How is the size defined for a gas planet? The gas density just keeps dropping, where do you draw the line (isosurface, rather)? Earth's radius is always the one without earth's atmosphere.
Two-body gravitational attraction is observed to be an inverse square power law; gravitational attraction decreases with the square of the distance.
g, the gravitational constant of Earth, is observed to be exponential; 9.8 m/s^2.
Atmospheric pressure: https://en.wikipedia.org/wiki/Atmospheric_pressure#:~:text=P... :
> Pressure (P), mass (m), and acceleration due to gravity (g) are related by P = F/A = (m*g)/A, where A is the surface area. Atmospheric pressure is thus proportional to the weight per unit area of the atmospheric mass above that location.
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> 40–1 The exponential atmosphere
The most important consideration for a definition is its practical consequence.
In this case, whether the line is drawn at 1 bar or an order of magnitude more or less doesn’t materially change that, on the same measure, Jupiter was 2x larger in the past. (Less than 1% in both cases.)
In a different context, that difference may be meaningful and should thus be noted and tested for robustness.