Here on Earth, our days are predictable, with sunrise following sunset each day at regular intervals. The Sun rises in the east and sets in the west. But what if that wasn’t the case? What if the Sun set in the west one day and in the north the next?
Inhabitants of two of Pluto’s moons—Nix or Hydra—would not know when each day would begin and from which direction the Sun would rise. This is because, like two cosmic footballs, these moons are wobbling erratically as they travel through space. “Living on Nix, you literally would not know if the Sun was coming up tomorrow,” said Mark Showalter of the SETI Institute and co-author of a new study.
The Pluto system is unique in that it contains the only binary planet system in our solar system. Pluto and its largest moon Charon are what scientists refer to as a double-dwarf system. Pluto is currently classified as a dwarf planet; while Charon is technically labeled a moon, both bodies orbit the same point in space—a point that isn’t within the circumference of either body.
Just like the Earth and the moon, Pluto and Charon are tidally locked in an orbital dance. The two orbit like unbalanced weights on a dumbbell, each pulling on the four smaller moons—Nix, Styx, Hydra, and Kerberos—that orbit the pair.
Using the Hubble Space Telescope, the researchers conducted a comprehensive analysis of the system and concluded that the two largest moons, Nix and Hydra, wobble chaotically as they orbit. Styx and Kerberos are expected to behave in the same manner, although further observations are needed to confirm this prediction. We can tell that the wobble is intensified by the fact that the four moons are not spherical in shape; they’re elongated, much like a football.
"Hubble has given us new clues about these mysterious moons as they dance around Pluto," said John Grunsfeld, associate administrator of NASA's Science Mission Directorate. "New Horizon’s flyby of Pluto in July will give us a chance to see what they really look like."
Hubble has been orbiting the Earth for a quarter of a century, revolutionizing the way we see and understand our universe. We don’t know much about Pluto’s tiniest moons yet, but what we do know, we’ve learned from Hubble. The largest of the four, Nix and Hydra, were detected by Hubble in 2005, with Kerberos following in 2011 and Styx in 2012.
"Prior to the Hubble observations, nobody appreciated the intricate dynamics of the Pluto system," Showalter said. "Our report provides important new constraints on the sequence of events that led to the formation of the system."
Nine years ago, in 2006, NASA launched the New Horizons spacecraft to the outer reaches of the Solar System with the intent to study the Pluto system. In the weeks leading up to a July 14th flyby, New Horizons will beam back unprecedented data and images.
But to do so, it will have to safely navigate through the Pluto system. In an effort to scout the area around Pluto for any debris that might be hazardous to the spacecraft, researchers have been reexamining Hubble data collected from 2005 to 2012. During their search, the researchers made some exciting observations.
The majority of moons in the solar system are tidally locked to their host planet, like the Moon is to the Earth—we only see one side of the Moon. By examining the Hubble data, researchers were able to determine that Pluto’s four outer moons do not play by the same rules.
"Like good children, our Moon and most others keep one face focused attentively on their parent planet," said Doug Hamilton, a University of Maryland astronomer and co-author of the study. "What we've learned is that Pluto's moons are more like ornery teenagers who refuse to follow the rules."
Astronomers cannot directly measure the shape of Pluto’s moons, but they are able to measure their apparent brightness. If the moons were tidally locked with the host planet, they would notice an increase in brightness at two distinct points in the moon’s orbit. After analyzing the data, they found this wasn’t the case—the moons were not tidally locked and they were not spherical.
The data also showed that three of the four moons—Nix, Styx, and Hydra—were orbiting in sync, meaning there was a precise ratio to their orbital periods, 3:4:6. This provides stability and is the reason why such a tiny planet (like Pluto) can have so many moons.
“If you were sitting on Nix, you would see that Styx orbits Pluto twice for every three orbits made by Hydra,” Hamilton added. This same orbital resonance can be seen in the Jovian system as Ganymede, Europa, and Io exhibit a resonant pattern.
Hubble data also showed that Nix, Styx, and Hydra have more in common than orbital resonance. The three are as bright as white sand, while Kerberos is very dark, like a piece of charcoal, with only four percent reflectivity.
So how does a system like this form in the first place? Astronomers are hoping that this question can be answered by New Horizons. Pluto and Charon are thought to have formed in a collision between a “proto-Pluto” object and an impacting body. The massive collision would have scattered debris around the system, and the family of moons are thought to have formed from that material.
The chaotic behavior of the Pluto-Charon system could have broader implications in the Universe. “We are learning that chaos may be a common trait of binary systems,” Hamilton said. “It might even have consequences for life on planets in such systems.”
The most well-known example of such a planet is not an actual planet but a fictional example in the Star Wars franchise. Tatooine, Luke Skywalker’s home world, orbits two stars—this is called a circumbinary planet. NASA’s Kepler space telescope has found 17 examples of actual circumbinary planets.
The new findings will appear in the journal Nature on June 4 and their announcement comes just six weeks ahead of the New Horizons flyby. "Pluto will continue to surprise us when New Horizons flies past it in July," Showalter said. "Our work with the Hubble telescope just gives us a foretaste of what's in store."