12 New Moons For Jupiter Makes 79!

A page from Galileo’s journal with drawings of Jupiter and its four bright moons made in early January 1610 when he discovered and tracked their movements.

Galileo was the first person to see Jupiter’s moons. He eyeballed the four brightest through his small telescope on winter evenings in early 1610. When I was a boy Jupiter had 12 moons. More than 275 years passed before a fifth, Amalthea, was spotted in 1892. In the 1960s, a dozen were known. And by 2016, with the advent of large ground-based and orbiting telescopes as well as in situ observation by spacecraft, Jupiter’s family had expanded to 67.

After a brief pause of several years, a team of astronomers with the Carnegie Institute of Washington announced they’d discovered a dozen more, bringing Jupiter’s count to 79 in one fell swoop. The team, led by Scott S. Sheppard, first spotted the moons in the spring of 2017 while looking for very distant solar system objects as part of the hunt for a possible massive planet far beyond Pluto nicknamed Planet Nine. After a year of observation and confirmation they announced the discoveries this week.

79 known moons keep things hopping around the Jovian planet!  The inner or prograde moons (purple, blue) move in the same direction as the planet rotates, while the outer moons (red) move in the opposite direction or retrograde. Valetudo (green) is the exception, a prograde-moving body that’s far out. The new finds’ orbits are shown in brighter colors.
Carnegie Institute for Science / Roberto Molar Candanosa

“Jupiter just happened to be in the sky near the search fields where we were looking for extremely distant solar system objects, so we were serendipitously able to look for new moons around Jupiter while at the same time looking for planets at the fringes of our solar system,” said Sheppard.

He and the team hit the jackpot. Nine of the new moons are part of a distant outer swarm of moons that orbit the giant planet in retrograde, opposite to the direction of the planet’s rotation. These distant retrograde moons are grouped into at least three groups with related orbits that are thought to be the remains of three once-larger bodies that broke apart during collisions with asteroids, comets, or other moons.

See how Jupiter’s moon orbit the planet in this animation

The nine are so far away from the planet they take about two years to orbit. Compare that to the four bright inner moons that you and I can see in a small telescope. They take from just under 2 days for Io to a little more than 16 days for Callisto to orbit the giant planet. The furthest of them, temporarily designated  S/2017 J2, lies 14.5 million miles (23.3 million km) from Jupiter or more than 60 times the distance of the moon from Earth.

Two of the new discoveries are part of a closer, inner group of moons that orbit in the prograde, or same direction as the planet’s rotation. Because the two have similar distances and orbital tilts to Jupiter’s equator, they’re also appear to be related fragments of a larger moon that was broken apart. Each takes a little less than a year to travel around the big guy. Did I mention all the new discoveries are small? Each spans from about about a half-mile to two miles (1-3 km) across. Given that they’re fragments, they probably have a variety of different shapes — anything but spherical.

These images of Valetudo from the Magellan telescope in May 2018 show the moon moving relative to the distant stars. Jupiter is not in the field but off to the upper left.

The 12th new moon is just plain kooky with an orbit unlike any other Jovian moon. It’s also the planet’s smallest known with a diameter of less than 0.6 miles (1 km). For now, it’s being called Valetudo after Jupiter’s great-granddaughter, the goddess of health and hygiene. Valetudo is more distant than the prograde group — it takes about 1½ years to orbit Jupiter — so it crosses the orbits of the outer retrograde moons.

This is an unstable situation with head-on collisions much more likely to occur between Valetudo and the retrograde moons, which are moving in opposite directions. Any collision would shatter it to pieces. Sheppard thinks it’s possible that the moon groupings we see today resulted from ancient impacts of exactly this kind. That would make Valetudo the last-remaining remnant of a once-larger prograde-orbiting moon that formed some of the retrograde moon groupings during past head-on collisions. In other words, it’s tiny today because it’s been chipped away through impacts over the last several billion years. It’s a survivor — barely!

In this way and a multitude of others, the solar system continues to sort itself out during the past 4.5 billion years. And it’s not over yet … by a long shot. For the particulars on each of the newly-discovered moons, click here.

5 Responses

  1. caralex

    Bob, how does a planet have both prograde and retrograde moons? Do collisions cause the retrograde ones? How exactly?

    1. astrobob


      I’ve always understood that the outer moons of Jupiter, the retrograde ones, are captured asteroids. For various rather complicated reasons, prograde orbits are more stable near the planet and retrograde ones provide more stability at large distances from Jupiter.

      1. caralex

        Oh, I didn’t know that! Seems counter-intuitive that retrograde orbits would be stable at all!

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