Jupiter is a gas giant with a volume of 1321 Earths and a mass of 318 Earths. It has 63 moons, including the Galilean moons, and a powerful magnetic field that causes volcanic activity on Io. Jupiter’s Great Red Spot is a circular storm many times the size of Earth. It absorbs comets and asteroids that might otherwise impact Earth.
Jupiter is the most massive planet in the Solar System, orbiting about 5.4 AU (Earth-Sun lengths) from the Sun. It has a volume of 1321 Earths, but a mass of only 318 Earths, which puts it just above the density of water . Its large size and relatively low mass, as well as its phase, make Jupiter a gas giant. Other gas giants include Saturn, Uranus, and Neptune, although the latter two are sometimes referred to as ice giants instead.
To become large enough to fuse hydrogen and become a star, Jupiter would need to be 75 times more massive. Ironically, even a planet of that much larger mass would be smaller than Jupiter, as its gravity would compress it.
Jupiter has a composition similar to that of the Sun and other large stars and planets: about 90% hydrogen, 9% helium and 1% other elements. It has more hydrogen than a typical star because it lacks the ability to fuse its hydrogen into helium.
Jupiter has at least 63 moons, including four, Io, Ganymede, Europa and Callisto, which were observed by Galileo Galilei when he invented the telescope. These are named the Galilean moons in his honor and were closely observed by the Voyager space probe when it made a flyby in 1979. Io was found to have volcanic activity, in part caused by its proximity to Jupiter’s powerful magnetic field, which it stirs the heart of the planet like a washing machine.
Jupiter’s most distinctive feature is the Great Red Spot, a circular storm many times the size of Earth. It’s been known to have been around since at least the 1600s, but we’re not sure how much older it is.
We Earthlings have Jupiter very much to thank for, as it absorbs many large comets and asteroids that might otherwise impact our tiny, fragile planet. One such event occurred in 2005, when comet Shoemaker-Levy broke up and impacted Jupiter’s atmosphere, leaving a series of holes in Jupiter’s atmosphere hundreds of kilometers in diameter.
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