Evolution of the semi-major axis of a planet embedded in an isothermal disk for different values of the disk mass. The graph in red corresponds to the simulation of the standard disk model with the planet initially starting at a₀ = 1.0 AU. The graph in blue shows a similar simulation but with half the disk mass. The graph in green corresponds to a simulation with a quarter of the initial disk mass that is continued from the model in blue at t = 4300.