So close, yet so far away

Long-distance relationships are tough enough when partners are a continent apart. Consider the plight of two solar system bodies separated by a distance more than 20 times as great.

Astronomers have found such a partnership: two equal-mass members of the Kuiper belt, the reservoir of frozen objects beyond Neptune, that are gravitationally bound to each other but separated by more than 100,000 kilometers.

Known collectively as 2001 QW322, the frozen couple is the most widely separated binary system known in the solar system.

The system’s very existence poses a challenge because no one had predicted two bodies could survive as a pair with such a wide separation, says co-discoverer Jean-Marc Petit of the Besancon Observatory in France. Indeed, the two bodies, which each have an estimated diameter of 54 kilometers, are so weakly bound that they would be easily disrupted by the gravity of an interloper, he notes.

The two objects take 25 to 30 years to orbit each other and are separated by between 105,000 to 135,000 km, Petit reported October 13 in Ithaca, N.Y., at the annual meeting of the American Astronomical Society’s Division for Planetary Sciences. Details will also appear online in the Oct. 16 Science. He and his colleagues tracked the orbit of 2001 QW322 for six years.

The widely spaced binary may reveal what kind of conditions existed in the Kuiper belt early in the solar system. Because the formation of this pair probably required the interaction of three or more bodies, it’s highly unlikely to have been created recently, because the Kuiper belt is now sparsely populated, Petit says. But the belt was originally 50 to 100 times more crowded, theorists believe. “One needs a more populated belt, as was the case in the early solar system,” says Petit.

“When we design models of the Kuiper belt and its history, we must make sure that the early conditions allowed the formation of the large-separation binaries,” he notes. To show how some of these binaries could survive to the present, models must simulate a rapid elimination of most of the early belt’s population. Otherwise, too many collisions would have destroyed such fragile partnerships.

Most researchers have assumed that 2001 QW322 formed from the start as two objects with the wide separation the now have. But a few scientists have suggested that the pair could have originated as a much more tightly bound system that gradually grew farther apart due to the interaction of passing interlopers. “Such a scenario needs to be studied and quantified, and we would need better statistics on [the population of] intermediate-separation binaries with equal mass in the Kuiper belt,” says Petit.