur near the ice-line and in the inner parts of the disk. The pressure gradient also decreases late in the disk's evolution as the accretion rate and the temperature decline. [77] A major source of turbulence in the protoplanetary disk is the magnetorotational instability.
The most widely accepted model of planetary formation is known as the nebular hypothesis. This model posits that, 4.6 billion years ago, the Solar System was formed by the gravitational collapse of a giant molecular cloud spanning several light-years. Many stars, including the Sun, were formed within this collapsing cloud. The gas that formed ...
Although more recent models including pebble accretion allow for faster growth the inward migration of the planets due to interactions with the gas disk leave them in closer orbits. [46] It is now widely accepted that the Solar System was initially more compact and that the outer planets migrated outward to their current positions. [47]
Models of how Siletzia formed are of two general types: [73] (1) Formation well offshore (possibly as seamounts, like the Hawaiian-Emperor seamount chain, or a hotspot at a spreading ridge, like Iceland) and then accretion to the continent; (2) formation inshore, on or near the continental margin (perhaps as a result of transcurrent extension ...