On Tuesday, scientists led by Scott S. Sheppard of the Carnegie Institution for Science announced the discovery of a dozen moons around Jupiter, bringing the total number orbiting the solar system’s largest planet to 79.

Nine of the newly discovered moons have retrograde orbits, meaning that they orbit in the opposite direction of the planet’s spin. These satellites are part of a large group of moons that orbit in retrograde far from giant planet. In fact, of Jupiter’s 67 previously discovered moons, the 33 outermost moons all have retrograde orbits.

Two of the newly discovered moons orbit much closer to Jupiter and have a prograde orbit, meaning that they orbit in the same direction as the planet. These are part of a group of prograde moons that orbit closer to Jupiter than the retrograde moons do. Most of these prograde moons take less than a year to travel around the planet.

As telescopes get better, astronomers will assuredly find more and more moons, smaller and smaller, around Jupiter and other giant planets orbiting the sun. When the count rises into the hundreds, maybe thousands, scientists might start to wonder whether it’s worth keeping track.

Jupiter is often described as a solar system in miniature. The gas giant is made out of the same basic ingredients as the sun — hydrogen and helium — and is surrounded by an array of geologically diverse moons.


Images taken in May 2018 with Carnegie’s 6.5-meter Magellan telescope at the Las Campanas Observatory in Chile. Lines point to Valetudo, the newly discovered “oddball” moon.

Credit: Carnegie Institution for Science