Why do planets close to the sun have thin atmospheres and the planets far away from the sun have thick atmospheres?

There are two primary factors for planets having thin atmospheres: size and distance from the Sun. Gravity helps planets and moons to hold on to their atmospheres, so small planets/moons such as Mars and the Moon have thin atmospheres.

Like the sun, the gas giants are composed mainly of hydrogen and helium. Because they are so massive, they exert a much stronger gravitational force than the terrestrial planets. This prevents their gases from escaping, so they have thick atmospheres.

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