Question

there is no atmosphere around many planets,then why there is atmosphere around earth

Answer 1

Venus,which is about the same size and mass as earth has a thick atmosphere made up of carbon dioxide.Mars which is 15% the mass of the earth has a thin atmosphere,because it can only retain the more massive molecules like CO2.The pressure at the surface of mars is 100 times smaller than on earth.Whether or not a planet retains an atmosphere over geologic time depends on the mass of the planet,the radius of the planet the temperature of the atmosphere and the composition of the atmosphere.