Some of the best things come in small packages, as the saying goes, and that certainly holds true for Pluto and its system of moons. A new comprehensive analysis of archival observations of the Pluto system, undertaken with NASA’s Hubble Space Telescope during the last decade, have revealed the dwarf planet and its moons comprise a fascinating mini planetary system of their own which is entirely unique in the Sun’s planetary family, while also providing many important insights not only about the physical processes that take place in our own Solar System, but about those that govern extrasolar ones as well.
Discovered in 1930 by American astronomer Clyde Tombaugh, Pluto was originally thought to be the long-sought-for hypothetical massive Planet X in the outskirts of the Solar System, which astronomers had extensively searched for many decades during the mid-19th and early 20th century. As telescopes were becoming steadily more powerful in the years following the discovery of Pluto, it was eventually determined that the latter was in reality just a diminutive world no more than 2,300 km across, which was also found in the late 1970s to be accompanied by a comparatively large moon named Charon that had a diameter of approximately 1,200 km, almost half that of Pluto. The Hubble Space Telescope completed the picture of Pluto’s moon system by detecting an additional four much smaller satellites around the planet, Nix and Hydra in 2005, as well as Kerberos and Styx in 2011 and 2012 respectively.
One of the more interesting aspects of Pluto and Charon is the fact that, due to their comparative sizes and close distance between them, both objects orbit their common center of mass thus comprising a double-planet system with the rest of the smaller moons revolving around the latter, essentially making Pluto and Charon the only binary planet in the entire Solar System, with the exception of the Earth and the Moon, whose own center of mass lies within our home planet. Ever since Pluto’s smaller moons were discovered, astronomers have been actively studying their fascinating orbital dynamics with the help of Hubble, which is currently the only space telescope capable of observing their motions from a distance of more than 5 billion km away. Now, in a new paper which is being published today (June 4) in the journal Nature, planetary astronomer Mark Showalter, a senior research scientist at the SETI Institute in California and co-discoverer of Kerberos and Styx, and Dr. Douglas Hamilton, a professor of astronomy at the University of Maryland, present the results of a comprehensive four-year study of the entire Pluto system, based on archival Hubble data collected by the orbiting observatory between 2005 and 2012. The analysis of the data revealed quite unexpected and fascinating findings that were contrary to previous theoretical predictions, including the extremely small surface brightness of Kerberos compared to that of other satellites in the Pluto system, as well as the chaotic nature of the orbital revolutions of Nix and Hydra.
More specifically, the researchers studied the brightness variations of Pluto’s moons throughout their entire orbital periods, under the assumption that the latter were locked in synchronous orbits around Pluto, as is the case with Charon and most of the other moons in the Solar System, which causes them to constantly point one hemisphere toward their respective planets. Since Pluto’s smaller moons are irregularly shaped due to their very small sizes, the researchers expected that their brightness would change in a predictable manner during their orbital revolutions around Pluto, if they were indeed locked in such a synchronous rotation. Yet what Showalter and Hamilton found to their great surprise was there appeared to be no correlation whatsoever between the moons’ brightness and their position along their orbits, indicating that Nix and Hydra definitely weren’t locked in a synchronous rotation around Pluto. After running a series of numerical simulations based on these observations, the researchers soon realised that these orbital wobbles of Nix and Hydra were the result of the irregular gravity field they were embedded in, which was caused by the Pluto-Charon binary planet system itself, leading the smaller moons to exhibit a fundamentally chaotic and unpredictable orbital spins over longer periods.
“[Pluto and Charon] whirl around each other rapidly, causing the gravitational forces that they exert on the small nearby moons to change constantly,” says Hamilton. “Being subject to such varying gravitational forces makes the rotation of Pluto’s moons very unpredictable. The chaos in their rotation is further accentuated by the fact that these moons are not neat and round, but are actually shaped like rugby balls! 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.”
These chaotic orbital dynamics elevate Pluto and its moons from the status of just a set of inconspicuous lesser bodies in the outskirts of the Solar System, to that of a very unique and fascinating mini planetary system in its own right. “Prior to the Hubble observations, nobody appreciated the intricate dynamics of the Pluto system,” adds Showalter. “Our research provides important new constraints on the sequence of events that led to the formation of the system.”
As for the results of these strange orbital dynamics, the view from Nix and Hydra would be unlike anything seen anywhere else in the Solar System. “You would literally not know if the Sun is coming up tomorrow,” explained Showalter during the presentation of the study’s results at a NASA teleconference that was held yesterday at the agency’s headquarters, in Washington, D.C. “For that matter, the Sun might rise in the west and set in the east. In fact, if you had a real estate on the north pole of Nix you might suddenly discover one day that you’re on the south pole instead. This is the environment we’re talking about for Nix and Hydra and we believe for the other moons as well, although we don’t [currently] have the data to show that.”
