Mean temperature, in the context of planets, refers to the average temperature across the surface or atmosphere of a planet. It provides a measure of the overall thermal condition of a planet and is often used to characterize and compare the climates of different celestial bodies.

The mean temperature of a planet is influenced by various factors, including its distance from the Sun, its atmosphere, greenhouse gases, albedo (reflectivity), and internal heat sources. These factors interact to determine the distribution of heat and energy across the planet.

The distance from the Sun plays a significant role in determining the mean temperature of a planet. Planets closer to the Sun, like Mercury and Venus, receive more solar energy and tend to have higher mean temperatures. Conversely, planets farther from the Sun, such as Neptune and Pluto, receive less solar energy and exhibit lower mean temperatures.

The presence and composition of an atmosphere also affect the mean temperature. An atmosphere can trap heat through the greenhouse effect, where certain gases like carbon dioxide and methane absorb and re-radiate thermal radiation, warming the planet's surface. This is evident in the case of Venus, which has a thick atmosphere that leads to extremely high mean temperatures due to the greenhouse effect.