What is one another way besides low tempurature that hydrogen and helium can remain on a planet?

Answer 1

Exactly. Here on Earth there is a lot of hydrogen in the form of water in all the oceans, though for industrial purposes it is usually derived from oil, which is composed of carbohydrates. Gas giants have much hydrogen trapped in methane. But helium is completely unreactive, and by itself it is so light that at medium temperatures the velocity of some helium atoms in the atmosphere exceeds escape velocity and soon they are all gone, unless the temperature is very low or gravity is very strong. And it is unlikely that helium would get trapped in air pockets beneath the surface, because it can diffuse through solid matter over long periods of time due to its small size. However, there is still a little bit of helium here on Earth, which comes from nuclear alpha-decay in the core and trickles up to the surface.

Answer 2

Hydrogen reacts with almost everything, forming heavy molecules that don't easily leave the atmosphere. Helium is light and can easy leave the atmosphere, so the only way to prevent this is to have either a very think atmosphere, or a very large planet.

Answer 3

yes get a large enough planet to where the gravity will hold a substantial atmosphere, think about mercury or the moon, not enough mass to hold onto anything substantial, other than a tiny amount of sodium and that's only because sodium is a heavier element and will more willingly stay.

Answer 4

I'd put them in closed bottles.