Why does Jupiter have so many darn moons?

Jupiter is monstrously large, so much so that it is more than twice as massive as all other planets in the solar system combined. Consequently, it has an equally massive gravitational pull that helped it capture a myriad of satellites.

By the latest count, the gas giant has 79 confirmed moons. That’s two moons shy of Saturn’s count of 82 confirmed moons. But the race is still on for which planet has the largest entourage since astronomers keep constantly discovering new ones.

Saturn stole the crown from Jupiter in 2019 when astronomers discovered 20 new moons. But Jupiter isn’t done for — far from it. Twelve new satellites were discovered in 2017 alone around Jupiter by researchers affiliated with the Carnegie Institution for Science thanks to recent advances in large digital cameras and astronomical techniques. What’s more, there are good reasons to believe that Jupiter has, by far, the largest number of moons in the entire solar system.

In September 2020, astronomers from the University of British Columbia identified 45 candidate moons with a diameter of over 800 meters. The researchers only surveyed a tiny area of the sky though and when they extrapolated their data, they concluded that there could be more than 600 of these tiny moons orbiting Jupiter.

These candidates are currently under investigation, which will take a lot of time since it takes quite a lot of telescope time to reliably verify their orbits. But while scientists are kept busy with cataloging huge boulders zipping across Jupiter’s motion, it’s perhaps a good time to find out why Jupiter has many moons. What’s so special about it?

Thanks for the free lunch, sucker!

The reason why Jupiter is such a moon magnet has a lot to do with its stupendous mass, equal to more than 300 Earths. Earth’s one and only Moon likely formed billions of years ago after a huge proto-planet slammed into primordial Earth. But most moons, especially around gas giants, don’t have such an exciting history. Like a school bully, Jupiter and other gargantuan planets like it are in the habit of ‘capturing’ rocky objects ranging from smallish asteroids to full-fledged mini-planets with volcanic activity, such as Io.

Jupiter, as well as other distant gas giants in the solar system such as Saturn and Neptune, has another ace up its sleeve. Not only does it have a very strong gravitational pull thanks to its mass, but it is also quite far away from the Sun. It’s about 5 times farther away from the Sun than Earth is, completing a full orbit every 11.86 years.

This great distance allows Jupiter to exert a larger area of influence or control as the Sun’s gravitational influence weakens the farther away you travel from it. With such a wide net cast, it’s no wonder that Jupiter has moons orbiting it as far away as 23.5 million miles, as is the case for Pasiphae and Sinope. Meanwhile, Venus and Mercury, the two closest planets to the sun in the solar system, have no moons at all, while Earth has a measly one to speak of and Mars as two tiny satellites.

Also important, albeit to a lesser degree, is the gas giant’s shape. A cosmic body that is regularly round will have a more stable orbit than a potato-shaped one. Jupiter is almost perfectly round and that may have helped it capture some additional small satellites, especially in its lower orbit.

Big rocks around a gas giant

Of Jupiter’s many moons, collectively known as Jovian moons, the four largest particularly stand out. These moons — Callisto, Io, Europan, and Ganymede — are often called Galilean moons in honor of Galileo Galilei, the Italian astronomer who first discovered them in 1610. The moons themselves are each named after lovers and favorites of the god Jupiter (Zeus); and since 2004, also after their descendants. In the future, however, astronomers may run out of mythological beings to name Jovian moons by.

Three of these four moons are larger than Earth’s Moon and one, Ganymede, is the largest moon in the solar system. In fact, all you need is a pair of good binoculars or a retail telescope to see all four of these largest moons of Jupiter, which are all at least 3,100 kilometers (1,900 mi) in diameter.

Although the four Galilean moons comprise a small proportion of Jupiter’s of 79 confirmed satellites, they collectively sum 99.999% of the total mass orbiting Jupiter, including the ring system. The Galilean moons are also a lot more quirky than their more puny Jovian cousins. For instance, Io is packed with active volcanoes and Europa may harbor life in a liquid ocean covered by thick ice.

All other Jovian moons are less than 250 kilometers (160 miles) in diameter, although most barely exceed 5 kilometers (3.1 miles).

While the Galilean moons are believed to have formed along with Jupiter from its circumplanetary disk — a ring of gas and dust — the outer, irregular moons are believed to have originated from captured asteroids.

And over the course of its long history, Jupiter likely harbored many other moons, which are now long gone. Some were destroyed by mechanical fracturing during the capture or during collisions with other asteroid-like objects, some simply drifted away out of Jupiter’s clutches. You win some, you lose some.

In the dark and gloomy part of the solar system that Jupiter calls home, there is an abundance of large asteroids (with a diameter greater than one kilometer), so it’s reasonable to believe the number of Jovian moons could swell in the future. But in the meantime, scientists are still busy identifying the gas giant’s existing moons. It may not be long before Jupiter reclaims its crown, rising once again to the top as the planet with the most moons.