The mass of the planet is 6.67×10^25 kg
let the mass of the planet be M and r be the radius of the planet and g be the gravitational acceleretion.
then:the gravitational acceleration is given by:
g = G×M/(r^2)
M = g×r^2/G
= [(18)×(1.2×10^7)^2]/(6.67408×10^-11)
= 6.67×10^25 kg
therefore, the mass of the planet is 6.67×10^25 kg
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