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Looking at the latest theories, I've noticed that most rocky planets are below a certain mass range. I speculate that maybe the way gas giants and gas dwarfs form is a rocky planet during the planetary formation phase of its history accumulates enough mass to begin gaining a hydrogen atmosphere that keeps building until it's no longer a rocky planet.

What would be the critical mass of a rocky planet before it would start down the path of becoming a gassy planet?

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    $\begingroup$ Temperature is also an important factor. $\endgroup$ – PM 2Ring Jul 12 at 13:08
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In most planet formation theories, the boundary is around 10 Earth masses - the build up of the core mass before that is relatively slow, but once it crosses that threshold, the planet gains mass quickly via attracting gas from the surrounding nebula via the core’s gravitational pull, a process called “runaway accretion.” As this summary shows, you can still end up with some planets in between.

One interesting result that has emerged from recent studies of exoplanets is that small planets seem to be divided into two groups, those with radii around 1-1.5 Earth radii, and those with radii of ~ 2-3 Earth radii, but with a gap in between where there are few planets:

Planet radius histogram

The current theory explaining this is that the larger group of planets managed to accrete some atmosphere, while the smaller group are bare, rocky worlds with little or no atmosphere.

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