Astronomers find 12 more moons orbiting Jupiter

Jupiter’s count of 92 knocks Saturn out of first place for most moons in the solar system.
Jupiter and its four planet-size moons, called the Galilean satellites, were first photographed in early March 1979 by Voyager 1 and assembled into this collage. They are not to scale but are in their relative positions.
Jupiter and its four planet-size moons, called the Galilean satellites, were first photographed in early March 1979 by Voyager 1 and assembled into this collage. They are not to scale but are in their relative positions. NASA/JPL

The planet Jupiter is famed for its immense size (a radius of 43,440.7 miles or 11 times wider than Earth) and its Giant Red Spot, a storm that has raged on the planet for hundreds of years. The fifth planet from the sun is not only the biggest in our solar system, but it also, according to the International Astronomical Union’s Minor Planet Center (MPC), has the most moons. 

Astronomers discovered 12 new moons around Jupiter over the past two years, making the total number of Jovian moons 92.

The discovery knocks Saturn and its 83 confirmed moons out of first place. Both Jupiter and Saturn have tons of small moons that are believed to be fragments of bigger moons that have collided with comets, asteroids, and each other. 

[Related: We just got our most detailed look yet at Jupiter’s icy moon, Europa.]

As for the other planets in our solar system, Mercury and Venus are moonless, Earth has one, Mars has two moons, Uranus has 27 confirmed moons, and Neptune clocks in at 14.

The MCP recently added the new moons to their list, team member Scott Sheppard of the Carnegie Institution told the Associated Press (AP). The team’s observations have also been submitted for publication.

“I hope we can image one of these outer moons close-up in the near future to better determine their origins,” Sheppard said in an email to the AP.

Telescopes in Chile and Hawaii discovered the moons in 2021 and 2022 and follow-up observations confirmed their orbits. Sheppard says that they range from 0.6 miles to 2 miles in size. 

According to Sky and Telescope, all of the newly discovered moons circle Jupiter far from its surface and take over 340 Earth days to complete a single orbit. Nine out of the 12 moons are particularly distant, with MPC estimating that they have orbits longer than 550 Earth days. They are also quite small—only five out of the nine distant moons are believed to have a diameter more than five miles.  

[Related: Dark matter, Jupiter’s moons, and more: What to expect from space exploration in 2023.]

These same nine moons also have retrograde orbits. The moons circle Jupiter in the opposite direction of its rotation. By comparison, the inner Jovian moons have prograde orbits, or orbits in the same direction of the planet’s rotation. 

The retrograde orbits mean that the huge gravitational influence of the planet may have captured the moons and the smaller ones might be the remains of largest celestial bodies that were broken apart by collisions, according to Sheppard.  

We can also expect to learn more about Jupiter’s moons over the next few years. The European Space Agency (ESA) is sending the Jupiter Icy Moons Explorer (aka Juice) into space in April to study the gas giant and some of its largest moons. NASA is scheduled to launch the Europa Clipper in October 2024 to explore Jupiter’s icy moon Europa,  which might have an ocean beneath its frozen crust.