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Planetary Migration Earth Origin | Article 5

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In planetary migration the outer two planets may be in the wrong place.

According to the nebular hypothesis, the outer two planets may be in the wrong place. Uranus and Neptune exist in a region where the reduced density of the solar nebula and longer orbital times render their formation highly implausible. The two are instead thought to have formed in orbits near Jupiter and Saturn, where more material was available, and to have migrated outward to their current positions over hundreds of millions of years. 

Planetary Migration
Planetary Migration
The migration of the outer planets is also necessary to account for the existence and properties of the solar systems outermost regions. Beyond Neptune, the solar system continuous into the kuiper belt, the scattered disc, and the Oort cloud, sparse population of small icy bodies thought to be the points of origin for most observed comets. At their distance from the sun, accretion was too slow tu to allow planets to phone before the solar nebula dispersed, and thus the initial disc lacked enough mass density tu consolidate into a planet. The creeper belt lies between 30 and 55 AU from the sun, while the farther scattered extends to over 100 AU, and the distant Oort cloud begins at about 50, 000 AU. Originally, however, the kuiper belt was much dancer and closer to the sun, with an outer age at approximately 30AU. Its inner edge would have been just beyond the orbits of Uranus and Neptune, which were in turn are closer to the sun when the formed,and in 50% of simulations and up in opposite location, with Uranus farther from the sun than Neptune.

According to the nice model,the formation of the solar system, the orbits of all giant planets continued to change slowly, influenced by their interaction with the large number of remaining planetesimals. After 500-600 million years Jupiter and Saturn well into a 2:1 resonance . Saturn orbited the sun once for every two Jupiter orbits. This resonance created a gravitational push against the outer planets, possibly causing Neptune to surge past Uranus and plough into the ancient kuiper belt. The planets scatered the majority of the small icy bodies inwards, wild themselves moving outwards. These planetesimals then scattered of the next planet the encountered in a similar manner, moving the planets orbit outward while they move inwards. This process continued until the plantesimals interacted with Jupiter, whose immense gravity sent them into highly elliptical orbits or even ejected them outright from the solar system. This caused Jupiter to move slightly inward. Those objects scattered by Jupiter into highly elliptical orbits formed the Oort cloud; those objects scattered to a lesser degree by the migration Neptune formed the current kuiper belt and scattered disc. This scenario explains the kuiper belt's and scattered disc present low mass. Some of the scattered objects, including Pluto, became gravitationally tied Neptune's orbit, forcing them into mean-motion resonances. Eventually, friction within the planetesimals disc made the orbits Uranus and Neptune circular again.
In contrast to the outer planets, the inner planets are not thought to have migrated significantly over the age of the solar system, because their orbits have remained stable following the period of giant impacts.

Another question is why mass came out so small compared with Earth. Study by Southwest research institute, San Antonio, Texas, published June 6, 2011, proposes that Jupiter had migrated inward to 1.5 AU. After Saturn formed, migrated inward, and established the 2:3 mean motion resonance with Jupiter, the study assumes that both planets migrated back to their present positions. Jupiter thus would have consumed much of the material that would have created a bigger mars. The same simulations also reproduce the characteristics of the modern asteroid belt, with dry asteroids and water rich objects similar to comets. However, it is unclear whether conditions in the solar nebula would have allowed Jupiter and Saturn to move back to their current position, and according to current estimates this possibility appears unlikely.moreover, alternative explanation for the small mass of mars exist.

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