Q. Why the Earth is having its own atmosphere?

Explanation by: Madan

Atmosphere contains particles and gases which do not escape to outer space because they are bounded by the force known gravity. Earth's gravity comes from all its mass. All its mass makes a combined gravitational pull on all the mass in your body. That's what gives you weight. And if you were on a planet with less mass than Earth, you would weigh less than you do here.