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Of course it's gravity- how can magnetism act on gases? Other factors are needed because gravity is constant; rotational speed, greenhouse effect, solar intensity, etc
AFAIK, and as b_b suggests, the magnetic field can protect the atmosphere from the solar wind. According to Wikipedia: Not something I knew about.The solar wind affects the other incoming cosmic rays interacting with the atmosphere of planets. Moreover, planets with a weak or non-existent magnetosphere are subject to atmospheric stripping by the solar wind.