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The Chaotic Moons of Pluto

June 15, 2015

In the later part of the 20th century, advances in instrumentation and our ability to send spacecraft to other planets in our Solar System corrected many misconceptions in planetary astronomy. One of these involved the rotation rate of the planet, Mercury. Mercury was always thought to be tidally locked to the Sun in having a rotational period the same as its orbital period of 88 days; that is, it would always present one face to the Sun.

Radar observations of Mercury in 1965 with the Arecibo Observatory radio telescope showed a rotational period of 59 days, demonstrating an unusual 3:2 spin-orbit resonance in which Mercury rotates three times for every two orbits around the Sun.[1] It's easy to see why such a discovery didn't happen until 1965. The observation required a two megawatt UHF transmitter beaming a 430 MHz signal through Arecibo's 300 meter dish antenna.[1]

The prime example of tidal locking is our Moon. Only the near side of the Moon is visible from Earth. Our planetary neighbor, Mars, has a tidal lock on both its moons, Phobos and Deimos. In fact, the majority of moons in our Solar System are tidally locked, including Saturn's largest moon, Titan; and Ganymede, the largest moon of Jupiter and the largest moon in the Solar System. Nearly all others are suspected to be locked to their planets, while a few outer satellites of the gas giants, distant from their host planet, are known not to be locked.

Phobos, Io, Europa, and Callisto.
The moons in our Solar System are quite varied, as these photos of Phobos, Io, Europa, and Callisto show. (Phobos image by NASA/JPL-Caltech/University of Arizona; Io image by NASA/JPL/University of Arizona; Europa image by NASA/JPL/Deutsche Forschungsanstalt fuer Luftund; and Callisto image by NASA/JPL/Deutsche Forschungsanstalt fuer Luftund; all via Wikimedia Commons. Click for larger image.)

Although it's now denigrated to the status of "dwarf planet," Pluto was hailed as a new planet when it was discovered in 1930. Pluto is so distant, located between thirty and fifty astronomical units from the Sun in a highly elliptical orbit, and so small (less than 20% the mass of our Moon), that observations are difficult. From its discovery until 1978, Pluto was thought to be devoid of satellites. Then, Charon, with about 11.6% Pluto's mass, was discovered.

The close proximity of Charon to Pluto, coupled with Charon's considerable mass, make the Pluto-Charon couple an extreme example of a tidal lock. Pluto and Charon are mutually locked, so each presents the same face to the other. Their barycenter, the center of their orbits around each other, is outside of Pluto.

The Hubble Space Telescope has considerably expanded our knowledge of Pluto with the discovery of four additional moons. Nix and Hydra were discovered by Hubble in 2005, followed by Kerberos in 2011 and Styx in 2012. Aside from always reminding me of the Care Bear plush teddy bears my daughter had in the 1980s, Kerberos has the darkest surface of all Pluto's satellites.

Orbits of Pluto's satellitesThe Pluto satellite system, showing the orbits of its five satellites, Charon, Nix, Hydra, Styx, and Kerberos.

(Illustration by the author using Inkscape. Click for larger image.)

The impending arrival next month of NASA's spacecraft, New Horizons, has motivated recent observations of Pluto and its moons by the Hubble Space Telescope, and these have just been reported in the journal, Nature.[2-7] New Horizons visited Jupiter in February, 2007, when it got a gravitational assist to propel it towards Pluto.

New Horizons will make the first flyby of Pluto by any spacecraft on July 14, 2015. The Hubble observations of Pluto and its moons were by Mark Showalter, a senior research scientist at the SETI Institute (Mountain View, California), and Douglas Hamilton, professor of astronomy at the University of Maryland (College Park, Maryland).[2-3]

As can be seen from the plan of the Pluto system, Charon orbits quite close to Pluto. This fact, combined with the more distant orbits of the other moons, has the effect that Nix, Hydra, Styx, and Kerberos are actually in orbit around both Charon and Pluto, which act as a binary planet.[2] As a consequence, Pluto's moons are subject to unusual gravitational forces.

