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تسجيل مستخدم جديدThe Formation of Uranus and Neptune in Solid-Rich Feeding Zones: Connecting Chemistry and Dynamics
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Sarah E. Dodson-Robinson
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The core accretion theory of planet formation has at least two fundamental problems explaining the origins of Uranus and Neptune: (1) dynamical times in the trans-Saturnian solar nebula are so long that core growth can take > 15 Myr, and (2) the onset of runaway gas accretion that begins when cores reach 10 Earth masses necessitates a sudden gas accretion cutoff just as the ice giant cores reach critical mass. Both problems may be resolved by allowing the ice giants to migrate outward after their formation in solid-rich feeding zones with planetesimal surface densities well above the minimum-mass solar nebula. We present new simulations of the formation of Uranus and Neptune in the solid-rich disk of Dodson-Robinson et al. (2009) using the initial semimajor axis distribution of the Nice model (Gomes et al. 2005; Morbidelli et al. 2005; Tsiganis et al. 2005), with one ice giant forming at 12 AU and the other at 15 AU. The innermost ice giant reaches its present mass after 3.8-4.0 Myr and the outermost after 5.3-6 Myr, a considerable time decrease from previous one-dimensional simulations (e.g. Pollack et al. 1996). The core masses stay subcritical, eliminating the need for a sudden gas accretion cutoff. Our calculated carbon mass fractions of 22% are in excellent agreement with the ice giant interior models of Podolak et al. (1995) and Marley et al. (1995). Based on the requirement that the ice giant-forming planetesimals contain >10% mass fractions of methane ice, we can reject any solar system formation model that initially places Uranus and Neptune inside the orbit of Saturn. We also demonstrate that a large population of planetesimals must be present in both ice giant feeding zones throughout the lifetime of the gaseous nebula.
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Uranus and Neptune form a distinct class of planets in our solar system. Given this fact, and ubiquity of similar-mass planets in other planetary systems, it is essential to understand their interior structure and composition. However, there are more
open questions regarding these planets than answers. In this review we concentrate on the things we do not know about the interiors of Uranus and Neptune with a focus on why the planets may be different, rather than the same. We next summarize the knowledge about the planets internal structure and evolution. Finally, we identify the topics that should be investigated further on the theoretical front as well as required observations from space missions.
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