<P> The region of a planetary system adjacent to the giant planets will be influenced in a different way . In such a region, eccentricities of embryos may become so large that the embryos pass close to a giant planet, which may cause them to be ejected from the system . If all embryos are removed, then no planets will form in this region . An additional consequence is that a huge number of small planetesimals will remain, because giant planets are incapable of clearing them all out without the help of embryos . The total mass of remaining planetesimals will be small, because cumulative action of the embryos before their ejection and giant planets is still strong enough to remove 99% of the small bodies . Such a region will eventually evolve into an asteroid belt, which is a full analog of the asteroid belt in the Solar System, located from 2 to 4 AU from the Sun . </P> <P> Thousands of exoplanets have been identified in the last twenty years . The orbits of many of these planets and systems of planets differ significantly from the planets in the Solar System . The exoplanets discovered include hot - Jupiters, warm - Jupiters, super-Earths, and systems of tightly packed inner planets . </P> <P> The hot - Jupiters and warm - Jupiters are thought to have migrated to their current orbits during or following their formation . A number of possible mechanisms for this migration have been proposed . Type I or Type II migration could smoothly decrease the semimajor axis of the planet's orbit resulting in a warm - or hot - Jupiter . Gravitational scattering by other planets onto eccentric orbits with a perihelion near the star followed by the circularization of its orbit due to tidal interactions with the star can leave a planet on a close orbit . If a massive companion planet or star on an inclined orbit was present an exchange of inclination for eccentricity via the Kozai mechanism raising eccentricities and lowering perihelion followed by circularization can also result in a close orbit . Many of the Jupiter sized planets have eccentric orbits which may indicate that gravitational encounters occurred between the planets, although migration while in resonance can also excite eccentricities . The in situ growth of hot Jupiters from closely orbiting super Earths has also been proposed . The cores in this hypothesis could have formed locally or at a greater distance and migrated close to the star . </P> <P> Super-Earths and other closely orbiting planets are thought to have either formed in situ or to have migrated inward from their initial locations . The in situ formation of closely orbiting super-Earths would require a massive disk, the migration of planetary embryos followed by collisions and mergers, or the radial drift of small solids from farther out in the disk . The migration of the super-Earths, or the embryos that collided to form them, is likely to have been Type I due to their smaller mass . The resonant orbits of some of the exoplanet systems indicates that some migration occurred in these systems, while the spacing of the orbits in many of the other systems not in resonance indicates that an instability likely occurred in those systems after the dissipation of the gas disk . The absence of Super-Earths and closely orbiting planets in the Solar System may be due to the previous formation of Jupiter blocking their inward migration . </P>

Use the nebular hypothesis to explain why the planets all orbit the sun in the same direction