Why doesn't the earth become superheated?
A body has to emit as much thermal energy as it absobs to remain in thermal equilibrium. The Earth has been doing the same since  its formation, i.e, it absorbs solar  shorwave radiation by its atmosphere, solid earth and water body, and releases it in the form of longwave radiation. The amount that is released has to be equal to the amount that is absorbed to maintain same temperature. However, due to much emission of greenhouse gases in to the atmosphere (or according to sceptics of such concept due to solar activity, volcanic emission), some amount that the Earth has to release is absorbed by the gases in the atmosphere and are causing the Earth to warm by few degrees every year.