Moving the Orbits of PlanetsThe mechanism was described in 1984 by Julio Fernandez and Wing Ip. In a one-planet solar system, the outward scattering of comets by the planet would cause the planet to spiral in towards the sun. In the real solar system, the motions of the planets are inter-dependent, causing some to move outwards and others inwards. What connects the planets is the transfer of comets (and angular momentum) from one to the other. Neptune scatters comets out to the interstellar medium and inwards, where some meet Uranus and are scattered again. The process repeats down to the innermost big planet Jupiter. Fernandez and Ip showed that massive Jupiter anchors the flow of angular momentum caused by the ejection of comets. It spirals towards the sun but Saturn, Uranus and Neptune drift outwards. It is the outward migration of Neptune that has had (apparently) observable effects on the Kuiper Belt by trapping the Plutinos...
by David Jewitt
September 2004
Meanwhile, the Doppler discovery of extrasolar planets orbiting very close to their parent stars has raised a different problem. Many of the planets are so close to their stars (<0.1 AU), and so hot, that they cannot be supposed to have formed where we now observe them. By inference, they could have formed at larger distances (several AU) and then migrated inwards. What would cause this inward migration? As with the solar system case, the root cause may be an exchange of angular momentum with material surrounding the planets at their formation. In particular, if the extrasolar planets formed in massive disks, then torques between the planets and the disks could drive the former inwards.
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