© Scott SheppardThe great majority of Jupiter's 69 known moons travel in retrograde orbits, meaning they travel in the direction opposite the planet's spin.
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:
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 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. (
See Animation)
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 (Gemini Observatory), who swept it up six weeks ago with the
8.2-m Subaru reflector on Mauna Kea.
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.(
See Animation)
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. These 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.
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.
The mass of all the planets combined equals the mass of 446.58 Earths. Jupiter represents 71.16 % of that total. Thus it is the big honcho in terms of its mass; and it seems that its far shorter year and faster speed would place it in more places/windows of opportunity to to catch more moons... I think. (I’ll defer to a knowledgable astronomer, but am not going to do the research.)
Jupiter’s orbital period (its ‘year’) equals 11.9 earth years. (EY).
Saturn: 29.4 EY
Uranus: 83.7 EY
Neptune: 163.7 EY.
[Link]
Thus it seems to me that an incoming planet killer has a far greater chance of being caught by Jupiter than any other and that Jupiter's ever increasing* number of moons (both actual and discovered) tends to show that more and more are coming in, and are likely accompanying something.... e.g., Nemesis or Nibiru, or who knows what.
Note also that the 1994 Come t Shoemaker–Levy 9 which broke up and ran into Jupiter, was at first simply an acquired, orbiting moon, but it broke apart and instead of staying a moon, it insteach 'became one' with Jupiter. If its original trajectory had been slghtly different. it could have become a distant moon of Jupiter, or it could have slid by and instead destroyed earth.
I bet that a fair analysis of the rate of the number of discovered moons of Jupiter, (even when adjusted/lowered to acknowledge the factors of increasingly accurate measurement tools and tech); would show us a true ‘hockey stick’ graph of accelerated increase of Jupiter’s acquired moons.
It seems to me that a study of all the outer gas giants, (J, S, U & N) - as properly adjusted per their speeds and periods would correlate very very well with the number and severity of comets and meteor showers.
It wouldn’t be hard to do. Go to, someone.
One thing that would be hard to do would be to bring in another planet to assist with the Solar Systems’ housekeeping. However, I fear Nemesis would be like hiring a bull to clean your china shop.
R.C.
*(I believe that's correct odds 85%)
** The other large outer planets all play a role, but Jupiter is the big vacuum cleaner per its mass and faster rotation, allowing it to pass by more 'roids and grab them. (Again, that latter point seems logical but hasn't been researched by me.)
*** I think there have been short times where a near pass of a planet or asteroid has broken loose one of two of them, but generally, it’s always acquiring more and more and more.
[Link]