Moons of Pluto

• 1. Introduction

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  Pluto has five known moons, and the story of how we came to know that fact is a lesson in how wrong our assumptions can be. For decades after Pluto's discovery, astronomers looked at the faint light coming from that distant point in the solar system and assumed everything they saw belonged to Pluto alone. Then, on the 22nd of June 1978, astronomer James Christy spotted something that changed the picture entirely. There was a companion. That companion, Charon, turned out to be so large relative to Pluto that the two bodies have since been seriously considered for reclassification as a binary dwarf planet. And Charon was just the beginning. Four more moons followed, the last two found not by accident but by people actively searching for dangers that might destroy a spacecraft. How did five moons end up orbiting a dwarf planet at the edge of the solar system? Why do they cluster so tightly when there is room for so much more? And what does the sky look like, standing on Pluto, when Charon rises?

• 2. Charon, the Companion

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  James Christy's 1978 discovery did not just add a moon to Pluto's profile. It forced a complete recalculation of Pluto's size. Before Charon was identified, every estimate of Pluto's dimensions had lumped the reflected light and observed mass of the entire Pluto-Charon system together and attributed it all to Pluto. The error was substantial.

  Charon is about half the diameter of Pluto. Its mass is nearly one eighth of Pluto's. That mass ratio is significant enough that the gravitational center of the Pluto-Charon system, called the barycenter, does not lie inside Pluto at all. It sits in open space roughly 960 kilometers above Pluto's surface, between the two bodies.

  Both worlds are also tidally locked, meaning each permanently faces the other. Charon always sees the same face of Pluto, and Pluto always sees the same face of Charon. No other planet-moon pair in the solar system shares this mutual tidal lock.

  At the International Astronomical Union's General Assembly in August 2006, a formal proposal was put forward to reclassify Pluto and Charon together as a double planet. The proposal was ultimately abandoned. Charon is, like Pluto, a perfect sphere to within current measurement uncertainty, which is itself a mark of how massive it is: only bodies with enough gravity collapse into that shape under their own weight.

• 3. Four Small Outer Moons

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  Nix and Hydra were spotted on the 15th of May 2005 by astronomers on the Pluto Companion Search Team, a group preparing for the New Horizons mission and working with the Hubble Space Telescope. They carried the provisional labels S/2005 P 1 and S/2005 P 2 until the International Astronomical Union formally named them Nix and Hydra on the 21st of June 2006.

  Kerberos came next. Its discovery was announced on the 20th of July 2011, found while researchers were looking for ring systems around Pluto. Styx was announced last, on the 7th of July 2012, spotted during a search for hazards that might threaten the New Horizons spacecraft on its approach.

  All four orbit well beyond Charon, ranging from Styx at 42,700 kilometers from the system's barycenter out to Hydra at 64,800 kilometers. Their sizes are modest. Nix and Hydra, the two larger ones, measure roughly 42 and 55 kilometers along their longest axes respectively. Styx measures about 7 kilometers and Kerberos about 12. None of the four is round; all are irregularly shaped.

• 4. A Compact and Crowded System

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  Pluto's gravitational reach, called the Hill radius, extends roughly 6 million kilometers. Prograde moons could theoretically orbit stably out to 53 percent of that distance. Retrograde moons could reach even farther, up to 69 percent. The actual moons occupy none of that available space. Only the innermost 3 percent of the stable prograde region contains any satellites at all.

  What is more, the region between Styx and Hydra is so tightly packed that there is little room for additional moons with stable orbits inside it. An exhaustive search conducted by New Horizons confirmed that no moons larger than 4.5 kilometers in diameter exist out to 180,000 kilometers from Pluto. That represents 6 percent of the full stable region for prograde orbits.

  Mark R. Showalter speculated about what life on Nix might actually look like, describing a day there in striking terms: "Nix can flip its entire pole. It could actually be possible to spend a day on Nix in which the sun rises in the east and sets in the north. It is almost random-looking in the way it rotates."

  Before New Horizons arrived, scientists predicted that Nix, Hydra, Styx, and Kerberos would all rotate chaotically or tumble. The spacecraft's imaging confirmed they had not tidally spun down the way Charon had. Instead, all four were found at high obliquity. Only one other moon in the solar system, Saturn's Hyperion, is known to tumble in this way, though Haumea's moons are suspected to do so as well.

• 5. Near Resonances with Charon

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  Styx, Nix, Kerberos, and Hydra sit in a 1:3:4:5:6 sequence of near resonances with the Pluto-Charon orbital period. None of them is in perfect resonance. Hydra is the closest, sitting only about 0.3 percent away from its resonance. Kerberos is about 0.6 percent off. Nix is about 2.7 percent away, and Styx is the farthest from resonance at about 5.4 percent.

