Outer-Planet Moons Found — and Lost

The orbital positions of more than a dozen small moonlets around Jupiter and Saturn are so uncertain that they are effectively lost.

Prior to the year 2000, Jupiter had 17 known moons, about half of which moved in "irregular" orbits that were highly inclined and eccentric and often traveling in reverse (retrograde), with respect to Jupiter's spin. Saturn had 18 moons in all, though only Phoebe occupied an unusual orbit.

Jupiter's irregulars had paths that seemed to be clustered, suggesting that these bodies were related — most likely chunks set loose during long-ago collisions.

Tiny moonlet of Jupiter
Discovery images, taken in 2001, of a 22.1-magnitude moonlet around Jupiter that's just 2½ miles (4 km) across. Spotted as part of a survey for new Jovian moons, this one — later named Hermippe — didn't get away. But some of its siblings are now considered lost.
S. Sheppard / D. Jewitt
To track down more members of these oddball families, University of Hawaii graduate student Scott Sheppard and his adviser, David Jewitt, started an ambitious project to find new irregular satellites around Jupiter and Saturn. Using the big guns on Mauna Kea — first the university's 2.2-m reflector and the 3.6-m Canada-France-Hawaii Telescope, and later the 8.2-m Subaru telescope, Sheppard and Jewitt were wildly successful. In a single 2003 article, they announced the discovery of 23 irregular Jovian satellites, clustered into five distinct dynamical groups.

Meanwhile, another observing group, led by Canadian astronomers Brett Gladman and JJ Kavelaars, had likewise been quite successful tracking down new outer-planet satellites, particularly around Saturn. NASA's Cassini orbiter also chipped in a few Saturnian moonlets after its arrival in late 2004.

Jupiter's outer satellites

The great majority of Jupiter's 67 known moons travel in retrograde orbits, meaning they travel in the direction opposite the planet's spin. Click on the image for a larger view..
Scott Sheppard

The upshot of this finding frenzy is that the count of planetary satellites now stands at an amazing 172: Earth has the Moon, Mars has 2, Jupiter 67, Saturn 62, Uranus 27, and Neptune 13. Among the dwarf planets, Pluto has 4, Eris 1, and Haumea 2.

However, just because all these moons have been found doesn't mean astronomers actually know where they are right now. Dynamicists Robert Jacobson and Marina Brozović (Jet Propulsion Laboratory), along with four collaborators, have analyzed the uncertainties of some 100 small outer-planet satellites whose orbits are distant, highly inclined, and eccentric.

Their surprising conclusion, summarized here and detailed in an article submitted last month to Astronomical Journal, is that the locations of 10 Jovian and seven Saturnian satellites are known so poorly that they are effectively lost. "We have very little idea of where to point a telescope to observe them," the authors conclude. "A new survey of the planetary environs will be required to find them."

"Lost" Satellites with Highly Uncertain Orbits
Designation Mean distance (106 km) Eccentricity Inclination Period (years) Magnitude
2003 J 12 17.83 0.49 151° 1.34 23.9
2003 J 3 20.22 0.20 148° 1.60 23.4
2003 J 15 22.63 0.19 146° 1.89 23.5
2003 J 10 23.04 0.43 165° 1.96 23.6
2003 J 9 23.28 0.26 165° 2.01 23.7
2003 J 5 23.49 0.25 165° 2.02 22.4
2003 J 19 23.53 0.26 165° 2.03 23.7
2003 J 23 23.56 0.27 146° 2.01 23.6
2003 J 4 23.93 0.36 150° 2.07 23.0
2003 J 2 28.38 0.41 157° 2.68 22.8
2007 S 2 16.72 0.18 174° 2.21 24.4
2004 S 13 18.41 0.26 169° 2.56 24.5
2006 S 1 18.78 0.14 156° 2.63 24.6
2007 S 3 18.94 0.18 178° 2.68 24.9
2004 S 17 19.45 0.18 168° 2.78 25.2
2004 S 12 19.89 0.33 165° 2.86 24.8
2004 S 7 21.00 0.53 166° 3.12 24.5

Source: R. Jacobson & others

Moreover, the team finds that 11 additional outer-planet moonlets could become lost over the next decade unless someone steps up to the eyepiece, so to speak, to pin down their orbits better.

