The atmosphere is held around the Earth by gravity. Gravity pulls gas molecules in the atmosphere toward the Earth's surface, causing air pressure. Air pressure is the measure of the force with which air molecules push on a surface.
Our atmosphere is a mixture of gases that surround Earth. It is kept in place by the pull of Earth's gravity. If Earth was a much smaller planet, like Mercury or Pluto, its gravity would be to weak to hold a large atmosphere.
