Only weeks before NASA’s New Horizons spacecraft is due to give us our first close-up view of Pluto and its moons, scientists using the Hubble Space Telescope have discovered this far-distant family is different from anything else so far discovered in the Solar System.
On Earth, the firm tug of our planet’s gravity has pulled our Moon into a “synchronous lock”, so that we only see one face of the Moon as it orbits around us. It’s the same with most other moons and their planets in the Solar System, says Mark Showalter, a planetary astronomer at the SETI Institute in Mountain View, California.
"The Sun might one day rise in the east and set in the north – and the next day it might do something entirely different."
But this type of steady face-to-face interaction is not how at least two of Pluto’s five known moons go about their business, Showalter has reported in Nature, with astronomer Douglas Hamilton from the University of Maryland. Instead, he says, the Pluto moons Nix and Hydra tumble about chaotically. So much so that if you lived on one, the Sun might one day rise in the east and set in the north – and the next day it might do something entirely different. “If you had real estate at the north pole you could suddenly discover it was at the south pole,” he says.
Showalter and Hamilton made the discovery by examining brightness variations in tiny specks of light – the best images Hubble has been able to capture of these two moons. Small moons such as Nix and Hydra are often oblong because of their weak gravity. “They’re not big enough to crush themselves into spheres,” Showalter says.
That’s useful to astronomers because it means their brightness waxes and wanes depending on how much of their profiles are facing us. (Imagine looking at a distant, spinning egg, trying to figure out which side is pointed toward you.) The astronomers’ analysis show that Nix and Hydra appear to be changing their orientations almost at random.
Two of Pluto’s other two small moons, Styx and Kerberos, are too small and faint for even the Hubble’s best data to reveal their rotations. New Horizons should tell us more about them soon.
The reason for Nix and Hydra’s tumbling orbit, Showalter and Hamilton realised, is that Pluto’s innermost moon, Charon, is a giant, weighing in at nearly one-eighth of Pluto’s own mass.
Hamilton compares it to Jupiter having a close-in moon larger than Neptune. The result, the two scientists say, is that Pluto and Charon essentially orbit each other at the centre of this system. “You can think of Pluto and Charon as a double planet,” Showalter says.
The continually shifting gravitational forces generated by this pair of bodies is the reason the smaller orbiting moons spin so chaotically.
And yet there’s some order to the Pluto system. Hamilton says the same shifting gravitational forces have put three of the small moons into “wobbly orbits” that have become synchronised so that all three never wind up on the same side of Pluto/Charon. “That helps stabilise [the system],” he says.
All of this makes Pluto, Charon, and their outer moons the Solar System’s only known example of how distant planets orbit double-star systems , says Heidi Hammel, a planetary astronomer and executive vice-president for the Association of Universities for Research in Astronomy, Washington, D.C. “We know that double stars are ubiquitous throughout our galaxy,” she says, “and we know that many do host planets. Pluto and its complex, chaotic moons can provide a direct analogue”.
Meanwhile, New Horizons’ Pluto flyby is approaching. “New Horizons will fly through [the Pluto/Charon system] on July 14,” says John Spencer, a planetary scientist as the Southwest Research Institute in Boulder, Colorado, and a member of the mission’s science team.
“We will get incredibly detailed pictures of Pluto and Charon, but we’ll also look at these small moons.” And, he notes, as the spacecraft draws nearer it is sending better pictures. “We’ll be learning a lot more as we go along,” he says.