Asteroid and Gas Giant Eras
The first 2-4 million years since the formation of the Solar System were the time of the first asteroid formation. The first asteroids have been accreted less than a million years after the Solar System formation, and they have undergone different processes throughout the next several million years. Some of the asteroid-forming processes include melting through impacts or internal nuclear reactions. The 26Al decay that heated the asteroids has been depleted within approximately a million years after their formation, and they began differentiating into different types (Pfalzner et al., 2015). Those that possessed Al and Ca inclusions are called chondritic asteroids and have not differentiated; they have been largely unchanged since their accretion. The position of the asteroids relative to the center of the System also influence their mineral composition. The chondritic asteroids developed near the frost line contained different secondary minerals that formed with the help of water ice (Jogo et al., 2017). Several million years after accretion, asteroids began to form terrestrial planets, which is a process that stretched past 10 million years after Solar System formed.
The Jovian planets have formed much further away from the center of the System than the terrestrial ones, and they have done so outside the frost line. Their formation is presumed to be due to heavy elements being aggregated into a core, which then began to gradually accrete gas from its surroundings. The dense core of a gas giant acquires the mass several times larger than Earth within a couple of million years. Jupiter formed in a similar process, and some of the heavy elements present in the primordial planetesimals have remained in its envelope (Lozovsky et al., 2017). The gas and heavy element envelopes of the gas giants catch other planetesimals and dissolve them before they reach the core, enriching itself. Jupiter’s presence pulled and halted the dispersal of planetesimals and gas, which caused Saturn to form uncharacteristically quickly (Kobayashi, Ormel, & Ida, 2012). That process allowed Saturn to keep its signature rings, which were not pulled into the envelope.
Planetary Formation
The frost line is a boundary between the two regions, which separates the System’s inner region where water and other elements are volatile because of the heat and get pushed out by solar winds, and the outer region, where they freeze into solid ice. That solid ice becomes available for the accretion of planetesimals, which allows them to grow larger and denser, eventually drifting into each other and forming gas giants. The terrestrial planets are smaller and rockier because there was less matter to accrete, and it was drier. A colder star would likely move the frost line closer, and vice versa. The expansion of the disc of matter around the star also heats it, which moves the frost line further from the star.
Planetary Migration
The formation of gas giants occurred much earlier than that of the terrestrial planets. In the astrophysical models, it is very likely that gas giants then migrate closer to the star. There is some debate in the field on the topic, but it seems likely that Jupiter has migrated inwards, and then was pulled back by Saturn. The early migration of Jupiter to the distance approximating 1.5 AUs from the Sun would create conditions that support the accretion of terrestrial planets, and explain why Mars is smaller than the rest of them, being at the outer edge of the truncated disc. Jupiter would then move back beyond the frost line, closer to where it was initially formed (Walsh, 2011). Other similar planets form stable orbits much closer to their stars, making the Solar System an anomaly.
References
Jogo, K., Nakamura, T., Ito, M., Wakita, S., Zolotov, M. Y., & Messenger, S. R. (2017). Mn–Cr ages and formation conditions of fayalite in CV3 carbonaceous chondrites: Constraints on the accretion ages of chondritic asteroids. Geochimica et Cosmochimica Acta, 199, 58–74.
Kobayashi, H., Ormel, C. W., & Ida, S. (2012). Rapid formation of Saturn after Jupiter completion. The Astrophysical Journal, 756(1), 70.
Lozovsky, M., Helled, R., Rosenberg, E. D., & Bodenheimer, P. (2017). Jupiter’s formation and its primordial internal structure. The Astrophysical Journal, 836(2), 227.
Pfalzner, S., Davies, M. B., Gounelle, M., Johansen, A., Münker, C., Lacerda, P., … Veras, D. (2015). The formation of the solar system. Physica Scripta, 90(6), 068001.
Walsh, K. J., Morbidelli, A., Raymond, S. N., O’Brien, D. P., & Mandell, A. M. (2011). A low mass for Mars from Jupiter’s early gas-driven migration. Nature, 475(7355), 206–209.