Scientists Have the Answer Why There are No Gas Moons
- NASA
When gas is hot, it floats, so to continue shrinking, the gas blob must release its heat. As a result, astronomers often see collapsing gas clouds glowing in hot infrared light.
However, there is a limiting factor called the blur limit for fragmentation. Sam Pearson from the European Space Agency (ESA) says that this process will emit enough heat to cool the gas and still collapse, depending on the opacity of the dust, as well as its temperature and density.
The smaller the volume, the denser and more opaque the dust, and the less efficient the process of releasing excess heat due to gravitational contraction. So, nobody smaller than 3 times the mass of Jupiter can form in a top-down process.
The answer to why the solar system has no gas moons. Most of the moons in the solar system formed through a bottom-up core accretion process. This happens in the disk of leftover material that surrounds the parent planet.
Since the host planet has absorbed most of the material, there is no material left to form massive moons, as it does not have strong gravity to pull in much gas.