Among the strangest denizens of the solar system are contact binaries. In these systems, two minor planets orbit each other so closely that they literally or nearly touch end-to-end — resulting in a peanut-like overall shape. Until now, astronomers had found only two possible contact binaries of relatively large size: the main-belt asteroid 216 Kleopatra and the Trojan asteroid 624 Hektor. Now Scott S. Sheppard (Carnegie Institution of Washington) and David C. Jewitt (University of Hawaii) may have found a third example: a Kuiper-Belt object orbiting beyond Pluto.
The object, 2001 QG298, orbits so far from the Sun that even the Hubble Space Telescope has no chance of resolving a peanut shape. But after measuring the object's changing brightness in 2002 and 2003 with the University of Hawaii's 2.2-meter telescope and the 10-meter Keck I telescope, Sheppard and Jewitt noticed something unusual. Its brightness varies by a whopping 1.14 magnitude every 6.89 hours. Yet the object's colors do not change, which suggests that dark spots rotating in and out of view are not causing the brightness changes.
Only three other solar-system objects larger than 50 kilometers across range in brightness by more than 1 magnitude. Two of them are Kleopatra and Hektor; the third is Saturn's peculiar moon Iapetus, which displays a very dark leading hemisphere and a very bright trailing hemisphere. But Iapetus represents an unusual case because it is locked in synchronous rotation with Saturn, so its leading edge may be sweeping up dark material kicked off from Saturn's outer satellites.
With an average diameter of about 180 kilometers, 2001 QG298 is large enough that it should be nearly spherical. But the object is not spinning fast enough for rotation to whirl it into an elongated shape. The simplest explanation for the brightness variations is that two roughly spherical and equal-sized bodies eclipse each other periodically every 6.89 hours, which means they must be very close together. We view them along their equators, which maximizes the eclipsing effect.
"We believe 2001 QG298 is a contact binary," said Sheppard, as he reported the team's findings at the November meeting of the American Astronomical Society’s Division of Planetary Sciences in Louisville, Kentucky.
Sheppard and Jewitt have found other possible contact-binary Kuiper Belt objects (KBOs). Given the number of KBOs they have observed, and the fact that other KBO contact binaries might be viewed pole-on (which makes them harder to categorize), Sheppard and Jewitt estimate that at least 10 to 20 percent of all large KBOs might be contact binaries with similarly-sized components. "The number of contact binaries with one component much larger than the other is probably much higher," says Sheppard, "but these don't make as large brightness variations and thus are not as easy to distinguish."
These close pairs probably formed early in the solar system's history when two bodies approached each other and went into mutual orbit after exchanging orbital energy with other bodies nearby.