The ratio of the weights of a body on Earth's surface to that on the surface of a planet is $9 : 4$. The mass of the planet is $\frac { 1 } { 9 }$th of that of the Earth. If $R$ is the radius of the Earth, what is the radius of the planet? (Take the planets to have the same mass density)\\
(1) $\frac { R } { 4 }$\\
(2) $\frac { R } { 2 }$\\
(3) $\frac { R } { 3 }$\\
(4) $\frac { R } { 9 }$