Have you guys seen the SUPER SUPER LARGE sealed ecosystem. It's called earth. It's amazing how long it has lasted without any significant mass coming in or going out.
Earth also loses helium and hydrogen. The current rate of loss is about three kilograms (3 kg) of hydrogen and 50 grams (50 g) of helium per second, according to Wikipedia. http://en.wikipedia.org/wiki/Atmospheric_escape
