JOVIAN PLANETS

In terms of both mass and radius, the jovian planets are the true giants of the solar system. The mass of Jupiter alone is 318 times that of Earth! These planets are so massive because they were able to incorporate volatiles when they formed.

During the formation of the solar system, most of the material present was H (hydrogen) and He (helium), with significantly smaller amounts of C (carbon), N (nitrogen), and O (oxygen), and even smaller amounts of things like Si (silicon), Mg (magnesium), and Fe (iron).

Consequently, 98% of the nebula was in for form of gaseous H₂ and He. Of the other 2%, most of that was ices of H₂O, CH₄, and NH₃, and even smaller amounts were in the form of rocks and metals (olivines, pyroxenes, etc.).

Beyond the “snow line” the ices and rocky material could form larger planetary cores (from icy planetesimals) than the terrestrial planets could from rock and metal alone. Once they grew to a critical size, they could also gobble up the nebular gas of H₂ and He. Jupiter and Saturn did this before the nebula dissipated and grew very large. Uranus and Neptune, forming in the more distant low-density part of the nebula, were not so lucky, having been deprived of most of the H₂ and He before they reached the “critical mass” needed to accrete the H₂ and He.

Actually, objects even more massive than Jupiter can form. Astronomers now know of many Brown Dwarfs. These are objects that, although like normal stars in some ways, were never massive enough to undergo the fusion of H into He in their cores (the reaction that powers the Sun and most stars). If they are more than 13-14 times the mass of Jupiter, they may undergo a phase where they fuse deuterium (²H, a heavier isotope of ‘normal” ¹H, and not to be confused with molecular hydrogen H₂, which as 2 separate H atoms bonded together). For convenience, astronomers have begun to adopt this as the “dividing line” between brown dwarfs and planets.

INTERIORS

The physical size of the planet depends on both its mass and its chemical composition. Most materials will compress into higher densities when forced to do so. Adding lots of mass may not make the planet all that much bigger in size, as the added mass contributes more gravity to compress the planet. Based on our knowledge of how various planet-making materials work, we can estimate the internal composition.

However, the situation inside these planets is more complex than this figure would indicate.

Chemical differentiation, driven by gravity, would cause the rock and metal to sink to the core, surrounded by the ices of H₂O, CH₄, and NH₃ (although these might be considered hot compressed liquids that are crushed together so tightly that they are essentially solids). Everything else is H and He (with traces of other stuff) in different forms.

There is no solid surface to these planets. Instead, we usually take the level where P=1 bar as our “reference” level for discussion of the interiors and atmospheres. Here, the H is gaseous H₂, but as we go deeper, the pressure increases to the point where the H₂ is a liquid with its molecules touching. But it is so hot that at no point is there a well-defined liquid-gas surface, just a gradual increase in density with depth. This is referred to as the liquid molecular hydrogen layer.

Eventually, however, the pressure crushes the H molecules so close together that their H atoms start to overlap. Under these conditions, the electrons that formerly orbited the nucleus of the atoms flow from one atom to another, just as a metallic electric conductor like copper. Here, we have a liquid metallic hydrogen layer, and it is indeed a good conductor, and responsible for generating the tremendous magnetic fields of both Jupiter and Saturn. This layer makes up most of the mass of Jupiter, and a good fraction of Saturn.

Uranus and Neptune, with less H and less gravity, probably lack this layer (although they might contain pockets of it very deep). Instead, the H is confined to a relatively thin outer envelope of “normal” liquid H₂.

Three of these planets, Jupiter, Saturn, and Neptune, also emit more energy than they receive from the Sun. Most of this derives from the conversion of PE into TE as they slowly contract. The amount generated must have been very large when these planets were forming, but the tail-end of this process is still apparently continuing today.

This alone is not able to account for the output of Saturn, which requires an additional energy source. The extra heat is believed to come from He condensing and raining down deep inside the planet, re-vaporizing as it goes. (It releases both PE plus heat from friction as it falls). Not surprisingly, the atmosphere of Saturn is depleted in He compared to Jupiter.

Uranus is the heat-wimp of the jovian planets, with little or no extra energy output. No one knows why.