Jupiter’s moons have long piqued our interest. Europa, one of the planet’s most interesting moons, is thought to hold a subterranean ocean under its ice surface, making it a possible contender for extraterrestrial life. The famous moon has appeared in works of popular science fiction, such as Futurama, Star Trek, and Arthur C. Clarke’s 2001: A Space Odyssey.
Scientists are particularly interested in Jupiter’s moons because they are varied habitats with the potential to support life. New moons are constantly discovered around the planet, provoking a question with an ever-changing answer: How many moons does Jupiter have?
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More Moons are on their way
Sheppard expects continual additions to Jupiter’s and Saturn’s moon counts. He believes that these gas giants are home to several minor moons, which are thought to be remnants of bigger moons, comets, or asteroids that collided in the past.
While Uranus and Neptune have comparable moon-spawning mechanisms, their distance hampers identification. In comparison, Uranus has 27 verified moons, Neptune has 14, Mars has two, and Earth has one. Surprisingly, Venus and Mercury have no moons.
The naming of the newly discovered moons around Jupiter has yet to be completed. Sheppard emphasizes that only half of these distant moons are large enough to deserve a formal designation (at least 1 mile (1.5 kilometers).
The European Space Agency will launch a probe from French Guiana in April 2023 to examine Jupiter and its several ice moons. NASA’s Europa Clipper mission will examine Jupiter’s moon of the same name in 2024, with implications for the presence of an ocean under its frozen surface.
The Discoveries of Galileo
Galileo Galilei, the famous astronomer, observed four celestial planets that appeared to orbit around Jupiter in 1610. Io, Europa, Ganymede, and Callisto are Jupiter’s four largest moons (Ganymede being the largest). These first Galilean satellites were also found.
As stargazing technology advanced, it became evident that the four had plenty of company. These new moons are tiny and have orbits that last one or two years, as opposed to the “big four,” which are massive and have orbits that last less than 17 days.
Jupiter had always had the most moons in the solar system, until 2019 when Saturn momentarily surpassed it with 82 moons, only to reclaim it in 2023. (At the moment, we know of 146 moons around Saturn.)
There’s a reason Jupiter has so many satellites while other planets, including ours, have so few. Everything boils down to gravity.
The Effect of Gravity on Moons
Astronomers classify the planets in our solar system into two groups. Mercury, Venus, Earth, and Mars are known as “terrestrial” or “inner” planets, whereas Jupiter, Saturn, Uranus, and Neptune are known as “outer planets.” (Uranus and Neptune were previously categorized as gas giants, but Uranus and Neptune have recently been reclassified as ice giants.)
The size difference between those groups is rather enormous; while being the smallest outer planet, Uranus is still 14.5 times more massive than Earth, the largest of the inner planets. However, none of the other planets can rival with Jupiter in terms of sheer size. To equal Jupiter’s massive mass, more than 300 doubles of our little home planet would be required. It’s a whole beast.
Jupiters in Flare
But it isn’t the only reason why planets like Jupiter have so many moons. The gas giants in our solar system are quite far from the sun. Some stars beyond our solar system, on the other hand, contain enormous, Jupiter-like exoplanets known as “Hot Jupiters.” These are essentially gas giants that circle their stars in close proximity. (Imagine if Saturn and Mercury traded positions.)
According to a 2010 article by French astronomer Fathi Namouni, Hot Jupiters have few, if any, moons. These planets are expected to form in remote portions of their solar systems before migrating inward.
One Big Jupiterian Family
A Jovian moon, often known as a Galilean moon, is any of Jupiter’s four biggest moons: Io, Europa, Ganymede, and Callisto. The four Galilean moons are notable because of their large sizes and different features. Io has active volcanoes; Europa has a secret ocean that might contain extraterrestrial life; and Ganymede is the largest satellite in the solar system, measuring two-thirds the size of Mars.
These three moons, together with Callisto, most likely formed simultaneously with Jupiter. The large planet most likely began as a disc of gasses and dust that evolved into the gas giant we know today. While Jupiter formed, part of the material spinning around it formed the four moons seen by Galileo in 1610. A youthful Saturn may have aided the process.
The Moon Crashes
Before we conclude, we should discuss lunar behavior. Many of Jupiter’s moons orbit in the same direction as Jupiter rotates. However, some have retrograde orbits, which means they move in the opposite direction. Collisions are unavoidable with so many bodies spinning in various directions.
Moons colliding with one another may be destroyed in the process. Jupiter is discovering ways to shed some of its older moons while it gets new ones.
This recent find does not mark the end of the moon search. New technology has made it simpler to detect weak moving objects against a background of stars. Sheppard and his colleagues believe there are many more moons to be discovered orbiting Jupiter and Saturn, as well as Neptune and Uranus, though their great distance from Earth (and our lone moon) makes confirmation difficult.