Excluding 'fluffy' or 'puffy' gas giants that are 'inflated by heat from their stars, what is the maximum radius of a gas giant planet. I keep reading things like "...Jupiter is 'about' as big as they get...", but I can't seem to find a specific number to go with that word "about".

Assuming a gas giant was made up of the highest plausible ratio of Hydrogen, and the lowest plausible ratios of other elements (and not significantly heated by the star), just how big could a gas giant get before it started shrinking again under its own weight/gravity?

Is it really just 1 Jupiter radius? 1.0001? 1.1? 1.3?

  • $\begingroup$ What is the "lowest plausible ratio"? $\endgroup$
    – ProfRob
    Commented Dec 26, 2018 at 22:58
  • $\begingroup$ @RobJeffries The different color sections in the image from your answer is exactly what I was referring to with regards to 'plausible ratios'. Mostly, I just meant it to exclude compositions that are not likely to be possible in natural planetary formation $\endgroup$
    – Harthag
    Commented Dec 28, 2018 at 17:10

1 Answer 1


According to the models of Mordasini et al. 2012 it is about 13 Earth radii (about 1.2 Jupiter radii) for gas giants older than a billion years.

Younger gas giants that are still contracting significantly can be bigger. How much bigger is wrapped up in uncertainties in the formation mechanism and the core mass and composition.

Gas giant mass-radius relationships from Mordasini et al. 2012. The colour coding corresponds to the fraction of heavy elements in the planet. The larger planets have smaller fractions of heavy elements.

Planet models


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