The image associated with this post is a slide taken from a MOOC called BUx: ASTR105x Alien Worlds: The Science of Exoplanet Discovery and Characterization offered at EdX. What it shows is the speed of various gas molecules at various temperatures. These are shown at lines showing the speed the molecules move at those temperatures. Notice that heavier gases move slower at a given temperature but all increase at a similar rate as temperature increases. Also plotted on the chart are various solar system objects at their temperature and on their escape velocity. A planet will hold all the molecules which are moving slower than the escape velocity of the planet at the planets temperature for as long as the solar system exists. Gasses above that will eventually drift away.
I just found this graph to interesting to pass up.