The Cryptic World of the Satellites of Distant Pluto

Designated 134340 Pluto, this second-largest dwarf planet in the solar system is one with a history of no other planet, such as its downgrade and climatic discovery in the mid twentieth century; but one of the most remarkable aspects of this cryptic world are its specialized brood of moons which it harbors so well astronomers have never been able to photograph them, or even Pluto itself (in high resolution) to say the least! So what makes this distant, icy world so fascinating it's rather arcane? The answer to that question lies in your perspective of the planet: probably because we know nothing about it, or because of its hidden mystery and history of its creation (etc). Pluto is a rather special dwarf-planet; it lies within the Kuiper Belt, a belt of objects beyond the orbit of Neptune and orbit the sun in irregular orbits, along with many other know minor and dwarf planets such as Eris (the most massive dwarf planet in the solar system), Makemake, and Haumea to name a few.

The green objects represent the wide variety of objects in the Kuiper Belt, Pluto including. This map shows the area of the Kuiper Belt and the comparison of its distance from the gas giants (such as Jupiter) in the center.

Although not the only minor planet to have a collection of moons, Pluto has the most known of all dwarf planets. With a count from March 2012, Pluto has four moons, three of which have been named officially by the International Astronomical Union (IAU). Its most famous would be Charon, which was discovered June 22, 1978 (publicly announced July 7) by James Christy at the United States Naval Observatory Flagstaff Station while he was carefully observing photographic plates of the minor planet. A bulge was noticed periodically while plates were being taken and it was obvious this bulge was not a result of an error on the plate, but rather a moon orbiting the planet. Although this was rather controversial for a while, any doubt of no lunar presence was annulled during the eclipse period of Charon during the 1980s (specifically 1985-90).

"It was fortuitous that one of these intervals happened to occur so soon after Charon's discovery," Swarthmore College Computer Society writes in their article of Charon, and it was beneficial! The eclipses of Pluto are an extremely rare phenomena which can occur only twice per the planet's 248 year revolution around the sun, the next ones to occur start October 2103, peak in 2110, and will end January 2117, as observed from earth. It will be Charon who does most of the eclipsing for Pluto. More can be read here about the phenomena.

The Sun disappears behind Charon's surface during the total solar eclipse on Pluto of 23rd December 2111 (computer simulation) Thanks to Wikipedia and JPL simulator

The complete Plutonian system (as of 2005) thanks to the Hubble

Charon is not the only moon Pluto has, however! Three others have been discovered since the days of Charon's discovery and people are becoming less and less more skeptical about the existence of such objects. Nix (also spelled Nyx) and Hydra are two others discovered in July of 2005 by the HST Pluto Companion Search Team. The discoveries of both were publicly announced on October 31, 2005 after more than five works of research and confirmation from precoveries done in 2002.

Nyx, named after the Greek goddess of the night (all moons of Pluto were named in accordance to deities of the Underworld, as Pluto was the god of the Underworld in Greek mythology; Charon was named after the ferryman who took souls into Hades across the river Styx), orbits in the same plane Charon does and orbits every 24.9 days with a unique 1:4 orbital resonance with Charon (although not quite). The Nictian surface is quite unknown (like Pluto's) and is usually at the measured magnitudes of 23.38 to 23.7, almost 6300 times dimmer than the dwarf planet itself! But, its orbital resonance was a problem (because it was not perfect). In a paper presented by the discoverers of the moons entitled Orbits and Photometry of Pluto's Satellites: Charon, S/2005 P1, and S/2005 P2. This paper explained that although the orbital resonance between these objects were mostly 1:4, there was a 2.7% timing discrepancy, proving no resonance existed. The below portion of text discusses this on a higher level. 

The orbital period of P1 is 38.2065 ± 0.0014 days, while 6 times the period of Charon is 38.3234 days. This is the period ratio most nearly commensurate, and from the 0.3% difference from a 6 : 1 period ratio we get a circulation of the resonant argument in 2090 ± 80 days, less than 6 yr. Likewise, our period of P2 is 24.8562 ± 0.0013 days, compared with 4 times the period of Charon, which is 25.5489 days. This difference corresponds to a 2.7% difference, and thus, the resonant argument will circulate in only 229 ± 2 days. Comparing the periods of P1 and P2, we find that their ratio is 1.53710 ± 0.00006, not the exact ratio of 3/2. Again, circulation would be quite rapid, at just 515 ± 6 days. These circulation periods are all of comparable timescales to the duration of the constraining astrometry for the two-body orbits we have derived. We do not see any obvious periodic deviations from a two-body Keplerian orbit and thus argue that perhaps there are no active resonances.

Hydra (named after the chthonic beast of nine heads in the Grecian underworld) was discovered with Nyx and remains virtually unknown as the others. Its magnitude is just a tad brighter than Nyxs' at 22.9 to 23.3, and its orbital resonance with Charon made an issue as well as Nyx's. The Hydrian revolution was 1:6 to the Charonian one and (not suprisingly!) there was a 03.% timing discrepancy between the two, making no resonance at last.  

