The formation of Uranus and Neptune in solid-rich feeding zones: Connecting chemistry and dynamics

Abstract

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 ∼ 10M ⊕ necessitates a sudden gas accretion cutoff just as Uranus and Neptune's 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 using the initial semimajor axis distribution of the Nice model 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 of Saturn's orbit. 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. This research marks a substantial step forward in connecting both the dynamical and chemical aspects of planet formation. Although we cannot say that the solid-rich solar nebula model of gives exactly the appropriate initial conditions for planet formation, rigorous chemical and dynamical tests have at least revealed it to be a viable model of the early solar system.

The PLATO 2.0 mission

“…Another important open question regarding these planets is their formation process. It is still unclear what conditions and physical mechanisms lead to the formation of these fairly low-mass objects, especially at the large radial distances we find them today in the solar-system (e.g., Dodson-Robinson & Bodenheimer 2010). It was suggested by so called `Nice model' (Tsiganis et al 2005) that the architecture of the Solar System changed over time thanks to dynamical interactions of the giant planets with the dense planetesimal disc.…”

Last giant impact on the Neptunian system

“…Assuming that the GC with Neptune occurred at 30 AU, v iM is ∼30 km s −1 . If Neptune had been formed between 12 and 30 AU (Dodson-Robinson & Bodenheimer 2010;Benvenuto et al 2009) and the GC had occurred before or during outward migration (Tsiganis et al 2005), v iM would had been between 35 and 30 km s −1 . It should be noted that our estimate of v iM in Eq.…”

Last giant impact on Uranus

“…But within ten MMSN, Jupiter falls like a stone into the Sun due to type III migration (Crida 2009). On the other hand, a solid surface density five to ten times the MMSN would lead to the formation of about five ice giants instead of two, which occurred with the three other giants; i.e., whether they were ejected or if they were simply spread out and all retained is a matter of debate (Goldreich et al 2004;Dodson-Robinson & Bodenheimer 2010;Ford & Chiang 2007;Levison & Morbidelli 2007). In the last case, then, where are they?…”

“…Modern models shorten the timescale for giant planet formation if taking a higher initial surface density into account well above that of the MMSN and/or the formation of all giant planets in an inner compact configuration (e.g. Dodson-Robinson & Bodenheimer 2010;Benvenuto et al 2009; Thommes et al 2003;Tsiganis et al 2005). Other effects include planetesimals migration due to gas drag and the small size of the accreted planetesimals, which accelerates the accretion rate (e.g.…”

“…To avoid stochastic giant collisions, an initial surface density ∼5−10 times that of the MMSN would be required to produce oligarchs with masses similar to the cores of Uranus and Neptune (e.g. Dodson-Robinson & Bodenheimer 2010). But within ten MMSN, Jupiter falls like a stone into the Sun due to type III migration (Crida 2009).…”