Helium confined to a gravity well It's a well known problem that Earth is slowly losing its supply of helium due to helium's ability to "bubble off" the atmosphere.  All the gas giants have significant percentages of helium in their atmosphere so they are big enough.  What's the minimum planet mass to hold onto atmospheric helium?
 A: You have to compare the most probable speed of the atoms of He with the escape velocity from the planet.
So you are working out whether
$$ \left(\frac{2kT}{m} \right)^{1/2} > \alpha\ \left(\frac{2GM}{R}\right)^{1/2},$$
where $T$ is the local temperature, $m$ is the He atom mass, $M$ is the planet mass and $R$ is the radius at which you are considering the escape speed.
The parameter $\alpha$ is there because even if the average thermal velocity is well below the escape speed, there will still be some fraction of those atoms in a Maxwell-Boltzmann distribution that will have enough kinetic energy to escape. But the escape also needs to occur in a region of the atmosphere that is sufficiently sparse that an energetic atom can escape before interacting with something else.
The value of $\alpha$ is not an exactly determined quantity, the rate of "Jeans escape" (as it is known) will increase with decreasing $\alpha$. Often a number like $\alpha \sim 0.2$ is used if the atmosphere is to escape within hundreds of millions of years. 
Anyway, the bottom line to this is that although planet mass features in this criterion, it is not the only parameter. The radius of the atmosphere from which an escape is hypothesised and its temperature are equally influential.
