The Proceedings of the Eighth International Conference on Creationism (2018)

Theory from various researchers may not be in complete agreement regarding the applicability of this mode of migration. Usually the scenario for Type III migration is described as being after Type II migration has taken place and a gap has set up in the disk. Gases can move across the gap and cause an accelerating migration of the planet. Though Type III migration can be either inward or outward, it is usually inward toward the star in simulations. Type III migration requires a more massive disk. If Type III migration is possible it often implies the planets will spiral into the star, but it depends on how the model is applied. Migration Types I, II, and III have all been applied in modern theories of the formation and evolution of our solar system (Goldreich and Tremaine, 1980, Papaloizou, et. al. 2007, Fog and Nelson, 2007, Hasegawah and Ida, 2013). Another form of migration considered theoretically possible is where solid planetesimals cause the planet orbit to change (Levison, et. al. 2007). This requires that there be enough planetesimals that their collective mass is comparable to multiple additional planets. Planetesimals can be scattered by the planets as they pass near them. Each time a planetesimal has its orbit altered by a planet, it transfers a small amount of its orbital angular momentum to the planet. This is the primary mechanism of planetesimals causing planet migration. Thus, it requires a large number of planetesimals to cause a significant sustained effect on a planet’s orbit. Planetesimals can also cause a planet’s orbit to round so that it is less eccentric, if there is a sufficient number of objects near the planet’s orbit. This process is referred to as dynamical friction. Planetesimals can continue having an effect on the orbits of planets after gases have dissipated in the disk. Thus, planetesimals are understood as being able to exert a slow effect that changes planet orbits over tens to hundreds of millions of years. The third type of planetary migration mechanism is from planet- planet interactions and orbit resonances. In a system of multiple planets which may be migrating, the planets may migrate at different rates due to their varying masses and may come into orbit resonances (Morbidelli, et. al. 2007). Orbit resonance refers to conditions where two or more objects have orbital periods which are small integer multiples of another orbiting body. Resonances can alter orbits over time because the two objects in resonance come nearer to each other in a repeating manner as they complete many orbits. Determining if two planets (or moons) are actually in a resonance requires good observations and analysis of their orbits. Sometimes apparent resonant motion can be explained as some type of temporary oscillation in the orbit. An orbit resonance may be a very stable configuration but it can also be a migration mode in which two or more planets migrate together. There are many known orbit resonances in our solar system but normally observed resonances are between a planet and smaller bodies such as moons, asteroids, or in some cases comets. If a massive body comes into resonance with a very small body, the small object’s orbit can be dramatically altered by the massive body. In a similar manner, a large planet can have a significant effect on smaller planets that may come into resonance with it. Thus, gravity can “nudge” the two objects closer when they are at their closest relative positions. If planets have eccentric orbits, resonance tends to cause the orbit of the planet of the lower mass to become more eccentric. Thus, if one large planet is migrating and it is in a resonance with another planet, it may cause the smaller planet to migrate with it. If sufficient planetesimals are present and gas is still present in the disk, there could be a combined effect of all these mechanisms on planet orbit migration. NEW SOLAR SYSTEM THEORIES 1. The Grand Tack Model The Grand Tack model and the Nice model apply planet orbit migration theory to our own solar system. These new models are believed by some scientists to address many limitations and difficulties with solar system theories of the past. Because gas in a disk will dissipate in a few million years it is believed that large gaseous planets form first. This process is believed to have started without any planet migration but then as the forming planet gets larger the gas and other material in the disk may cause it to migrate (in Type I, II, or III migration above). The Grand Tack model begins with Jupiter having formed and nearly at its full mass and it begins to migrate inward toward the Sun. Initially Saturn accretes at a slower rate than Jupiter, and Uranus and Neptune accrete at rates slower than Saturn because the density of the disk trails off with distance from the Sun. The Grand Tack scenario applies to the period after Jupiter has formed for a period of 600,000 years. In the Grand Tack scenario Jupiter is assumed to have initially formed at approximately 3.5 A.U. from the Sun. The Grand Tack addresses the inner solar system and defines a set of conditions where it is thought the four inner planets as well as the asteroid belt would form. The Nice model essentially starts where the Grand Tack ends and addresses the outer solar system, including the migration of Saturn, Uranus, and Neptune, and effects of this in the outer planetesimal belt. In the Grand Tack scenario Jupiter would begin migrating inward by either the Type I or Type II mechanism above, accreting some material as it goes. Saturn does not migrate until it nears its present mass and size. Then it begins to migrate faster than Jupiter and it catches up with Jupiter when Jupiter is at approximately 1.5 A.U. from the Sun. This distance is chosen to allow for the formation of Earth and Mars at approximately the right distances from the Sun and to allow for Mars forming with approximately the correct mass. In this approach, the mass of Mars is much less than that of Earth because Jupiter had scattered away much of the solid material in the zone from approximately 2 A.U. to 5 A.U. Jupiter’s movement in to 1.5 A.U. causes large planetesimals and planet embryos to be pulled inward with it. So, the result of Jupiter’s inward migration is to form a belt with many of the largest planetesimals and a limited number of larger planet embryos in the region from approximately 0.3 to 1.0 A.U. from the Sun. There would be many collisions and interactions of objects in this inner planetesimal belt. Some planetesimals and planet embryos would fall into the Sun and some could be ejected from the solar system. But the collisions with planet embryos are believed to lead to a small number of surviving rocky planets. Thus Mercury, Venus, Earth, and Mars are believed to have formed from this inner belt of objects. Jupiter’s movement inward also causes planetesimals that were in the region from 2 to 5 A.U. to move inward and they collide with the planet embryos forming in the inner belt. This provides volatile compounds such as water to Earth and the other Spencer ◀ Origin of our solar system with planet migration ▶ 2018 ICC 74