Implications of the Giant Planets 75 IMPLICATIONS OF THE GIANT PLANETS FOR THE FORMATION AND EVOLUTION OF PLANETARY SYSTEMS

The giant planet region in our Solar System appears to be bounded inside by the limit of water condensation, suggesting that the most abundant astrophysical condensate plays an important role in giant planet formation. Indeed, Jupiter and Saturn exhibi~ evidence for rock and/or ice cores or central concentrations that probably accumulated first, acting as nuclei for subsequent gas accumulation. This is a "planetary" accumulation process, distinct from the stellar formation process, even though most of Jupiter has a similar composition to the primordial Sun. Uranus and Neptune are more complicated and imperfectly understood, but appear to exhibit evidence of an important role for giant impacts in their structure and evolution. Despite some interesting systematics among the four major planets and their satellites, no simple picture emerges for the temperature structure of the solar nebula from observations alone. However, it seems likely that Jupiter is the key to our planetary system and a similar planet could be expected for other systems. The data and inferences of these data are summarized for the entire known Solar System beyond the asteroid belt.