Why does earth have atmosphere whereas other don't?

Answer 1

Earth has an atmosphere due to its gravity and magnetosphere. Other planets do have atmospheres (Venus, Mars, Jupiter, Saturn, Uranus, etc.) and even some moons have been shown to have them. Some planets lose their atmospheres or don't have them at all due to either their small size preventing them from holding onto the gases required to form an atmosphere, or their lack of geological activity (a molten core like earth possesses) meaning there is no magnetosphere (a magnetic field surrounding the planet that redirects particles from the sun around the planet) which causes the atmosphere to be eroded away over time by solar wind (a constant stream of charged particles emitted by the sun) and lost into space. Our magnetosphere and the interaction with particles from the sun and those in the upper atmosphere are the cause of the Aurora Borealis.

This is a very basic explanation, for more information on the topic look to wikipedia.

Answer 2

Earth has an atmosphere primarily because of its gravity, which enables it to hold onto gases such as nitrogen and oxygen that make up the atmosphere. Other celestial bodies like the moon or Mars have weaker gravity which results in thinner atmospheres or none at all.