In a mysterious region beyond the orbit of the beautiful, banded, blue ice-giant planet Neptune–the most distant of the eight major planets from our Sun–there is a dark and frigid domain called the Kuiper Belt. Within this remote region, where our Sun shines with only a weak fire, and appears to be merely a particularly large star suspended in the black sky, a multitude of strange, icy worldlets tumble around our Star. Pluto, a large icy denizen inhabiting the Kuiper Belt, was originally classified as the ninth major planet from our Sun after its discovery in 1930. However, with the realization that this frozen “oddball” is really only one of several large, icy inhabitants of the Kuiper Belt, the International Astronomical Union (IAU) found it necessary to formally define “planet” in 2006–and poor Pluto was unceremoniously ousted from the pantheon of major planets. Pluto, now freshly reclassified as a dwarf planet, nonetheless remains a small world of great interest, debate, and affection. Scientists will soon learn much more about this beloved, distant, ice-ball so far away, when, after a treacherous nine-year journey of three million miles through interplanetary space, NASA’s hearty New Horizons spacecraft arrives at Pluto on July 14, 2015.
However, weeks before New Horizons’ scheduled arrival at its main target, it managed to reveal some strange secrets long-held in the deep freeze of our Solar System’s outer limits. For their new study, released in June 2015, a team of planetary scientists collected all available NASA and European Space Agency (ESA) Hubble Space Telescope (HST) data on the four outer moons of Pluto in order to study this strange system in greater depth. Their observations reveal that at least a duo of Pluto’s moons are not rotating on their axes in an orderly manner at all, but are instead in chaotic rotation while circling Pluto and its large companion moon, Charon–a moon-world that weighs-in at a hefty 11% of Pluto’s mass. Charon is commonly thought to have once been a part of Pluto itself, before it was blasted off in a catastrophic collision with another object billions of years ago. The new study also suggests that one of the mystery moons has a bizarre jet-black coloring of unknown origin and composition. These truly amazing results appear in the June 4, 2015 issue of the journal Nature.
Almost every moon in our Sun’s family of orbiting objects, including Earth’s own bewitching, large Moon, rotates on its axis at the same speed as it orbits its parent-planet. It is for this reason that we always observe the same side of our Moon facing us on Earth. But on Pluto, things work a bit differently. Astronomers have now discovered that there are no hidden sides to its moons!
In this fascinating new research, the astronomers announced that they have discovered that a duo of Pluto’s moons, dubbed Nix and Hydra, are actually in a chaotic rotation. This basically means that an observer standing on the alien surface of icy, strange Pluto, would not observe the same face of its moons from one night to the next. For those observers visiting the two bizarre moons themselves, things would be even weirder because every day would be a different length from the one that came before it.
The other duo of Pluto’s five known moons being studied, Kerberos and Styx, are also currently thought to be in chaotic rotation as well! However, Pluto well may have a few more small moons that have not as yet been spotted–at least, as of this writing.
“Prior to the Hubble observations, nobody appreciated the intricate dynamics of the Pluto system,” Dr. Mark Showalter explained in a June 3, 2015 HST Press Release. Dr. Showalter is of the Search for Extraterrestrial Intelligence (SETI) Institute in Mountain View, California. He is lead author of the Nature paper.
The truth is that Pluto’s large moon Charon is a freak. Pluto and Charon do not behave like a “normal” planet-and-moon duo. In fact, the system is unique in our Solar System because the two small, icy worlds face each other and spin together around a fixed point. For this reason, many planetary scientists have suggested that Pluto and Charon actually form a binary system–rather than that of a moon and planet pair. The new research shows that the chaotic movements of Pluto’s smaller moons are caused by this weird Pluto-Charon relationship.
“These two bodies whirl around each other rapidly, causing the gravitational forces that they exert on the small nearby moons to change constantly. Being subject to such varying gravitational forces makes the rotation of Pluto’s moons very unpredictable. The chaos in their rotation is further intensified by the fact that these moons are not neat and round, but are actually shaped like rugby balls,” explained Dr. Douglas Hamilton in the June 3, 2015 HST Press Release. Dr. Hamilton is of the University of Maryland in College Park, and co-author of the study.
The Outer Limits
Pluto itself is a relatively large denizen of the distant Kuiper Belt, that orbits our Sun in the frigid company of a vast multitude of other bewitching and mysterious icy objects. Like other Kuiper Belt Objects (KBOs), Pluto is thought to be composed primarily of ice and rock. It is an intriguing frozen “oddball”, a mere 1/6 the mass of Earth’s own Moon and 1/3 its volume. Pluto also has a highly inclined, eccentric orbit that carries it from 30 to 49 Astronomical Units (AU) from our Sun. One AU is equal to the mean Earth-Sun separation of 93,000,000 miles. As a result, Pluto periodically moves towards our Sun at a distance that is closer to our Star than Neptune. Very fortunately for both Neptune and Pluto, an orbital resonance with Neptune prevents the duo from crashing into each other.