With that having been said, Pluto's three moons discovered 2005 and before have become a pivotal part of the history of Pluto, the planet we know almost nothing about. But then, in July of 2011, another moon was discovered. This moon was not the farthest moon out, as most moons are discovered in their order from closest to out-most of the planet [probably because the closest moons are the largest and most prominent and the outer are less-brighter and more hard to detect], but rather it was found to orbit between Hydra and Nyx. This moon, labeled S/2011 P1 was discovered (announced) on July 20, 2011, and is relatively dimmer than the other moons, at magnitude 26.1 ± 0.3. The plus/minus symbols details that the magnitude has been seen to shift as it orbits the planet. You can read more about Pluto's fourth in our article we published back in July when it was discovered. [Below is an image of P4 orbiting the planet]. 

When we image what Pluto may look like, we imagine a gelid, ice-ridden world void of life and any consolation. But amid the depression that lurks throughout the planet, we will always have the exciting anticipation of one thing: New Horizons. This discussion of Pluto's moons would never be complete without NASA's excellent space-probe sent out to explore than planet and its moons, hoping to discover new ones. It won't reach the planet until (proposed) July of 2015, so although the wait is burdensome, the rewards will be plenty! Pluto is not an icy world but a world of discovery, something we can all take part in.

Hubble Discovers the Fourth Moon of Dwarf Planet Pluto

Discovered by the Hubble Space Telescope through images taken of Pluto on June 28, 2011 (verified July 20); Hubble has discovered that Pluto has a fourth companion. Though this may not seem very impressive or such a big deal, finding moons around labeled dwarf planets is something rare, especially for those who only knew about Charon. Currently labeled S/2011 134340 1 P4 (or P4 for short), Hubble discovered this icy moon in a search for rings around the planet. This is peculiar, becasue so far, only gas giants are known to have rings, and Pluto is definitely not a gas giant!

According to recent statistics, P4 has an estimated diameter of 13 to 34 kilometers (8 to 21 miles) making it the smallest known moon  and orbits Pluto at a rapid rate: about every 32.1 Earth-Days, between the orbits of Nix and Hydra, the other moons. Its full orbit reaches 59,000 kilometers, which is about 37,000 miles in length; and is remarkably in a 1:5 resonance with Charon, despite the 0.6 % leftover time involved. This means every time P4 orbits, Charon does five times. Nix and Hydra are also in orbital resonance with Charon, 1:4 and 1:6, respectively.

Image of Pluto's full family; credit Hubble. The black strip down the center was placed to block out the unnecessary light that Pluto emitted. The glare prohibits the moons being seen. The 'cross' shape is an imaging error.

A few NASA scientists comment about the new moon: "I find it remarkable that Hubble's cameras enabled us to see such a tiny object so clearly from a distance of more than 3 billion miles (5 billion kilometers)," said Mark Showalter of the SETI Institute in Mountain View, Calif., who led this observing program with Hubble.

"Could this planet get any more interesting?" says New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute, Boulder Colorado. "We already know that when New Horizons provides the first close-up look at Pluto in July 2015, we'll see planetary wonders we never could have expected. Yet this discovery gives us another hint of what awaits us in the Pluto system, and we're already thinking about how we want to study this new moon with New Horizons. What a bonus for planetary science and for New Horizons!"

Hubble helped to discover three of the four moons of dwarf planet Pluto. Nix and Hydra were discovered in 2005, and now P4 in 2011. Hubble could have discovered P4 in 2005, but there was a simple complication. When the images of Nix and Hydra were released in 2005, NASA scientists noticed a small 'smudge' in the picture (which we now know was P4), but disregarded it, for which I can assume for 'logical' reasons. Apparently, it was ignored on account of the fact that the speck was drowned out by a diffraction spike, or an error in imaging.

This is the new model for the Plutonian system. P4 is the third closest object in the image.

Something interesting to concern yourself with - what will this new moon be named? Of course we have to name it (who would wish to keep it at S/2011 134340 1 P4?!); in the most probable sense, it will be another name from mythology, like most moons and other astronomical bodies are; observe: Io, Callisto, and Europa (a few of Jupiter's satellites) come from Ovid's Metamorphoses (Roman Mythology), you have asteroid 2 Pallas (has a myriad of names for a myriad of people), and 1 Ceres (Roman goddess of the field), to name a few.

In naming of the Plutonian system, P4 will be named soon a name that follows after the 'tradition' of naming the Plutonian moons. It will have as name that reflects Hades and the Underworld in Greek Mythology. Pluto was named after the god of the Underworld; Charon, the ferryman that would take one's soul to the Underworld; Nix, (spelled Nyx in mythology), was the goddess of the night; and Hydra, an mythical creature with seven heads, one (the center) immortal. I am thinking that P4 will be named one of the following, but have no idea what it will be. When P4 is named, I'll post it. I am thinking possibly: Medusa, Persephone, or Styx. Medusa is a horrible monster that would turn onlookers into stone, living on an island in the river Styx; Persephone was the goddess of the Underworld, who was captured by Pluto to be his bride (it really is a myth in itself); and Styx, is a river in which leads to the underworld, where ferryman Charon rides.

"Pluto continues to amaze us," says Hal Weaver, a New Horizons project scientist. "Who would have thought a dwarf planet could support such a complex satellite system? The hunt will now be on for similarly complex systems around other Kuiper Belt objects."More news can be found here.

More awaits for us from the Plutonian system! It's peculiar to find a dwarf planet that has this many moons, but anything is possible. New Horizons will arrive in 2015, but until then; P5 may be around the corner...