  We present a massive accreting gap (MAG) planet model that ensures large gaps
in transitional disks are kept dust free by the scattering action of three
co-planar quasi-circular planets in a 1:2:4 Mean Motion Resonance (MMR). This
model uses the constraint of the observed gap size, and the dust-free nature of
the gap, to determine within ~10% the possible orbits for 3 massive planets in
an MMR. Calculated orbits are consistent with the observed orbits and H-alpha
emission (the brightest line to observe these planets) for LkCa 15 b and PDS 70
b and PDS 70 c within observational errors. Moreover, the model suggests that
the scarcity of detected H-alpha planets is likely a selection effect of the
current limitations of non-coronagraphic, low (<10%) Strehl, H-alpha imaging
with Adaptive Optics (AO) systems used in past H-alpha surveys. We predict that
as higher Strehl AO systems (with high-performance custom coronagraphs; like
6.5-m Magellan Telescope MagAO-X system) are utilized at H-alpha the number of
detected gap planets will substantially increase by more than tenfold. For
example, we show that >25 new H-alpha "gap planets" are potentially
discoverable by a survey of the best 19 transitional disks with MagAO-X.
Detections of these accreting protoplanets will significantly improve our
understanding of planet formation, planet growth and accretion, solar system
architectures, and planet disk interactions.
