Could a Gas-planet the size of Jool exist?

[KASASpace]

Everything in KSP is scaled down, but when I looked at Jool's radius, I noticed it was around 6,000 kilometers. That's ten times Kerbin, which if I recall is supposed to be one-tenth the size of Earth. So, Jool is small.

Now, on to my question, could a gas-planet the size of Jool even form in a young solar system?

[Armchair Rocket Scientist]

Everything in KSP is scaled down, but when I looked at Jool's radius, I noticed it was around 6,000 kilometers. That's ten times Kerbin, which if I recall is supposed to be one-tenth the size of Earth. So, Jool is small.

Now, on to my question, could a gas-planet the size of Jool even form in a young solar system?

Hmm...

The smallest I know of is Kepler-11f at 2.3 earth masses, but its radius is much bigger than Earth due to its low density.

The mass the wiki gives for Jool is 4.23*10^24 kg, or about 71% of earth's mass. At a density of 1.00 g/cm^3 - around what would be expected if it had a large core - it would have a radius of about 10,000 km. According to this little widget that plots whether an object will retain gases, it could only retain helium up to the temperature of Saturn, and hydrogen would escape into space below the temperature of Neptune. On the other hand, if we take the radius of 6,000 km and scale the mass to give a gas planet like density, it would be unable to retain either gas even at the temperature of Neptune.

If we use both Jool's "canon" mass and its "canon" radius, it could actually retain helium up to a little below the temperature of Jupiter, but hydrogen would escape into space. However, with a density of 4.68 g/cm^3, it wouldn't really be a gas planet anymore. That said, a rocky or icy planet can acquire an atmosphere so thick that viewed from space it LOOKS like a gas giant. The obvious examples are Venus and Titan.

Now, Venus's atmosphere is only about 0.01% of the planet's mass. But we can imagine a planet with an atmosphere that's, say, 1% of its mass. For Jool, this atmosphere would have a mass of 2.23*10^22 kg, spread over a surface area of 4.52*10^14 m^2, with a gravitational acceleration of 7.85 m/s^2. This gives a maximum atmospheric pressure of 3800 bars (as opposed to 92 bars on Venus, or 1000 bars at the bottom of the Mariana Trench). Such an atmosphere would crush any spacecraft before it could reach the surface, and might even trap enough internal heat to get rather toasty at depth. Such a planet would be rather similar to Jool.

Finally, if we "scale up" Jool to realistic size (multiply radius by 10 and mass by 100) we get a planet the size of saturn, but only 71 earth masses as opposed to 95, and a density under half that of water. This would imply an extremely light, fluffy planet, which would require the planet to be almost entirely hydrogen and helium an extremely small rocky and icy core. That might start to stretch the limits of planetary formation theories.

So basically, probably not.

[Nuke]

even a gas giant has considerable rocky material at its core. on a smaller gas giant you might see a higher ratio of this material and the gases and ices that make up the rest of it. so essentially what you get is a super earth with a very thick atmosphere. the line between rocky and gas is kind of fuzzy.

[lajoswinkler]

even a gas giant has considerable rocky material at its core. on a smaller gas giant you might see a higher ratio of this material and the gases and ices that make up the rest of it. so essentially what you get is a super earth with a very thick atmosphere. the line between rocky and gas is kind of fuzzy.

Using planetary geology terms, it's not fuzzy at all. Rocky means silicates and metals, gas means hydrogen and helium. Huge terrestrial planets will still have a solid surface and a thick icy (nitrogen, methane) atmosphere.

Gas giants are huge blobs. Gaseous envelope with icy clouds, gradual transition to supercritical fluid (gas/ice in variable ratios), highly compressed plasma with increasingly rocky content towards the center.

There can't be a gas giant the size of Jool. Considering normal matter we encounter in the universe, the densities aren't enough to cause one.