Just a mile across, a pair of moonlets found orbiting Jupiter bring the planet's total satellite count to 69.

The advent of monster telescopes equipped with super-sensitive, wide-field detectors has been a boon for astronomical discoveries, among them a bevy of tiny moonlets around the outer planets. For example, observations made from 2000 to 2003 yielded 46 moons around Jupiter — more than two-thirds of the planet's total!

Now astronomer Scott Sheppard (Carnegie Institution for Science) has added two more to the planet's extended family, bringing the total of known moons to 69. The announcements for S/2016 J 1 and S/2017 J 1 ("S" for satellite, "J" for Jupiter) came via Minor Planet Electronic Circulars issued on June 2nd and June 5th, respectively.

As Sheppard explains, "We were continuing our survey looking for very distant objects in the outer solar system, which includes looking for Planet X, and Jupiter just happened to be in the area we were looking in 2016 and 2017." So they took a minor detour to image some fields that were very close to Jupiter.

With magnitudes hovering near 24, these barely-there moonlets must be only 1 or 2 km across. So for now all that's really known is the character of their orbits:

Two of the discovery images for Jupiter's tiny moonlet S/2016 J1, acquired on March 3, 2016, with the 6.5-m Magellan-Baade telescope in Chile.
Scott Sheppard

S/2016 J 1: Sheppard discovered this moonlet during an observing run on March 8, 2016, with the 6.5-m Magellan-Baade reflector at Las Campanas Observatory in Chile. Averaging 20,600,000 km from Jupiter, it's in an elongated orbit inclined 140° with an eccentricity of 0.14. It takes 1.65 years to orbit the planet.

Although Sheppard first sighted this moon last year, its orbit remained uncertain until he teamed up with David Tholen (University of Hawai'i) and Chadwick Trujillo (Northern Arizona University), who swept it up six weeks ago with the 8.2-m Subaru reflector on Mauna Kea.

Discovery images for S/2017 J 1
This animation combines two discovery images for the new Jovian satellite S/2017 J 1, taken March 23, 2017, with the 4-m reflector on Cerro Tololo in Chile.
Scott Sheppard

S/2017 J 1: Sheppard and Trujillo recorded the second new find on March 23, 2017, using the venerable 4-m Victor Blanco reflector at Cerro Tololo Inter-american Observatory in Chile. It also turned up in images recorded with Subaru in 2016 and earlier this year, which allowed the team to confirm its existence. This moon likewise is far from Jupiter, at an average distance of 23,500,000 km. In this very elongated orbit, inclined 149° with an eccentricity of 0.40, the moonlet takes 2.01 years to go around Jupiter.

Both of these discoveries, as with the vast majority of Jupiter's moons, occupy retrograde orbits, with inclinations greater than 90°, meaning that they move in directions opposite that of the planet's spin. Such distant, irregular orbits imply that these bodies formed elsewhere in the outer solar system and were captured while passing by early in the planet's history.

According to an orbital assessment published in April by Marina Brozović and Robert A. Jacobson (Jet Propulsion Laboratory), 11 of Jupiter's irregular satellites have orbits known so poorly that they're considered "lost." Sheppard and his collaborators found all but one of those in 2003, and they haven't been observed since.

Jupiter's outer satellites
The great majority of Jupiter's 69 known moons travel in retrograde orbits, meaning they travel in the direction opposite the planet's spin.
Scott Sheppard

However, that's changing. The time Sheppard and Trujillo spent scrutinizing the region around Jupiter has already led to the recovery of S/2003 J 5, S/2003 J 15, and S/2003 J 18, as well as a better orbit for S/2011 J 2.

"We have for sure recovered five of the lost moons," Sheppard says, noting that the 2016 and 2017 observations could be easily linked to some of 2003's uncertain finds. "We have several more Jupiter moons in our new 2017 observations and likely have all of the lost moons in our new observations," he continues, but to ensure the identifications he'll need to return to those big telescopes for more observations in early 2018.

If you're interested in all these crazy outer-planet moons, be sure to check out Sheppard's compilations for Jupiter and the other outer planets.


Image of Jim-Baughman


June 9, 2017 at 4:54 pm

Typo alert! A word was left out, fifth paragraph down:

“Averaging 20,600,000 from Jupiter...” should be:

“Averaging 20,600,000 [miles/km] from Jupiter...”

(Don’t know whether the measurement was in miles or km.)

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Image of Vad-Falcone


June 12, 2017 at 4:53 am

Since the Astronomy Union limited what qualifies as a planet, I wonder why they don't divide satellites from dwarf satellites too. It's impossible to have up-to-date text books if the list of satellites grow longer and longer with small bodies that are discovered all the time.

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