In 2D models, the timescale of this process is proportional to the 1.75 power of a gravitational softening parameter and the square of the scale height-to-radius ratio. In one regime, this process occurs when density waves are excited by Lindblad and corotation resonances, creating a strong torque. Large objects can clear a gap in a gaseous disc to undergo the “type II” form of this process. A specific example of this process is thought to have reversed due to a transient (*) 2:1 orbital resonance in the “Grand Tack” model. The occurrence of this process in the outer solar system perturbed the orbits of many comets, causing the Late Heavy Bombardment, according to the Nice (rhymes (-5[1])with “geese”) model. For 10 points, name this process in which interactions with other celestial bodies cause a planet to change its orbit and move inwards (0[4])or outwards. ■END■