The solar system's current architecture of giant planets (Jupiter, Saturn, Uranus, and Neptune) is thought to have evolved from a different initial configuration. Scientists have proposed three competing models to explain the early migration of these planets.
Model 1 (Disk-Driven Migration)
During the first few million years of the solar system, a thick protoplanetary disk of gas and dust was present. Jupiter, forming first, experienced Type II migration, where viscous torques from the surrounding gas disk drove it rapidly inward from its birth site at to . As Saturn formed and also migrated inward, it was captured into a mean-motion resonance with Jupiter. In this configuration, the combined gravitational torques of the two planets cleared a gap in the gas disk, reversing their migration direction and forcing both planets to migrate outward until the gas disk dispersed.
Model 2 (Planet-Planet Scattering)
Giant planets formed in a very compact, unstable configuration immediately after the gas disk dissipated (around after solar system formation). The system remained dynamically stable for a short period until the gravitational influence of the planets on one another triggered a chaotic phase of direct planet-planet scattering. During this phase, close encounters between the planets rapidly modified their orbits. One ice giant was completely ejected from the solar system, while Jupiter was scattered slightly inward and Saturn, Uranus, and Neptune were scattered outward to their current, stable orbits.
Model 3 (Planetesimal-Driven Migration)
Following gas disk dispersal, the giant planets occupied a stable, compact, circular configuration surrounded by a massive outer disk of solid planetesimals. Over approximately , slow gravitational interactions between the outer planets and the planetesimal disk caused planetesimals to be scattered inward. In reaction to ejecting these planetesimals, Saturn, Uranus, and Neptune slowly migrated outward, while Jupiter migrated slightly inward. This slow migration eventually drove Jupiter and Saturn to cross a mean-motion resonance. The resonance crossing abruptly increased the eccentricities of Jupiter and Saturn, destabilizing the orbits of Uranus and Neptune and rapidly scattering them into the outer planetesimal disk.
According to the descriptions of the models, both Model 1 and Model 3 rely on a mean-motion resonance between Jupiter and Saturn to explain changes in planetary orbits. Which of the following statements best describes how the two models differ regarding the environment in which this resonance occurs and its primary effect on Jupiter's migration?
- In Model 1, the resonance occurs in a gas-rich protoplanetary disk and reverses Jupiter's migration from inward to outward, whereas in Model 3, the resonance occurs in a gas-free planetesimal disk and triggers an orbital instability that leads to Jupiter migrating slightly inward.Answer
- BIn Model 1, the resonance occurs in a gas-free planetesimal disk and reverses Jupiter's migration from inward to outward, whereas in Model 3, the resonance occurs in a gas-rich protoplanetary disk and triggers an orbital instability that leads to Jupiter migrating slightly inward.
- CIn Model 1, the resonance occurs in a gas-rich protoplanetary disk and triggers an orbital instability that leads to Jupiter migrating slightly inward, whereas in Model 3, the resonance occurs in a gas-free planetesimal disk and reverses Jupiter's migration from inward to outward.
- DIn Model 1, the resonance occurs in a gas-free planetesimal disk and triggers an orbital instability that leads to Jupiter migrating slightly inward, whereas in Model 3, the resonance occurs in a gas-rich protoplanetary disk and reverses Jupiter's migration from inward to outward.