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Giant protoplanets formed by gravitational instability in the outer regionsof circumstellar disks go through an early phase of quasi-static contractionduring which radii are large and internal temperatures are low. The main sourceof opacity in these objects is dust grains. We investigate two problemsinvolving the effect of opacity on the evolution of planets of 3, 5, and 7 M_J.First, we pick three different overall metallicities for the planet and simplyscale the opacity accordingly. We show that higher metallicity results inslower contraction as a result of higher opacity. It is found that thepre-collapse time scale is proportional to the metallicity. In this scenario,survival of giant planets formed by gravitational instability is predicted tobe more likely around low-metallicity stars, since they evolve to the point ofcollapse to small size on shorter time scales. But metal-rich planets, as aresult of longer contraction times, have the best opportunity to captureplanetesimals and form heavy-element cores. Second, we investigate the effectsof opacity reduction as a result of grain growth and settling, for the samethree planetary masses and for three different values of overall metallicity.When these processes are included, the pre-collapse time scale is found to beof order 1000 years for the three masses, significantly shorter than the timescale calculated without these effects. In this case the time scale is found tobe relatively insensitive to planetary mass and composition. However, theeffects of planetary rotation and accretion of gas and dust, which couldincrease the timescale, are not included in the calculation. The short timescale we find would preclude metal enrichment by planetesimal capture, as wellas heavy-element core formation, over a large range of planetary masses andmetallicities.

The Effects of Metallicity, and Grain Growth and Settling on the Early Evolution of Gaseous Protoplanets