  In the core accretion model of giant planet formation, a solid protoplanetary
core begins to accrete gas directly from the nebula when its mass reaches about
5 earth masses. The protoplanet has at most a few million years to reach
runaway gas accretion, as young stars lose their gas disks after 10 million
years at the latest. Yet gas accretion also brings small dust grains entrained
in the gas into the planetary atmosphere. Dust accretion creates an optically
thick protoplanetary atmosphere that cannot efficiently radiate away the
kinetic energy deposited by incoming planetesimals. A dust-rich atmosphere
severely slows down atmospheric cooling, contraction, and inflow of new gas, in
contradiction to the observed timescales of planet formation. Here we show that
photophoresis is a strong mechanism for pushing dust out of the planetary
atmosphere due to the momentum exchange between gas and dust grains. The
thermal radiation from the heated inner atmosphere and core is sufficient to
levitate dust grains and to push them outward. Photophoresis can significantly
accelerate the formation of giant planets.
