This is a drawing of possibilities for the atmosphere.
Click on image for full size
A big question for scientists is "where did a planets' atmosphere come from?"
There are four ideas for where the atmosphere comes from, which we will talk more about later. The four ideas have to do with whether the planet and its atmosphere formed together or whether the atmosphere was created within the planet itself.
This picture shows one idea for how an atmosphere may have formed around a planet. The atmosphere was drawn to the planet from the cloud out of which everything came. The atmospheres of the giant planets may still be largely made of that original material. The atmospheres of the Earth-like planets have almost certainly changed since then.
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