Pluto and its moon Charon (both pictured) are sometimes called a "double planet," because of Charon's large size relative to Pluto. Soon, however, they may be called something else: parents.
The Pluto-Charon system is embedded in a vast swarm of icy objects known as the Kuiper Belt. According to the "Pluto's family" hypothesis, some Kuiper Belt objects were produced by the same ancient collision that created the Pluto-Charon system.
"It's not a clincher yet. We need to make more observations to clinch it, but the evidence to date is promising," says astronomer Alan Stern, who developed the hypothesis with colleagues Robin Canup and Daniel Durda at the Boulder, Colorado-based Space Studies Department of the Southwest Research Institute.
The hypothesis will be presented at an American Astronomical Society conference in Padua, Italy on October 12.
The view is supported by several pieces of "circumstantial evidence," says Stern. First, he notes, some Kuiper Belt objects have orbits "within reach" of Pluto's orbit -- they are "where you'd expect to find debris" from a collision between an earlier version of Pluto and another object.
In addition, the estimated mass of debris from the ancient collision is compatible with estimates of the total mass of the Kuiper Belt, and the colors of some Kuiper Belt objects are similar to the colors of Pluto and Charon. Also, the size and speed of certain Kuiper Belt objects suggest that they resulted from the collision.
So far, about 200 Kuiper Belt objects have been observed, and the number is rising quickly, notes Stern, who is co-author with Jacqueline Mitton of a book titled Pluto and Charon: Ice Worlds on the Ragged Edge of the Solar System.
The total number of Kuiper Belt objects is believed to be about 100,000. Among these, an estimated 10,000 are "Plutinos" -- objects in the vicinity of Pluto's orbit -- and "Pluto's family" is a subset of probably fewer than 1,000 objects.
Yet even from the current small sample of observed Kuiper Belt objects, there are already "one or two" that fit all the criteria to be offspring of the Pluto-Charon system's creation, according to Stern. If the hypothesis were wrong, he says, it is unlikely that any such objects would have been found.
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