Re: does every planet have a atmosphere

The short answer is no! In our solar system, Mercury does not have an 
appreciable atmosphere and Pluto has a very thin one only when closest to 
the Sun.

Whether a planet is able to hang on to its atmosphere is mostly dependent 
on its size. On Earth, the 'escape velocity' is around 7 miles per second --
in other words, any molecule of gas moving at more than this speed will 
escape permanently. On Mars, where the escape velocity is much lower 
(around 3 miles per second) as the planet is smaller, the atmosphere is 
much thinner, much having been lost long ago.