Grand tack hypothesis
The reversal of Jupiter's planetary migration is likened to the path of a sailboat changing directions (tacking) as it travels against the wind.
[3] Debris produced by collisions among planetesimals swept ahead of Jupiter may have driven an early generation of planets into the Sun.
An overlapping gap in the gas disk then formed around Jupiter and Saturn,[7] altering the balance of forces on these planets which began migrating together.
Most of these embryos collide and merge to form the larger terrestrial planets (Venus and Earth) over a period of 60 to 130 million years.
[17] Others are scattered outside the band where they are deprived of additional material, slowing their growth, and form the lower-mass terrestrial planets Mars and Mercury.
[16] These may be reduced during the giant planet instability described in the Nice model so that the eccentricity distribution resembles that of the current asteroid belt.
As these eccentricities are damped by the denser gas disk of recent models, the semi-major axes of the embryos shrink, shifting the peak density of solids inward.
Simulations that instead reversed Jupiter's migration at 2.0 AU yielded a closer match to the current Solar System.
[9] When the fragmentation due to hit and run collisions are included in simulations with an early instability the orbits of the terrestrial planets are better produced.
This also results in a larger fraction of the terrestrial planets mass being concentrated in Venus and Earth and extends their formation times relative to that of Mars.
[28] The migration of the giant planets through the asteroid belt creates a spike in impact velocities that could result in the formation of CB chondrites.
[29] The presence of a thick atmosphere around Titan and its absence around Ganymede and Callisto may be due to the timing of their formation relative to the grand tack.
These perturbations cause material to escape from the orbit of Mars or to impact on its surface reducing the mass of the disk resulting in the formation of smaller moons.
[33][34] Later studies have shown that the convergent orbital migration of Jupiter and Saturn in the fading solar nebula is unlikely to establish a 3:2 mean-motion resonance.
[11] The type of nebula density required for capture in the 3:2 mean-motion resonance is especially dangerous for the survival of the two planets, because it can lead to significant mass growth and ensuing planet-planet scattering.
[41][42][43] A small Mars could be the result of its region having been largely empty due to solid material drifting farther inward before the planetesimals formed.
[44][45] Most of the mass could also have been removed from the Mars region before it formed if the giant planet instability described in the Nice model occurred early.
[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.
[50] Sweeping secular resonances during the clearing of the gas disk could also excite inclinations and eccentricities, increasing relative velocities so that collisions resulted in fragmentation instead of accretion.
[56][57] The eccentricities and inclinations of the asteroid could also be excited during the giant planet instability, reaching the observed levels if it lasted for a few hundred thousand years.
[67][68] The formation of super-Earths may require a higher flux of inward drifting pebbles than occurred in the early Solar System.
