Wouldn't gravity be a good explanation? Once your rocky core grows beyond a certain size, you start retaining some gas molecule X. If you are too small, the thermal velocity for the gas X is bigger than escape velocity and thus you can't retain it. So you generally don't have planets right at the small end of the X retaining size. They always grow bigger because they sweep all of X. Also it makes them heavier so they can hold onto X better, even if, say the sun becomes brighter. What is X? Helium?
This is indeed one of the possible theories to explain the Fulton gap, as far as I understand it. If you have a rocky core that accretes more and more atmosphere, the atmosphere collapses once it contains about as much mass as the core and you become much more efficient at accreting more gas and you form a gas giant.
If this is true, would that mean that life is rare in the Universe? I would mean that we are an anomaly in that we have the perfect size and core materials to be able to hold onto some of our atmosphere, but not so big so as to turn into a gas giant.
Kinda sad to think that there may be nothing but space out there.
A planet can have a atmosphere that is sufficient for life, but of negligable mass compared to the core. In our solar system we have at least 2.5 examples of that: Earth, Venus and Mars. And that is not counting ocean moons with an ice crust such as Europa that might be hospitable to life as well.
