Explain how you can weigh a planet?

( Is it by the combination of the netwons law of univeral gravitation and keplers third law; t^2/r^3 = 4(pi)^2/G(Msun) )

I assume you mean massing a planet, not weighing. Weight is the action between two masses due to gravity.

http://www.physics247.com/physics-tutorial/mass-planet.shtml