Styx, Nix and Hydra interact in a three-body resonance in which their orbits are synchronized with each other, and Nix and Hydra rotate chaotically, driven by the unusual gravitational influence of the Pluto-Charon binary planet.[2-3] These recent Hubble observations also confirmed the relative darkness of Kerberos compared with the other satellites.[3]

Figure captionThe shapes and relative sizes of Pluto's moons.

Kerberos is darker in color than the other moons.

(NASA/ESA image by A. Field (STScI), relabeled for clarity.)

Kerberos and Styx are also expected to exhibit chaotic dynamics.[4] Says co-author of the study, Douglas Hamilton,
"Like good children, our Moon and most others keep one face focused attentively on their parent planet... What we've learned is that Pluto's moons are more like ornery teenagers who refuse to follow the rules."[3]

Co-author Showalter observes that "Prior to the Hubble observations, nobody appreciated the intricate dynamics of the Pluto system"[3] As can be deduced intuitively, the orbital dance of the Pluto-Charon binary causes the remaining four moons to tumble unpredictably, and this effect is amplified by their oblate shapes.[3] If it wasn't for their orbital resonance in which Styx orbits Pluto twice, and Hydra three times, for every orbit of Nix, these moons would crash into each other.[3-4]

Three of Jupiter's satellites, Io, Europa and Ganymede, also have orbital resonance.[7] Mathematical models indicate that all five of Pluto's moons were likely locked into a 1:3:4:5:6 resonance after an impact on a proto-Pluto formed Charon and the other moons. These ratios are very close to the orbital periods now seen for Pluto's moons.[7]

Figure captionArtist's impression of a tumbling Nix.

(NASA/ESA image by M. Showalter (SETI Inst.), G. Bacon (STScI). Click for larger image)

The charcoal-like appearance of Kerberos is hard to reconcile with the proto-Pluto impact model for the system formation. If all the moons formed from the same material, why does Kerberos look different? As Douglas Hamilton summarizes, "...If they all formed together, they all formed out of the same stuff. It is extremely hard to understand how one of them is a charcoal briquette and it's orbiting between two snowballs."[6]

Most scientists are thinking that New Horizons will discover more moons orbiting Pluto. Alan Stern, principal investigator for the New Horizons mission, is quoted in Smithsonian Magazine as saying,
"If you polled my science team, I'm quite sure that the majority would be surprised not to find more moons... The question is are we going to find 2, or 10 or 20? I wouldn't put my bets on zero."[7]

Careful study of the Pluto system can lead to an understanding of circumbinary planets, planets that orbit a binary star. Seventeen such planets have been discovered to date.[3,6]


  1. G. H. Pettengill and R. B. Dyce, "A Radar Determination of the Rotation of the Planet Mercury," Nature 206, vol. 206, no. 4990, (June 19, 1965), pp. 1240ff., doi:10.1038/2061240a0.
  2. M. R. Showalter and D. P. Hamilton, "Resonant interactions and chaotic rotation of Pluto's small moons," Nature, vol. 522, no. 7554 (June 4, 2015), pp. 45-49, doi:10.1038/nature14469. A PDF copy of the paper is available from the Hubble Space Telescope web site, here.
  3. Pluto's Moons Seen in Highest Detail Yet, University of Maryland Press Release, June 3, 2015.
  4. Hubble observes chaotic dance of Pluto's moons, Hubble Space Telescope Press Release No. heic1512, June 3, 2015.
  5. Hubble Finds Two Chaotically Tumbling Pluto Moons, Space Telescope Science Institute Press Release No. STScI-2015-24, June 3, 2015.
  6. Jonathan Amos, "Hubble studies Pluto's wobbly moons," BBC Web Site, June 4, 2015.
  7. Jay Bennett, "Weird Orbital Behaviors Offer Clues to the Origins of Pluto's Moons," Smithsonian Magazine, June 3, 2015.

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