The Kuiper Belt, sometimes called the Edgeworth-Kuiper Belt, is a region located in our Solar System’s outer limits beyond the realm of the eight major planets. It extends from the orbit of Neptune to approximately 50 AU. Neptune’s average distance from our Sun is about 30.1 AU–its perihelion is 29.8 AU, while its aphelion is 30.4 AU.
Pluto has a tenuous atmosphere composed of nitrogen, methane, and extremely toxic carbon monoxide, which probably originates from the ice on its frigid surface. As Pluto wanders in its orbit ever closer and closer to our Sun, it becomes increasingly warmer and warmer. The ice on its strange surface evaporates as a result, and the gases flow into interplanetary space. This continues until Pluto starts to travel away from the Sun again, becoming increasingly colder and colder as it does so. Pluto’s bizarre atmosphere again freezes, and then floats down to its very alien surface as snow–but it will evaporate again when Pluto begins its long journey back towards our Sun. It takes 248 years for the frozen dwarf planet to complete a single orbit around our Sun.
Poor Pluto was discovered by the American Clyde Tombaugh in 1930, and it was appropriately named after the ancient Roman god of the underworld, shrouded as it is in the perpetual darkness of our Solar System’s distant deep freeze. Charon was discovered in 1978 by the astronomer James Christy, also an American.
For most of the 20th century, astronomers thought that Pluto was a lonely little world, a solitary ball of ice circling our Sun, so very far from the comforting warmth and delightful light of our brilliant Star. However, in 1992, the discovery of the first KBO (other than Pluto), made astronomers come to the realization that Pluto is not far from the madding crowd of a vast population of other Kuiper Belt ice balls.
The Kuiper Belt is populated by mostly small, frigid objects, which are relics of our Solar System’s formation 4.56 billion years ago. Most KBOs are primarily composed of frozen volatiles, such as water, methane, and ammonia. The Kuiper Belt is also the home of two other officially designated dwarf planets (in addition to Pluto): Haumea and Makemake. A few of our Solar System’s moons, such as Saturn’s Phoebe and Neptune’s Triton, are also commonly thought to have been born in this distant and mysterious region.
There are currently over a thousand known KBOs, and more than 100,000 KBOs over 62 miles in diameter are believed to exist. Pluto is compositionally similar to many other objects inhabiting the Kuiper Belt, and its orbital period is characteristic of a class of KBOs termed plutinos, that share the same 2:3 resonance with Neptune.
Dance Of The Distant Moons
The movement of the moons in the Pluto-Charon system provides precious insights into the way planets orbiting in a binary stellar system–or double star system–might behave. “We are learning that chaos may be a common trait of binary systems. It might even have consequences for life on planets orbiting binary stars,” Dr. Hamilton explained in the June 3, 2015 HST Press Release.
When the astronomers measured variations in the light reflected off Nix and Hydra, they obtained the first clues of the Pluto system’s chaos. When studying images obtained from HST between 2005 and 2012, the astronomers were surprised to find that the brightness changed unpredictably, rather than following a regular cycle. This weird discovery could only be explained by chaotic movement.
The HST images also showed that the moon Kerberos is charcoal black in color, which is in stark contrast to the brilliant white of the other moons of Pluto. It was predicted that dust blasted off the moons by meteorite impacts would blanket the moons, giving them a homogeneous appearance. However, the reason why Kerberos is black remains a mystery.
The chaotic rotation of the moons was not the only surprise that came from the study. HST’s monitoring also showed a link between the orbits of the three moons Hydra, Nix, and Styx.
“Their motion is tied together in a way similar to that of three of Jupiter’s large moons. If you were sitting on Nix, you would see Styx go around Pluto twice every time Hydra goes around three times,” Dr. Hamilton commented in the June 3, 2015 HST Press Release.
The chaotic motions of the bewitching, bewildering dance of the distant moons, is not necessarily an indication that the Pluto system is trembling on the brink of falling apart. Additional observations are necessary in order to determine the future and ultimate fate of the Pluto system.
“Pluto will continue to surprise us when New Horizons flies past it in July (2015). Our work with Hubble just gives us a foretaste of what’s in store,” Dr. Showalter commented to the press on June 3, 2015.