silent partners

FOUND: Several pairs of icy objects in the Kuiper Belt, past the orbit of Neptune. The binaries could help astronomers understand Pluto and its relatively large moon, Charon.

The Kuiper Belt is a rubble pile left over from the solar system's era of planet building some 4.5 billion years ago. It consists of a band of ancient bodies that surrounds the sun between the orbits of Neptune and Pluto, and possibly extending even farther out. This ring around the sun bears some resemblance to the asteroid belt. Its inhabitants, however, known as Kuiper Belt Objects (KBOs), are made up of rock and ice, not the rock and metal composition of asteroids.

Almost weekly since the first sightings of KBOs in 1992, the documented population has grown, until it now numbers more than 560. Each new discovery seems to yield more surprise and mystery. Researchers are continually updating their views on the orbital distribution of KBOs, as well as on the composition, colors and sizes of the bodies.

One of the most surprising announcements came in 2001, when astronomers reported the discovery of not just one but several KBOs with a difference. Unlike objects discovered earlier, these KBOs have companions orbiting around them. They are binaries, icy rocks with moons. The findings may hold more than passing interest--they may help shed new light on the nature of Pluto and its moon, Charon, which have not yet been visited by spacecraft. (For more about Pluto, see Journey to the Farthest Planet," by S. Alan Stern, Scientific American, May 2002.)

Paired Worlds

Several researchers have claimed that there could be a sharp edge to the Kuiper Belt located somewhere near the most distant point of Pluto¿s orbit from the sun. Mike Brown of Caltech argued in an Astrophysical Journal paper, published June 10, 2001, that the majority of the KBOs lie slightly inside Pluto¿s farthest point from the sun.

More recently, Brown has claimed that the classical KBOs might actually represent two distinctly different populations of objects. One grouping lies in the plane of the solar system and originates from its formation. The second is much more widely dispersed and formed more recently through dynamical interactions with the outer giant planets, Uranus and Neptune. In effect, what we see could be two superimposed Kuiper Belts.--M.A.G.

The first and most definite report of a binary came in April 2001 following observations by Christian Veillet and Alain Doressoundiram, which were described in International Astronomical Union Circular 7610. The images of the KBO known as 1998 WW31, obtained using the Canada-France-Hawaii Telescope, clearly show two bodies separated by about 40,000 kilometers. Comparing their images with others obtained earlier confirmed that this observation was neither a chance alignment between a single KBO and a star, nor an alignment between two separate KBOs. "WW31 is a binary beyond any doubt," Veillet says.

In the April 18, 2002, issue of Nature, Veillet and colleagues announced new details about the highly eccentric and long-period orbit of the pair. "It was a big surprise to find out that 1998 WW31 had such a high eccentricity," says Veillet, who cautions that it is too early to make judgments about how such an orbit could come to be.

On October 15, 2001, James Elliot of the Massachusetts Institute of Technology reported another potential binary. This one, 2001 QT297, is more tightly bound, with just 18,000 kilometers separating the two components. And less than one month later, J. J. Kavelaars of McMaster University added a third candidate pair: 2001 QW322, separated by about 120,000 kilometers and each about 200 kilometers across. The findings were reported in the International Astronomical Union Circular 7733 and 7749, respectively. There are now four others, for a total of seven: TC36 (IAU 7787), SM165 (IAU 7807), CQ29 (IAU 7824) and CF105 (IAU 7857). Veillet notes that these are 1 percent of all known KBOs.

Unlikely Pairs?

"The surprise in all these binaries is that they exist at all," Kavalaars says. The Kuiper Belt is sparsely populated because Neptune¿s gravitational tugs have, over time, ejected many KBOs clean out of the belt. "Somehow, these binaries have survived the large-energy encounters [with Neptune], which have excited, or gravitationally perturbed, the remainder of the belt, as well as the encounters that placed each of these systems into slightly noncircular orbits."

Kuiper Belt Objects
Image: Courtesy of NASA and the Space Telescope Science Institute
COMPOSITE IMAGE, made from six separate observations using the Hubble Space Telescope, shows one of a pair of Kuiper Belt Objects called WW31 (the six faint blobs) in position relative to the other (the large blob). The two revolve around a common center of gravity.

Moreover, in order to form binaries, two individual KBOs must come close enough together to exert strong tidal forces on each other. The energy dissipated by this interaction can slow the two objects to the point that they no longer possess the energy needed to escape from each other, so that they become binary KBOs. How this happens, given that the Kuiper Belt is so sparsely populated, is a complete mystery. And there could be more surprises in store. Kavalaars points out that many of the KBOs already found could be binaries, but with separations that we cannot as yet resolve with our limited telescopes.

As well as forcing yet another rethinking of the dynamics of the Kuiper Belt, these new findings could draw fresh fire from teams on both sides of the debate concerning Pluto¿s status as a planet. "A while back the discussion was to start calling Pluto a large planetesimal--in other words, a KBO--but people argued that Pluto had a moon and thus should be considered a planet," Kavelaars says. "The fact is that Pluto looks like a KBO, acts like a KBO and now we see that KBOs have moons, too. So I don¿t see why we can't start calling Pluto the biggest KBO known."