Dynamical Evolution of Planetary Systems

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The apparent regularity of the motion of the giant planets of our solar system suggested for decades that said planets formed onto orbits similar to the current onesand that nothing dramatic ever happened during their lifetime. The discovery of extrasolar planets showed astonishingly that the orbital structure of our planetary system is not typical. Many giant extrasolar planets have orbits with semimajor axes of ∼ 1 AU,and some have even smaller orbital radii, sometimes with orbital periods of just a few days. Moreover, most extrasolar planets have large eccentricities, up to values that only comets have in our solar system. Why is there such a great diversitybetween our solar system and the extrasolar systems, as well as among the extrasolar systems themselves? This chapter aims to give a partial answer to this fundamental question. Its guideline is a discussion of the evolution of our solarsystem, certainly biased by a view that emerges, in part, from a series of works comprising the “Nice model.” According to this view, the giant planets of the solar system migrated radially while they were still embedded in a protoplanetary disk of gas and presumably achieved a multi-resonant orbital configuration, characterized by smaller interorbital spacings and smaller eccentricities and inclinations with respect to the current configuration.The current orbits of the giant planets may have been achieved during a phase of orbital instability, during which the planets acquired temporarily large-eccentricity orbits and all experienced close encounters with at least oneother planet. This instability phase occurred presumably during the putative “Late Heavy Bombardment” of the terrestrial planets, approximately ∼ 3.9 Gy ago (Tera et al. 1974). The interaction with a massive, distant planetesimal disk (the ancestor of the current Kuiper belt) eventually damped the eccentricities of the planets, ending the phase of mutual planetary encounters and parking the planets onto their current, stable orbits. This new view of the evolution of the solar system makes our system not very different from the extrasolar ones. In fact, the best explanation for the large orbital eccentricities of extrasolar planets is that the planets that are observed are the survivors of strong instability phases of original multi-planet systems on quasi-circular orbits. The main difference between the solar system and the extrasolar systems is in the magnitude of such an instability. In the extrasolar systems, encounters among giant planets had to be the norm. In our case, the two major planets (Jupiter and Saturn) never had close encounters with each other: They only encountered “minor” planets like Uranus and/or Neptune. This was probably just mere luck, as simulations show that Jupiter-Saturn encounters in principle could have occurred. Another relevant difference with the extrasolar planets is that, during the gas-disk phase, our giant planets avoided migrating permanently into the inner solar system, thanks to the specific mass ratio of the Jupiter/Saturn pair and the rapid disappearance of the disk soon after the formation of the giant planets. This chapter ends on a note on terrestrial planets. The structure of a terrestrial-planet system depends sensitively on the dynamical evolution of the giant planets and on their final orbits. It appears clear that habitable terrestrial planets, with moderate eccentricity orbits, cannot exist in systems where the giant planets became violently unstable and developed very elliptic orbits. Thus, our very existence is possible only because the instability phase experienced by the giant planets of our solar system was of “moderate” strength.