How to Atmosphere's stay stable?

[whowhatwhy]

Ok, so I should know better. But after looking at some recent video footage of the space shuttle and, more precisely, the atmosphere surrounding our planet, it got me thinking about how it all works. Space is a (near as hell) vacuum so why don't gasses produced on earth escape into space? Ok, so we have gravity. Is this the only force in action here to prevent the gases moving from an area of high pressure into a low pressure are? How come the force of gravity is just strong enough to hold the gas particles at a perfect balance to create our atmosphere and is it just the mass of our planet that defines it's gravitational influence? Cheers!

[whowhatwhy]

sorry, i should have posted this thread in the science section. please just ignore this as I have reposted it there.