Areal velocity $\frac{\mathrm{dA}}{\mathrm{dt}} \&$ angular momentum $L$ of a planet are related by equation $\frac{d A}{d t}=\frac{L}{2 M}$, where $\mathrm{M}$ is the mass of planet. Since in planetary motion $\overline{\mathrm{L}}$ is constant $\left(\bar{\tau}_{\text {ext. }}=0\right)$, hence $\frac{\mathrm{dA}}{\mathrm{dt}}$ is also constant.