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A gas giant planet is large enough that it retains a lot of hydrogen and helium. A rocky planet is one with a solid surface. That's the rule of thumb to distinguish between the rocky and gaseous planets.

But gaseous planets can have a solid core and rocky planets can have a very thick atmosphere. So is there a clear-cut difference between the two categories of planets (perhaps linked to different formation histories), or is there a continuum of planets that goes from more rocky to more gaseous? If it's a continuum, where is the limit set between the two types?

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  • $\begingroup$ Answering this question would require knowing the bulk composition of lots of planets which I don't think we know. In most cases we only know the radius or an estimate of the mass but not both and even when we know both there are a range of compositions with the same radius and mass. As well as rocky and gaseous planets there are also possibly water worlds. There is the Fulton gap in the radius distribution with not many planets between 1.5 and 2 Earth radii so that could separate rocky from gaseous planets although there are outliers like rocky planets with mass greater than Neptune. $\endgroup$ – sno Mar 6 at 5:42
  • $\begingroup$ The article astronomy.com/news/2020/01/… claims there are only two types of planet: small rocky or large gassy. However this ignores gas dwarfs and mega-earths. $\endgroup$ – sno Mar 6 at 6:44
  • $\begingroup$ The density is known for many : for every transiting planet where also rv measurements are done $\endgroup$ – planetmaker Mar 6 at 7:06
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The currently-detected planets do not show a clear distinction between rocky and gaseous planets. While there seem to be somewhat two ensembles, the rocky planets of Earth-size and "super-earths" and the gaseous planets (Jupiter-size), there is a broad transition between them. The transition is roughly where we find Neptune and Uranus. Play around with plotting various parameters at exoplanet.eu

It's still subject to debate whether this apparent depletion in the 'ice giants' (cold, gaseous, but with heavier gases like Methane etc instead of Hydrogen and Helium as main constituents) is real or due to observational bias.

In brevity:

The argument for "is real" is that it is more likely that in the inner nebula, inside the snow zone you have enough material to form terrestrial planets, but being too hot and too few to grow to a size that a sizable atmosphere can be retained. Outside the snow zone you have plenty of more solids (all the ices), so that you form cores which become big enough to also accrete an atmosphere.

However, the counter-argument is that you do find planets of all sizes in all kind of stellar distances, you have various initial conditions with different densities of solids in protoplanetary discs, and thus any final result is possible with possibly various probability. The actual observations seem to support this.

Generally, in current literature and on conferences, planets are often distinguished further than just rocky and gaseous: you have terrestrial, super-earths, neptune-likes and the gas giants, and often then somewhat separated even the so-called hot jupiters and hot neptunes, the gas giants which migrated inwards after formation.

EDIT: one possible distinction which could be made between rocky and gaseous is looking at the atmosphere : if the atmosphere gets dense enough at the lower levels that the distinction between gas and liquid vanishes (thus the critical point reached) , it's a gas planet, otherwise a Rocky planet with atmosphere.

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  • $\begingroup$ Glad last paragraph was added as it specifically answers the question. $\endgroup$ – antlersoft Mar 6 at 15:44
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    $\begingroup$ The problem with that distinction is knowledge of the equation of state. $\endgroup$ – planetmaker Mar 6 at 16:20

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