Despite of what we might think, this phenomenon is not affected by the different physical properties of stars and planets and is not related to the fact that stars shine with their own light while planets reflect sunlight. The explanation lies in the properties of the Earth’s atmosphere and the different distances between planets and stars.
The Earth’s atmosphere is an extremely complex structure, which at first approximation can be described as composed of a series of parallel layers in turn divided into vortices of about ten centimeters in diameter (the ovals in the figure). Each of these vortices is characterized by physical properties (refractive index, temperature, etc.) that vary slightly from one another. When the wave front coming from a star (that is its light) passes into the atmosphere it is refracted and distorted. Atmospheric vortices are in constant motion due to the wind: in a subsequent instant of time the wave front will pass through a different atmospheric layer and will be refracted in a different direction. Integrated over time, this phenomenon gives rise to the scintillation of the stars, that is to their continuous variation in brightness.
According to what has been said so far we should observe the scintillation also for the planets, but we must take into account their distance.
The stars are in fact very far away and appear as point-like objects even at the most powerful of telescopes. Planets are not point-like, but they are extended objects that can be solved. By some calculation it is found that the average size of the wavefronts coming from the stars is about ten centimeters (comparable to that of atmospheric vortices), while the wave fronts of the planets have an extension on the order of the meter. These wavefronts when they pass into the atmosphere are spread over several vortices and the deviations induced by refraction are mediated, resulting null when integrated in time. Consequently, the light of the planets appears fixed.
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