The Planck length is defined as $l_p = \sqrt{\frac{\hbar G}{c^3}}$, where $\hbar \approx 1.05457 \times 10^{-34} \text{ J}\cdot\text{s}$, $G \approx 6.67430 \times 10^{-11} \text{ m}^3\text{kg}^{-1}\text{s}^{-2}$, and $c \approx 2.99792 \times 10^8 \text{ m/s}$. What is the value of the Planck length in attometers ($1 \text{ am} = 10^{-18} \text{ m}$)?