Definition
Jovian planets are huge planets with no solid rocky surface to stand on. Instead, they are wrapped in thick layers of light gases like hydrogen and helium, or icy materials. Jupiter, Saturn, Uranus, and Neptune, in the outer Solar System, are the Jovian planets. They are far bigger than Earth and have many moons and rings.
A Giant Planet with No Ground to Stand On
Earth is made of solid soil and rock. Jovian planets are more like giant bubbles of air. If a spacecraft tried to land on one, it would not touch down on a surface. It would sink endlessly into thick clouds. That is because there is no solid rocky surface to stand on.
When the Solar System was forming, water and gas could freeze into big lumps in the cold regions far from the Sun. These lumps had strong gravity. They pulled in huge amounts of hydrogen and helium gas around them and grew into giant planets.
As a result, Jovian planets have a radius about 4 to 11 times that of Earth, and they are overwhelmingly heavier. Their strong gravity also holds on to anywhere from a dozen to hundreds of moons. They also have beautiful rings made of ice and dust.
What Happens If You Go Deep Into the Clouds?
Being made of gas does not mean it is light and fluffy like cotton candy, or empty inside. The deeper a spacecraft sinks into Jupiter's atmosphere, the more gas piles up above it. The weight of that gas makes the pressure and temperature climb to unimaginable levels.
Deep down, the extreme pressure squeezes the hydrogen, which was a gas, into something closer to a liquid. There is no sharp line between gas and liquid. Go deeper still, and the electrons in the hydrogen atoms start to move freely. That creates a layer of liquid metallic hydrogen, which conducts electricity like a metal.
At the very center, scientists think there is a core where heavy materials like rock and ice gather. But observations from the Juno spacecraft suggest that Jupiter's core is not a solid lump with a sharp edge. It looks more like a "diluted core," where heavy materials are mixed with hydrogen and spread out widely. The center is crushed by heat of tens of thousands of degrees and pressure of tens of millions of atmospheres.
To Be More Precise: Gas Giants and Ice Giants
In the past, all four planets were lumped together as Jovian planets. Modern astronomy splits them into two groups based on what they are made of.
Jupiter and Saturn are mostly hydrogen and helium, so they are called "gas giants." Uranus and Neptune have a relatively smaller share of hydrogen and helium. They are built from lots of "icy" materials like water, ammonia, and methane. Scientists think these materials are now a hot, dense fluid inside the planets. That is why these two are classed separately as ice giants.
Their atmospheres look different, with different colors and bands. But both types have no solid ground, and both have huge atmospheres and rings. That makes them completely different worlds from Earth.
Common misconceptions
Jovian planets are made only of gas, so a spacecraft could fly straight through to the other side.
The deeper you go, the more the gas is packed tightly into liquid and metal forms by enormous gravity and pressure. A spacecraft would not pass through. It would be crushed and melted almost instantly by the pressure and heat.
Where you meet it
Jupiter's "Great Red Spot" is a giant swirling storm, bigger than the whole Earth.
Saturn's huge, bright rings are made of countless pieces of ice and rocky dust.
In one sentence
A Jovian planet is a giant planet with no solid surface. It is wrapped in a huge atmosphere of gas and ice, with a hot, high-pressure core inside.