Our solar system has four jovian or giant gas planets; Jupiter, Saturn, Uranus, and Neptune. They are not really made only of gas. They are named the gas giants because that is how they appear to the eye.

The large, outer gas layers are less than 10% of Jupiter and Saturn’s masses. In deep layers, under high pressure, the dense gas changes into a liquid. The transition from gas to liquid is gradual and occurs at about 1.4 million Earth atmospheres. For Jupiter, the transition occurs at about 4,500 miles under its surface (10% of its radius). For Saturn, the transition occurs at 19,000 miles (50% of its radius.) The liquid molecular hydrogen transforms into liquid metallic hydrogen, which conducts electricity. The currents in the liquid layer generate the powerful magnetic fields of Jupiter and Saturn.

Because of the tremendous pressures, Jupiter and Saturn have solid cores. Their sizes and densities have been calculated based on the rotational flattening at their poles. Jupiter’s polar diameter is 6% less than its equatorial diameter. Scientists believe Jupiter must have a rocky core 10 times more massive than Earth (3% of Jupiter) and about 1.5 times Earth’s diameter. Saturn is flattened by about 10% at the poles, so it probably has a larger core about 2.5 times the size of Earth with about 15 times the mass of Earth.

Jupiter’s internal pressure is 40 million atmospheres with the temperature at about 41,000 degrees Fahrenheit. An ice-rich outer core (made of different substances like water, ammonia, and methane) surrounds its inner rock core made of iron and silicon. Scientists call this ice-rock mixture a “clathrate”.

Uranus and Neptune are less massive than Jupiter and Saturn, so their internal pressures never reach more than 100,000 atmospheres. It’s too low for liquid molecular hydrogen to turn into metallic hydrogen and become electrically conductive. Both planets do have strong magnetic fields generated by their layers of hot, swirling of water, ammonia, and methane mixtures beneath their hydrogen layers.

Uranus and Neptune have rock-ice cores of about 1.2 Earth diameters and 10 Earth masses. Their cores are a large part of their interiors at 60 to 70% of their mass and 30 to 40% of their radius.