LOS ANGELES, May 10 (Xinhua) -- Neptune's largest moon Triton may have been a member of a binary pair orbiting the Sun before it was trapped by Neptune, scientists reported on Wednesday.

    Triton is unique among all the large moons in the solar system because it orbits Neptune in a direction opposite to the planet's rotation. About 40 percent more massive than the planet Pluto, Triton has an inclined, circular orbit that lies between a group of small inner moons with prograde orbits and an outer group of small satellites with both prograde and retrograde orbits.

    There are other retrograde moons in the solar system, including the small outer moons of Jupiter and Saturn, but all are much tiny compared to Triton. Astronomers suspect Triton was captured from elsewhere.

    According to Craig Agnor at the University of California, Santa Cruz, and Douglas Hamilton at the University of Maryland, a new model involving a three-body gravitational encounter between a binary and a planet may explain the origin of Triton.

    Their findings were published in the latest issue of the journal Nature.

    Triton may have come from a binary very similar to Pluto and its moon Charon, the researchers said. Charon is relatively massive, about one-eighth the mass of Pluto, so Charon does not actually orbit Pluto, but rather both move around their mutual center of mass lying between the two objects,

    In a close encounter with a giant planet like Neptune, the system of the binary can be pulled apart by the planet's gravitational forces, according to the researchers. The orbital motion of the binary usually causes one member to move more slowly than the other.

    Disruption of the binary leaves each object with residual motions that can result in a permanent change of orbital companions. This mechanism, known as an exchange reaction, could have delivered Triton to any of a variety of different orbits around Neptune, they said.

    According to Agnor and Hamilton, many binaries have been discovered in the Kuiper belt and elsewhere in the solar system in past decades. Recent surveys indicate that about 11 percent of Kuiper belt objects are binaries, as are about 16 percent of near-Earth asteroids.

    "These discoveries pointed the way to our new explanation of Triton's capture," Hamilton said. "Binaries appear to be a ubiquitous feature of small-body populations."

    The Pluto/Charon pair and binaries in the Kuiper belt are especially relevant for Triton, as their orbits abut Neptune's, headed.

    "Similar objects have probably been around for billions of years, and their prevalence indicates that the binary-planet encounter that we propose for Triton's capture is not particularly restrictive," Hamilton said.

    An earlier scenario proposed for Triton is that it may have collided with another satellite near Neptune.

    But this mechanism requires the object involved in the collision to be large enough to slow Triton down, while small enough not to destroy it. The probability of such a collision is extremely small, according to the researchers. Enditem