Years-worth of studies with the Hubble Space Telescope have revealed that the Jupiter and Pluto systems have much in common regarding their structure and that the moons of Pluto are not small in the relative sense. Seen at left is a montage of Jupiter and its four large Galilean moons as photographed by NASA’s Voyager 1 spacecraft in 1979. Scaling Pluto and its moons up to the size of Jupiter creates a system very similar to the Jupiter system we already know (seen at right), with the exception of Charon, which is the primary driver of the chaos that has been observed with Hubble in the Pluto system. Image Credits: NASA/Showalter/SETI Institute
One key question that naturally arose out of these findings is how the Pluto system is kept stable over long periods of time without having its moons fly apart or collide with one another due to these orbital instabilities. The answer came when Showalter and Hamilton determined through their numerical simulations that all four moons are being kept in a surprising near-3:4:5:6 orbital resonance relative to Charon, while Nix, Styx, and Hydra are tied together in a near-1:2:3 three-body orbital resonance, where for each orbital revolution that Nix makes around Pluto, Styx makes two and Hydra makes three, similar to the near-resonances that are shared between Io, Europa, and Ganymede around Jupiter. “What this relationship does, is to prevent these moons from ever getting very close together all at the same time and that helps to stabilise the system,” explains Hamilton. “The resonant relationship between Nix, Styx and Hydra makes their orbits more regular and predictable, which prevents them from crashing into one another. This is one reason why tiny Pluto is able to have so many moons.”
The brightness variation measurements which led the researchers to study orbital dynamics of Pluto’s smaller moons allowed them to make another important discovery about the latter as well. More specifically, by studying the light curves of the smaller moons, Showalter and Hamilton were able to determine that Nix and Hydra both shared an albedo (surface reflectivity) of approximately 40 percent, similar to that of Charon, which indicated they are all fairly bright objects. Yet the albedo of Kerberos, whose orbit lies between that of Nix and Hydra, was found to be no greater than 4 percent, indicating it was a surprisingly dark object. Such surface darkening has been observed on various Solar System moons as well and is thought to result from meteorite dust debris that covers the moon’s surfaces over time. But the fact that the surfaces of the neighboring moons Nix and Hydra are so bright presents an unexpected enigma for astronomers, who had theorised that due to the close locations, the surfaces of all three moons should have been coated with dark debris material in similar proportion. “Think of a charcoal briquette orbiting between two dirty snowballs,” says Showalter. “Now, that’s a very, very strange result.”
A numerical simulation of the orientation of Nix as seen from the center of the Pluto system. It has been sped up so that one orbit of Nix around Pluto takes 2 seconds instead of the actual 25 days. Large wobbles are visible, and occasionally the pole flips over. This tumbling behavior meets the formal definition of chaos; the orientation of Nix is fundamentally unpredictable. Video Credit: STScI and Mark Showalter, SETI Institute
The overall findings to come from this new study by Showalter and Hamilton not only help to shed more light to the intricate dynamics of the Pluto system, but they can help astronomers gain important insights to the inner workings of extrasolar planetary systems as well. “What if Pluto were the size of our Sun? Then Charon would be a small [neighboring] star, forming a double star system,” commented during yesterday’s teleconference Heidi Hammel, a planetary astronomer and executive vice president of the Association of Universities for Research in Astronomy in Washington, D.C. “And we actually know from many telescopic searches that double stars are ubiquitous throughout our galaxy. And thanks to NASA’s Kepler spacecraft and many other telescopic searches , we know that many of these double stars do host planets, so Pluto and its complex and chaotic moon system can provide a direct analog to these planetary systems we see around other stars … Everything we have learned about the Pluto system, we actually learned without resolving these moons – they are just points of light. And we’re developing now the capabilities in astronomy, of seeing exoplanets around other stars as points of light separated from their host stars … So, thanks to this very interesting moon system of Pluto, we will one day be able to apply the same kinds of tools and techniques for probing those moons, to study exoplanetary systems around other stars and learn about the characteristics of those planets in that kind of detail that you are hearing [about Pluto] today. And all of this is grounded in our knowledge of these four tiny moons travelling in their chaotic orbits and chaotic rotations around the distant around the binary Pluto and Charon system, as seen with the Hubble Space Telescope.”
“We are learning chaos may be a common trait of binary [exoplanetary] systems,” adds Hamilton. “It might even have consequences for life on planets if it is found in such systems.”
Despite its superior capabilities, Hubble can only reveal so much about Pluto and its menagerie of fascinating and chaotic moons. More detailed observations are bound to come just 40 days from now, when NASA’s New Horizons spacecraft will speed through the Pluto system on 14 July, for its eagerly anticipated and historic flyby of this distant realm of the Solar System. “Hubble has provided a new view of Pluto and its moons revealing a cosmic dance with a chaotic rhythm,” says John Grunsfeld, associate administrator of NASA’s Science Mission Directorate in Washington, D.C. “When the New Horizons spacecraft flies through the Pluto system in July we’ll get a chance to see what these moons look like up close and personal.”
And if there ever was a need to come up with a reason to make the upcoming New Horizons flyby more interesting than it already is, then the promise of a more detailed view of exoplanetary systems in the vast reaches of our Milky Way galaxy would be a fine reason indeed.
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