Jupiter’s moon count just hit 69! Scott Sheppard of the Carnegie Institution for Science added the latest two in announcements published in the Minor Planet Circulars on June 2nd and June 5th. Named S/2016 J 1 and S/2017 J 1 (S=satellite, J=Jupiter), they’re only about a mile across apiece and very, very faint, about magnitude +24. Let’s just say, they’re way beyond the reach of amateur telescopes.
Sheppard and his team were looking for extremely distant objects in the outer solar system including the putative Planet 9 with a 6.5-meter (255.9-inch) telescope at Las Campanas Observatory and a 4-meter (157.5-inch) telescope at Cerro Tololo, both in Chile. Jupiter happened to be near the survey area when they were searching in 2016 and early 2017, so the crew detoured to the planet and took photos to see if anything would turn up. And it did, or they did.
Not only have moons coalesced around Jupiter back when it was a young planet, but the behemoth has captured a swarm of stray asteroids and made them part of its family in the eons since. When I first got into astronomy, Jupiter had 12 moons — Io, Europa, Ganymede, Callisto, Amalthea, Himalia, Lysithea, Elara, Ananke, Carme, Pasiphae and Sinope. That lasted until the early 1970s. With the advent of spacecraft flybys and new, giant telescopes, moons popped up like dandelions in spring. In the four years from 2000-2003, 46 were discovered, most of them by Sheppard and his team. With the two latest discoveries, he now has 47 to his credit or about two-thirds of the planet’s total!
Jupiter’s inner moons orbit in the same direction as the planet rotates just like Earth’s moon, but most of the others are far from the planet and travel in the opposite direction or retrograde. Their remoteness and contrary motion imply they came from elsewhere (stray asteroids) and were captured by the gravitational might of the solar system’s largest planet.
We’re still in the dark about the physical characteristics of the newest members of the family, but we do know that S/2016 J1 orbits on average some 12.8 million miles (20.6 million km) from Jupiter and takes 1.65 years to complete its orbit. That’s incredibly long when you consider that the planet’s four brightest moons, easily visible in any telescope, orbit between 1.8 days (Io) and 16.6 days (Callisto).
S/2017 J1 takes 2 years to circle the planet at a distance of 14.6 million miles (23.5 million km). Both moons’ orbits stand out because they’re steeply inclined to Jupiter’s equator, 140° and 149° respectively. That’s really cockeyed compared again to both our moon and especially the four brightest, which orbit nearly in the plane of the equator. Astronomers take this as yet another sign that they were captured from elsewhere.
Since the first Galileo laid eyes on the planet in 1609, Jupiter’s has always been thought of as a miniature version of the solar system with the big mass in the middle orbited by numerous smaller orbs. These newly-found moons only reinforce Jupiter’s planetary dominance and hint that more moons may yet lie waiting for us in the darkness. For more about the discoveries, check out Scott Sheppard’s site.