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

gas and dust may be considered time zero for planet formation. The new planet migration models for our solar system begin with the formation of Jupiter within the first 10 million years. The disk around the star is referred to as the “protoplanetary disk,” “protosolar disk,” or sometimes as the “solar disk.” It is referred to sometimes as the “solar disk” because it is believed to initially be of the same overall composition as the Sun. The distribution of material in the disk as well its density is a critical issue for planet formation theories. The new Grand Tack model begins with the formation of Jupiter and deals with Jupiter and Saturn migrating inward toward the Sun, then Jupiter and Saturn enter a resonance and reverse their migration (Walsh, et. al., 2011, Walsh, et. al., 2012, Jacobson and Morbidelli, 2014, Isidoro, et. al., 2015). Thus, the Grand Tack has Jupiter and Saturn migrating first inward then outward. Then the end of the Grand Tack scenario becomes the beginning of the Nice model. The Nice model then proposed that Saturn, Neptune, and Uranus migrate outward to their present orbital positions over a period of 100 million years (Tsiganis, et. al., 2005, Levison, et. al., 2008, Batygin and Brown, 2010). These new models have generated great interest and enthusiasm among planetary scientists because of the apparent success from them in producing the characteristics of our solar system in computer simulations. This paper will do a review of the new migration theories for the formation of our solar system. In what follows, theoretical methods used in planetary science today will be summarized followed by an explanation of the new Grand Tack and Nice models for our solar system. Following this will be the author’s interpretation and evaluation of the Grand Tack and Nice models in the Discussion section. Lastly in the conclusions will be comments on the significance of this research to creationism. THEORETICALMETHODS IN PLANETARY SCIENCE 1. Accretion of Solid Bodies Planet formation theories have been developed that depend critically on the formation of sizable solid objects known as “planetesimals” and “planetary embryos.” The protoplanetary disk is initially composed of gas and dust. The dust is thought to become more concentrated from settling to the midplane and possibly from its tendency to spiral inward. Experiments have demonstrated that very small dust particles can stick together in collisions (Poppe, Blum, Henning, 2000 and Blum, et. al. 2006). Theories assume that agglomerated dust can eventually grow into larger solid objects on the order of 1 km in size and larger. These are the planetesimals. Planetesimals in turn grow through collisions and collecting material near them until some of the planetesimals grow to larger sizes from approximately 1000 km diameter to the size and mass of Mars. These are the planetary embryos. It is thought that the protoplanetary disk in the early solar system likely contained perhaps a few dozen planetary embryos, of which only a few survived to the present. It is believed growth of the largest gas giant planets would have been most rapid at the start, while gas is most readily available in the disk. Gas giant planets are believed to grow initially by a process called core-accretion (Matsuo, et. al., 2007) and later by absorbing planetesimals. In core-accretion, solid planetesimals and other material must combine to make a mass thought to be of a minimum of approximately 4 Earth masses (m E ). If this takes place quickly enough so that the gas in the disk is plentiful, the planet core can attract gas to it and it can grow rapidly until gas becomes depleted in the disk. If the planet core does not grow to about 4 m E in a sufficiently short time, then this will limit its size because of the dissipation of gas in the disk. In the first few million years of the disk, growth of planetary embryos is thought to be more rapid. The process of absorbing solid planetesimals is believed to form the terrestrial (rocky) planets. Zahnle, et. al. (2007, pp. 41- 42) summarized the early stages of planetary accretion of the rocky planets as follows: In the simplest terms accretion of terrestrial planets is envisaged as taking place in four stages: (1) Settling of circumstellar dust to the mid-plane of the disk. (2) Growth of planetesimals up to ~1 km in size. (3) Runaway growth of planetary embryos up to ~10 3 km in size. (4) Oligarchic growth of larger objects through late-stage collisions. Stage 1 takes place over time scales of thousands of years and provides a relatively dense plane of material from which the planets can grow. The second stage is the most poorly understood at present but is necessary in order to build objects that are of sufficient mass for gravity to play a major role. Planetesimals would need to be about a kilometer in size in order for the gravitationally driven stage 3 to start. We do not know how stage 2 happens, although clearly it must. Scientists have succeeded in making fluffy aggregates from dust, but these are all less than a cm in size. Planetary scientists commonly refer to objects larger than 1000 km as planet embryos. Thus, our Moon, whose diameter is 3,476 km, could be referred to as a planetary embryo. The rate of growth of the solid planetary embryos is thought to depend chiefly on their relative velocities in collisions and the relative numbers and masses of the embryos compared to the planetesimals. When the planetesimals are very numerous they tend to reduce the velocities of the embryos and the slower speeds of the embryos facilitates faster accretion. This is Stage 3 referred to above as “Runaway growth.” In this stage the planetary embryos are thought to grow relatively rapidly. In Stage 4, oligarchic growth, the number of planetesimals is not enough to affect the velocity of the embryos and the embryos are larger. Thus, in the oligarchic growth stage the gravity of the planetary embryos draws planetesimals to them. 2. Starting Assumptions of Current Theories Several overarching assumptions are made in current theories regarding the overall process of how the protosolar disk evolved into the current array of planets and small bodies in our solar system. First, the protosolar disk is initially assumed to contain sufficient material to form the planets and other objects in our solar system. Early work on modeling the protosolar disk was done from the late 1960’s through the mid-1980’s. Two significant papers on what has been called the Minimum Mass Solar Nebula Spencer ◀ Origin of our solar system with planet migration ▶ 2018 ICC 72