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We have generated an extensive new suite of massive giant planet atmospheremodels and used it to obtain fits to photometric data for the planets HR 8799b,c, and d. We consider a wide range of cloudy and cloud-free models. The cloudymodels incorporate different geometrical and optical thicknesses, modalparticle sizes, and metallicities. For each planet and set of cloud parameters,we explore grids in gravity and effective temperature, with which we determineconstraints on the planet's mass and age. Our new models yield statisticallysignificant fits to the data, and conclusively confirm that the HR 8799 planetshave much thicker clouds than those required to explain data for typical L andT dwarfs. Both models with 1) physically thick forsterite clouds and a60-micron modal particle size and 2) clouds made of 1 micron-sized pure irondroplets and 1% supersaturation fit the data. Current data are insufficient toaccurately constrain the microscopic cloud properties, such as composition andparticle size. The range of best-estimated masses for HR 8799b, HR 8799c, andHR 8799d conservatively span 2-12 M_J, 6-13 M_J, and 3-11 M_J, respectively andimply coeval ages between ~10 and ~150 Myr, consistent with previously reportedstellar age. The best-fit temperatures and gravities are slightly lower thanvalues obtained by Currie et al. (2011) using even thicker cloud models.Finally, we use these models to predict the near-to-mid IR colors of soon-to-beimaged planets. Our models predict that planet-mass objects follow a locus insome near-to-mid IR color-magnitude diagrams that is clearly separable from thestandard L/T dwarf locus for field brown dwarfs.

Model atmospheres for massive gas giants with thick clouds: Application to the HR 8799 planets and predictions for future detections