First family: Pluto-size body has siblings.

Shaped like a squashed football, the ice-covered body 2003 EL61 rotates faster and reflects more sunlight than any other object in the outer solar system, is about as big as Pluto, and even has two moons. Now, astronomers have discovered that this fringe object, located beyond Neptune in a region called the Kuiper belt, has another distinction. It's the first Kuiper belt denizen known to have an extended family.

Five smaller members of the belt, although not close to 2003 EL61, have nearly identical surface properties and orbits, Mike Brown of the California Institute of Technology in Pasadena and his colleagues report in the March 15 Nature. The researchers suggest that the family arose soon after the birth of the solar system, when a Pluto-size body smashed into 2003 EL61, creating the fragments that Brown's team has found.

The researchers, who discovered 2003 EL61, had already proposed that a giant impactor had pummeled the body. Such a collision could account for the 4-hour rotation of 2003 EL61, as well as its two moons and high density. The density indicates that the object was stripped of most of its ice, leaving just an icy glaze over a rocky core (SN: 1/14/06, p. 26).

The familial finding firms up the collision hypothesis and is "a milestone in Kuiper belt science--and by extension, in our understanding of the outer solar system's development," says Alessandro Morbidelli of the Observatory of the Cote d'Azur in Nice, France, in a commentary accompanying the Nature report.

Brown and his coworkers surveyed 50 Kuiper belt objects using the Keck 1 telescope atop Hawaii's Mauna Kea. Their results make a convincing case that five objects are chips off 2003 EL61, says theorist Hal Levison of the Southwest Research Institute in Boulder, Colo. The finding, he adds, "is also pretty shocking."

That's because theorists can't easily account for the collision, Levison says. Some models suggest that the Kuiper belt was once more crowded than it is now and had more-frequent impacts, but those crashes would not have been powerful enough to strip much material from 2003 EL61, he says.

Instead, Levison suggests that both 2003 EL61 and its impactor resided in a region of the outer solar system known as the scattered disk. That area, which intersects but is distinct from the Kuiper belt, consists of energetic objects on highly elongated orbits that are inclined relative to the plane in which the planets circle the sun. Collisions among objects in the scattered disk would pack enough punch to vaporize large chunks and hurl fragments of a battered body into the Kuiper belt, Levison calculates.