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. (15[2])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 (10[1])Heavy Bombardment, (-5[1])according to the Nice (rhymes 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 or outwards. ■END■