A THREE PHASE MODEL FOR PLANET FORMATION - THE FORMATION OF A PLANET IN THE EYE OF A HURRICANE

摘要

Neither one of the classical formation scenarios for giant planets seems sufficient. The disk instability model has problems to explain the cooling of the disk that is required, and the core accretion model is too slow for the standard solar nebula to lead to the formation of Jupiter before the disk has dissipated either viscously or via evaporation. One way out of this dilemma is a new scenario that combines the virtues of both models into a new one. We claim that vortices play a crucial role in the formation of planets. They are probably formed naturally in protoplanetary accretion disks from a so called Global Baroclinic Instability that arises if the radial entropy gradient is strong enough. The vortices show up as huge stable anti-cyclonic rotating gas masses, that can be regarded as planetary precursors for two reasons. First they produce peaks in the gas surface density about four times above the ambient medium, and second they concentrate very efficiently all solid particles above a certain size in their center. Thus vortices can be the preferred formation sites for planets. A three-stage formation scenario can be invoked: (1) the formation of vortices, (2) the concentration of solid particles in a vortex and the buildup of a planetary core, and (3) the accretion of gas onto the core. Using ALMA it will be possible to do direct imaging of vortices. In addition, we suggest that the eclipses of the object KH15D can be explained by the presence of a vortex in the inner region of a circumstellar disk.