if the sun fell into the pacific ocean?

If we look at neutral particles now, so only gravity (of the sun and earth):

All molecules have a

(mean) velocity in a gas, which is dependent on the temperature and their molar weight. Smaller molecules travel faster. The "rock-molecules" will be fairly heavy (being iron oxides and such), and will not have a high velocity.

I propose the following (I never read any theory about it, it just seems to make sense):

The point where a planet will lose its atmosphere is when the escape velocity of the planet is less than the average velocity of gas molecules in its atmosphere.

Therefore, an atmosphere of heavy molecules will not be easily removed from a planet as for example a hydrogen atmosphere.

(Now I'll prepare to get hammered down. I neglected incoming photons and other particles from space and the sun, I neglected electricity, magnetism... etc, etc )