Type I migration timescale from disk torque
= Type I migration timescale from disk torque
{c}
{title2=$t_a=a/|\dot a|$}
For a circular fixed-<mass> <planet>, $L=M_pa^2\Omega$ and $\dot a=2\Gamma/(M_pa\Omega)$. With $\Gamma=C(q/h)^2\Sigma a^4\Omega^2$, the radial migration timescale is $a/|\dot a|=(M_\star/M_p)[M_\star/(\Sigma a^2)]h^2/(2|C|\Omega)$. The factor of two distinguishes radial evolution from $L/|\Gamma|$. The dimensionless coefficient and its sign depend on the disk model.