Which Planets Have an Atmosphere? Effect of Radiation

All other things being equal, the more high-energy light a planet receives from its star, the less likely it is to have an atmosphere. When starlight — particularly X-rays and ultraviolet light — reaches a planet, gas molecules in the atmosphere heat up and move very fast. If they absorb enough energy to move faster than the escape velocity of the planet, they can fly off into space. The more energy the planet receives from its star over time, the harder it is for the planet to hold on to its atmosphere. This is one reason why Mercury has only a trace atmosphere.