Triton is an unusually big moon and is in fact about 40% bigger than the "planet" Pluto. It is unique among all the large moons in our solar system because it orbits Neptune in the opposite direction to Neptune itself -- a so-called "retrograde" orbit. Triton is also an "irregular" satellite that has an inclined, circular orbit around Neptune. It is highly unlikely that the moon formed in such a configuration and was probably captured from elsewhere. But how this happened is a mystery.

Over the years, scientists have proposed several mechanisms to explain how Triton was captured by Neptune. These include the "gas-drag" effect, whereby Neptune's atmosphere -- which was much more extensive than it is today -- slowed Triton down enough for it to be captured. Another idea is that Triton may have collided with another satellite near Neptune, causing it to slow down and be captured. However, none of these theories seems to hold up under close scrutiny.

Now, Agnor and Hamilton have proposed a new model that involves a three-body gravitational encounter between a binary and a planet. The physicists say that Triton was originally one of a pair of planet-like bodies, called planetesimals, both orbiting the Sun in the early solar system. Neptune's gravity then pulled Triton away from its companion when the duo ventured too close to the planet.

The new theory might satisfy two important conditions that should have been present when the capture occurred. The first is that the protoplanetary disk from which Neptune formed must have contained a large number of Pluto-sized objects. The second is that a significant fraction of the large objects in this disk must have been binaries. Indeed, scientists recently discovered that up to 15% of Kuiper-belt objects are binaries.

"We’ve found a likely solution to the long-standing problem of how Triton arrived in its peculiar orbit," says Agnor. "In addition, this mechanism introduces a new pathway for the capture of satellites by planets that may be relevant to other objects in the solar system."

Alessandro Morbidelli of the Observatoire de la Côte d'Azur in France agrees and says that the new model could even become a "mainstay for models of the origin of irregular satellites".