8. $\alpha, \beta$ 、 $\gamma$ are acute angles, and $\cos ^{2} \alpha+\cos ^{2} \beta+\cos ^{2} \gamma=1$, then the minimum value of $\operatorname{tg} \alpha \operatorname{tg} \beta \operatorname{tg} \gamma$ is $\qquad$.
$2 \sqrt{2}$.