The count of AWOL objects would be higher had the team not undertaken an intensive effort to recover faint outer-planet satellites during 10 observing runs in 2009-11 with the CFHT in Hawaii and Hale 5-m reflector on Palomar Mountain, California. Observers attempted to track down 40 objects with dicey orbits and found 38 of them. It helped that, in September 2010, Jupiter had one its closest-ever oppositions and Uranus was situated just 1° from it in the sky.

New-found satellites are given temporary identification codes: for example, two new ones swept up during the recovery work are designated S/2010 J 1 and J 2. Eventually, once their orbits are known precisely, they'll get mythologically relevant names.

Jovian moon Arche in 2011
B. Gladman & others
So how do "found" satellites become "unfound"? The simple answer is that their orbits were too poorly determined initially for long-term certainty. "You can't just find them," Gladman grumbles. "You have to find them and track them." Many of these irregular moonlets are very faint (23 to 25 in magnitude) and lie far from their planets, he explains, positional follow-ups require the use of big telescopes for long observing runs. Unfortunately, it's not the kind of scientific yield that's going to gain much support when observatory time gets allocated.

All of the lost and near-lost moons were discovered by Sheppard, Jewitt, and their collaborators, the most recent ones in 2007. "We wanted to complete all the known satellites with modern CCD technology," Sheppard told me, keying on dynamically related irregular clusters. "We definitely wanted to recover everything," he explained, but observations to follow up the Jovian finds were largely ruined by a run of bad weather. His team got more time last year on one of the twin Magellan telescopes, and although those observations haven't yet been analyzed, Sheppard expects that some of the missing moons will turn up.

And if not? It wouldn't be the first time an outer-planet satellite became lost. In 1975, Charles Kowal and Elizabeth Roemer announced their discovery of a tiny moonlet in an inclined, elliptical orbit averaging 4.6 million miles (7.4 million km) from Jupiter. Then its whereabouts became unknown — until 2000, when Sheppard, Jewitt, and two others spotted it in their survey. Named Themisto in 2002, this one is not in any danger of being lost again.

7 thoughts on “Outer-Planet Moons Found — and Lost

  1. Emilio Pisanty

    Are these moons simply lost, in that we don’t know where they are, or is there the possibility that they may have been kicked out of their systems? Were their orbits known precisely enough beforehand to rule out this possibility?

  2. Ed-MarshallEd

    Hi Emilio, Good question. Although the is a very small possiblility that these little guys are gone, it is WAY more likely that thay are still there if they exist. Like the article says, you really need to pin down an orbit to know where to look to reaquire them for a "real" discovery. There are some tiny moons that I’m sure get captured by Jupiter, stay for a few million years, nudged out of orbit, meander around in a similar Jovian orbit for while and maybe even get recaptured. They are in orbits that are quite far away for Jupiter, and would be in this sort of "dance" more than really being able to be "kicked out" in a more violent way. On the way back into the Jovian system, they could be kicked out into a solar orbit, but then they wouldn’t of been a moon at that point. So what is comes down to is the odds of something like that happening in the few years since we "lost" them. It’s possible, but highly likely. Ed

  3. John F. Tashjian

    I know that the explanation was given in the article, but it still begs the question: how on Earth (so to speak) can a planet’s moon become "lost" once it has been observed? I’ll concede that some moons’ orbits are quite irregular, but I cannot imagine how such moons can be lost…unless, of course, they fell out of orbit. Something to reflect on, and to think about, I suppose.

  4. darvasart

    any possibility that some of these moonlets with fringe orbits go “loose” … join another planet or even go into a solar orbit .. then return.. in some figure 8 or more complex path .. that may give argument for retrograde orbits .. some of these patterns could take millions of years to repeat?

  5. Rod

    As various reports like "Irregular Satellites of the Giant Planets", Publication Date: 00/2008, http://adsabs.harvard.edu/abs/2008ssbn.book..411N indicate, explaining the origin of the irregular moons of the giant planets is difficult. If we do not truly know how they formed then the question rises, when did the irregular moons form? Are the irregular moons evidence of past catastrophism in the solar system, e.g. < 1E+8 years ago?

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