I read that in 1986 Voyager 2 measured the composition of Uranus' atmosphere, which turned out to be composed of $85 \%$ hydrogen and $15 \%$ helium.
It's not clear to me how this relevant amount of He can be present. The force of gravity on Uranus ($8.69 \space \rm m/s^2$) is lower than that measured on Earth ($9.780 \space \rm m/s^2$). Why does helium remain in the atmosphere of Uranus?