  It has been suggested that the gaps and cavities recently discovered in
transitional disks are opened by planets. To explore this scenario, we combine
two-dimensional two fluid (gas + particle) hydrodynamical calculations with
three-dimensional Monte Carlo Radiative Transfer simulations, and study the
observational signatures of gaps opened by one or several planets, making
qualitative comparisons with observations. We find that a single planet as
small as 0.2 MJ can produce a deep gap at millimeter (mm) wavelengths and
almost no features at near-infrared (NIR) wavelengths, while multiple planets
can open up a few *10 AU wide common gap at both wavelengths. Both the contrast
ratio of the gaps and the wavelength dependence of the gap sizes are broadly
consistent with data. We also confirm previous results that NIR gap sizes may
be smaller than mm gap sizes due to dust-gas coupling and radiative transfer
effects. When viewed at a moderate inclination angle, a physically circular
on-centered gap could appear to be off-centered from the star due to shadowing.
Planet-induced spiral arms are more apparent at NIR than at mm wavelengths.
Overall, our results suggest that the planet-opening-gap scenario is a
promising way to explain the origin of the transitional disks. Finally,
inspired by the recent ALMA release of the image of the HL Tau disk, we show
that multiple narrow gaps, well separated by bright rings, can be opened by 0.2
MJ planets soon after their formation in a relatively massive disk.
