Other
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.u43b..02s&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #U43B-02
Other
5455 Origin And Evolution, 5749 Origin And Evolution, 6040 Origin And Evolution, 6207 Comparative Planetology
Scientific paper
Current observational evidence suggests that planetary systems are common. We do not yet know whether Earthlike planets are common. Many aspects of planetary system construction are easily understood, almost unavoidable. Other aspects are less obvious, even somewhat mysterious. In all likelihood, the diversity of outcomes is large and non-deterministic and our particular outcome is correspondingly uncommon despite the abundance of systems. Formation of a disk during star formation is almost unavoidable and provides the natural source of planet-forming material. Presence of solid particulate matter in the disk is assured. Overall abundances of "gas", "ice" and "rock" are also likely to be very similar from system to system. However, the aggregation of solid matter into planetesimals is not assured and remains poorly understood. Their formation in our solar system is not in doubt because of meteoritical evidence. Aggregation from planetesimals to larger solid bodies is unavoidable except at high (and improbable) relative encounter velocities. This process may proceed rapidly to bodies of Moon or Mars sized (or even larger in the giant planet zone). Perhaps surprisingly, the least well understood aspect of planet formation is the bodies that are evidently quite common: giant planets (including Uranus and Neptune ice giants). Direct gravitational instability from the disk might work but may fail to explain our giant planets, especially Uranus and Neptune. The more popular process of solid accumulation followed by gas inflow is potentially too slow relative to the time scale of gas loss from the nebula. Theory suggests that inward spiral of planets imbedded in gaseous disks is expected and in that sense the extrasolar systems found thus far are understandable. However, the range of possible outcomes is large, our knowledge of these is still quite primitive and there may be many surprises ahead. It's an exciting time in the planet system game!
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