r=Radius of planet
T1=Temperature of planet at which energy is radiated
Power radiated by Sun, P=4πR2σ T4o
Energy received by planet =P4πd2×πr2=R2T4oπr2σd2
Energy radiated by planet=4πr2σ T41
For thermal equilibrium, R2T4oπr2σd2=4πr2σT41
R2T4o=4d2T41
T1=T0√R2d