Planets cool off through:

conduction - heat flowing on the microscopic level
convection - heat flowing on the macroscopic level (bulk motions) - eurptions
eruptions - hot lava bursts through crust
radiation - Earth's surface loses both internal heat (and surface heating due to the Sun) into space by radiating in the INFRA-RED.

The larger the planet, the longer it takes to cool off

4 x pi x r2 / 4/3 x pi x r3 = 3 / r
Therefore, larger planets have comparatively smaller Surface Area-to-Volume ratios….and hence cool slower than smaller planets !