Suppose two planets (spherical in shape) of radii $R$ and $2 R$, but mass $M$ and $9 M$ respectively have a centre to centre separation 8 R as shown in the figure. A satellite of mass ' m ' is projected from the surface of the planet of mass ' M ' directly towards the centre of the second planet. The minimum speed ' v ' required for the satellite to reach the surface of the second planet is $\sqrt{\frac{a}{7} \frac{G M}{R}}$ then the value of ' $a$ ' is $\_\_\_\_$ .
[Given : The two planets are fixed in their position]