Quick notes:

  • Jupiter has dozens of moons, most of which are named.

  • In 2018, 12 new moons were discovered.

  • Not all of the Jovian satellites rotate in the same direction

Jupiter is the largest planet in our solar system and is easily visible with the naked eye during certain times of the year. With binoculars or a telescope, we Earthlings can bask in the majesty of the Jovian moons, or at least the largest and brightest among them. Most home telescopes will show you about half a dozen moons at most, but Jupiter holds onto many more satellites than meets the eye.


Moon magnet

Planets are big. The larger they are, the more they tend to warp the space around them. The more space is warped, the easier it is for stray bits to get caught. Think of it like balls on a taut sheet. A golf ball might not dip the fabric too much when placed in the center, but a baseball or a bowling ball would. Now, imagine a marble placed on the center of the sheet and call it Earth.

You could probably catch a few grains of sand with how much it warps the sheet. Bring in a bowling ball, and you’ve got yourself a crude comparison with Jupiter.

Because of Jupiter’s mass and subsequent influence on the space around it, Jupiter pulls in space debris (which occasionally includes comets) like metal filings to a magnet.


Ring around the gas giant

Although Saturn takes the cake when it comes to ring structure, all of the gas giants have rings. Saturn’s are so visible because they’re primarily made out of ice fragments. Jupiter’s, on the other hand, are considered “dark rings,” as they’re mainly composed of rocks and dust.

The longer we stare at Jupiter’s rings, however, the more large objects we’re noticing. In 2018, the massive gas giant “gained” 12 new moons, bringing the total up to 79.

Of those many, many moons, four stand out from the rest by their notable sizes and their spherical shapes. These four Galilean moons are Io, Europa, Ganymede, and Callisto. Io and Europa are particularly attractive to astronomers because of their unusual surfaces.

Io is covered in highly-active volcanoes while Europa is encased in fissured and cracked ice, showing evidence of tectonic activity. The four moons are more massive than any known dwarf planet. Ganymede, the largest Galilean moon, is almost the size of Mars and generates a magnetic field.


What makes a moon?

By definition, a moon is any celestial body that orbits a planet. For a world with a ring system, that definition gets a little bit sketchy. Jovian moons are larger than the surrounding rocky debris in the ring system and tend to fall into one of three zones. The innermost zone is home to the Galilean moons, followed by the prograde group farther out.

Both of these collections of satellites orbit Jupiter in a counterclockwise manner. The farthest group of moons, however, circle the gas giant in the opposite direction. This retrograde group is newly discovered and raises several questions about Jupiter’s moon capture process.

Of Jupiter’s 79 moons, only 53 are named and confirmed. The remaining 26 are considered “provisional moons” until astronomers know more about them. In 2018, five of the 12 newly-discovered moons were christened and welcomed into the full status family. Those moons are: Pandia, goddess of the full moon and daughter or Jupiter; Ersa, sister of Pandia and goddess of dew; Eirene, goddess of peace and daughter of Zeus; Philophrosyne, spirit of welcome and kindness and granddaughter of Zeus; and Eupheme, sister of Philophrosyne and the spirit of praise and good omens.


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