From top: Neptune, Uranus, Saturn, and Jupiter.
The term "Jovian planet" refers to one of 4 gas giants within Earth's solar system: Jupiter, Saturn, Uranus, and Neptune.
The term is in reference to the Roman god Jove, also known as "Jupiter". The term was intended to indicate that the Jovian planets were all similar to Jupiter, but it has become less popular after notable differences were found between these planets; mainly, Jupiter and Saturn are primarily composed of hydrogen and helium, while Uranus and Neptune have less hydrogen/helium, and more ice and rock. They are also know as "Uranian planets" or "ice giants".
With the discovery of massive extrasolar planets, the term has seen some renewed usage in describing planets of comparable mass to Jupiter. See also Hot Jupiter.
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