Final answer:
The Sun's heat is not trapped within a planet's atmosphere because of complex balancing factors like the radiation budget and greenhouse effect, which vary based on each planet's unique characteristics such as atmospheric composition, distance from the Sun, and surface reflectivity.
Step-by-step explanation:
The Sun's heat is not trapped inside a planet's atmosphere indefinitely because of several dynamical processes that balance heat absorption and heat loss. For example, when considering Earth's radiation budget, which relates to how much solar energy is absorbed versus how much is retained or reflected, the balance is crucial for maintaining surface temperatures necessary for liquid water. Earth achieves this balance through its greenhouse effect, which acts like insulation to keep the heat in. However, atmospheres differ from planet to planet, and factors such as distance from the Sun, atmospheric composition, and surface reflectivity affect the ability of an atmosphere to trap heat. On planets with weaker or no atmospheres, like Mars, any heat that is absorbed during the day is more easily lost to space at night, preventing the heat from building up and being retained. The greenhouse gases within an atmosphere absorb infrared radiation emitted by the planet's surface and reradiate it in all directions, which helps to warm the planet's surface. Without a sufficient atmospheric greenhouse effect, or if a planet is simply too far from the Sun, such as the outer gas giants, the heat is not retained efficiently, resulting in much cooler surface temperatures.