Pluto has at least five moons, and these distant satellites orbit through the collection of bodies in a highly complex manner. Now, data from the Hubble Space Telescope (HST) reveals surprising information about the dance of satellites around the icy dwarf planet.
Charon is the largest moon of Pluto, first seen by astronomers at the United States Naval Observatory in 1978. This body orbits so close to its icy parent body that the two are often described as a binary planet, although neither are true planets according to current standards. The term "binary planet" is an informal one, referring to a set of planetary bodies in which a satellite is of a significant size compared to its companion. Some astronomers believe the Earth and our Moon could be classified as such a system. Charon is 750 miles across, roughly half as wide as Pluto itself.
Nix and Hydra were discovered in 2005, followed by the smallest known moons, Kerberos, found in 2011, and Styx, discovered the following year.
The unusual movement of this "binary planet" appears to be driving complex gravitational forces that are creating strange paths for the smaller moons orbiting the icy pair.
"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," Douglas Hamilton, from the astronomy department at the University of Maryland, said.
The Hubble Space Telescope was used to determine that the smaller moons are roughly football-shaped and tumble erratically as they make their way around the binary planet. However, their orbits around the pair are surprisingly regular. The trio of Hydra, Nix and Styx are locked in a gravitational embrace with each other, regulating their orbits with clock-like precision. This is similar to the Jovian system, where three large moons — Europa, Io and Ganymede — are also locked by orbital resonance as they travel around Jupiter.
Kerberos appears to be, quite literally, the black sheep of Pluto's system. While the rest of the satellites appear to be colored white, this tiny body, estimated to be between eight and 21 miles in diameter, is much darker — perhaps black.
The New Horizons spacecraft is closing in on the network of planetary bodies and will soon provide the world with unprecedented images of Pluto and its network of satellites. This first-ever flyby of Pluto by a spacecraft could answer many questions about the far-flung system and could even discover new moons.
"Hubble has provided a new view of Pluto and its moons revealing a cosmic dance with a chaotic rhythm. 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," John Grunsfeld of NASA said.
Examination of Hubble data for what it can tell us about the chaotic dance of the moons of Pluto was detailed in the journal Nature.
