Jupiter is king of the planets. It’s huge, it’s bright in our night skies, and even four of its comparatively tiny moons are bright enough to see with the most basic of telescopes. We’ve sent nine probes either into orbit or on a close flyby of the planet. And yet, as recently as this past year, we discovered not one, but twelve new moons around Jupiter, bringing the total to 79. How haven’t we exhausted this particular moon mine yet?

The easy targets first

The answer is that most of Jupiter’s moons aren’t the grand companion that our own moon is to Earth, at nearly a quarter as wide as its host planet. The four moons first spotted by Galileo in 1610 — Io, Europa, Ganymede and Callisto — are big enough compared to our moon, but absolutely puny when compared to Jupiter, the planet they circle. And those are the easy targets. It makes discovering new moons against its bulk difficult.

It took the advent of photography before astronomers discovered any more moons around Jupiter, and the work over the next century or so was painstaking. By the time Voyager cruised by in 1979, the giant was up to 13 moons. Voyager added three to the count: Metis, Adrastea, and Thebe.

All three of these plus Amalthea (discovered in 1892 by famed astronomer E.E. Barnard) and the original Galilean moons comprise Jupiter’s regular moon group. This means they’re more or less spherical, orbit in the same direction that Jupiter spins and do so on well-behaved, near-circular orbits that don’t tip much out of the plane of Jupiter’s equator. In other words, what you probably imagine a moon to be.

The rest are the irregular moons, and these make up the vast majority of Jupiter’s satellites. These tend more toward potato shapes, and their orbits are often eccentric, tilted, or even retrograde, meaning they fly backwards to Jupiter’s spin. Most are probably captured asteroids or the results of long-ago collisions of larger bodies — perhaps past moons of Jupiter. They’re tiny and tend to orbit farther out from Jupiter than the regular moons. This makes them much harder to spot.

Astronomers found a few of these irregular moons. But after Voyager, the discoveries stopped for about two decades.