  One hypothesis ties this arrangement to Charon's early history. As Charon was tidally boosted into its current synchronous orbit, it may have dragged the outer moons into forced resonances. When Charon's orbital eccentricity was later tidally damped, the moons were released from those resonances, ending up near but not inside them. The Pluto-Charon pair generates gravitational forces that vary by about 15 percent peak to peak at the locations of the outer moons.

  However, the mathematics of resonance boosting presents a problem. A resonance with Charon could have pushed either Nix or Hydra into its current orbit, but not both. Sending Hydra outward to its current position would have required Charon's eccentricity to be nearly zero, at about 0.024. Sending Nix to its position would have required Charon's eccentricity to be at least 0.05. These two requirements are mutually exclusive. The implication is that Nix and Hydra were not boosted from closer in. They were captured material that formed around the Pluto-Charon pair and migrated inward until Charon's gravity trapped them near resonance. Kerberos and Styx may have followed the same path.

• 6. A Violent Origin

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  Pluto's entire satellite system is thought to trace back to a single catastrophic collision, an event compared directly to the Theia impact that created Earth's Moon. In both cases, the angular momenta of the resulting moons are only explainable by such a high-energy scenario.

  The nearly circular orbits of the four smaller moons argue against an alternative: that they were captured objects from the Kuiper Belt. Captured objects tend to arrive in eccentric and tilted orbits. The flatness and circularity of the system point instead to a shared formation event.

  The moons also show a color difference from Pluto that carries information. Pluto is one of the reddest bodies in the solar system. The moons are grey. The grey color is thought to reflect a loss of volatile materials during the original impact or in the coalescence that followed, leaving surfaces dominated by water ice.

  One alternative version of the collision describes an impact at roughly 2,000 miles per hour, not violent enough to shatter either Pluto or Charon entirely. In this scenario, the two bodies actually stayed in contact for as long as ten hours before separating. Pluto was rotating faster at that time, completing one rotation every third hour. That rapid spin created a centrifugal force strong enough to overcome the gravitational attraction holding the two bodies together. Charon separated but stayed bound. The same process might have ejected material that eventually formed Styx, Nix, Kerberos, and Hydra. New Horizons found no additional moons and no ring system, which is itself a puzzle: a collision large enough to create the known moons should have scattered more debris.

• 7. Eclipses Across the System

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  Standing on the surface of Pluto, Charon occupies an angular diameter of 4 degrees of arc. The Sun, by comparison, appears between only 39 and 65 arcseconds wide from that distance. Charon would therefore appear to span roughly eight times the diameter, or 64 times the area, of the Moon as seen from Earth. This is not because Charon is larger than Earth's Moon. It is because Charon is so much closer to Pluto. Earth's Moon is about 20 times farther from Earth's surface than Charon is from Pluto's.

  Because Pluto always faces Charon due to tidal locking, solar eclipses by Charon only happen on the Charon-facing hemisphere. The opposite side of Pluto never sees Charon at all and never experiences a Charon eclipse. The next major period of mutual events will begin in October 2103, peak around 2110, and end in January 2117. During that window, solar eclipses caused by Charon will occur once every Plutonian day, with the longest lasting up to 90 minutes.

  Nix and Hydra, despite their small sizes, are large enough as seen from Pluto to cause total solar eclipses. Their angular diameters, between about 2 and 9 minutes of arc, exceed the Sun's apparent width. Styx and Kerberos produce partial and hybrid eclipses rather than total ones. Kerberos, though slightly larger, has a minor axis of only about 32 arcseconds, which is not wide enough to fully block the Sun. Styx, with angular dimensions on the order of 77 by 39 arcseconds at its largest, produces no annular eclipses at all, its widest axis being more than 10 arcseconds wider than the Sun.

• 8. New Horizons Flies Through

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  In July 2015, the New Horizons spacecraft became the first craft to visit the Pluto system. The encounter returned images of Nix at resolutions as fine as 330 meters per pixel and Hydra at up to 1.1 kilometers per pixel. Lower-resolution images covered Styx and Kerberos.

  Beyond the images, New Horizons resolved several standing questions. It confirmed that no moons larger than 4.5 kilometers in diameter exist within 180,000 kilometers of Pluto. It found no ring system, despite earlier predictions that small-body impacts on the outer moons could eject enough debris to form one. A deep optical survey by the Advanced Camera for Surveys on the Hubble Space Telescope had already suggested no rings existed, and occultation studies supported the same conclusion. New Horizons settled it.

  The spacecraft also found the rotation states of the four small moons surprising. Scientists had expected chaotic tumbling, and in some sense they got it, but the moons had not tidally locked or spun down as expected. Styx may still be experiencing intermittent and chaotic obliquity variations, meaning its tilt toward the Sun shifts unpredictably over time. Whether Styx, Nix, Kerberos, and Hydra were born tilted or were tipped by a spin precession resonance with Charon remains an open question, one that the New Horizons dataset has narrowed but not yet closed.