By Paul Rincon BBC
May 11, 2006
Triton's capture by Neptune's gravity has been difficult to explain
Neptune's moon Triton used to be paired with another object, but was torn from its companion during capture by the eighth planet's gravitational field.
This is the new model put forward to explain the moon's present location.
The capture of such a large object as Triton by the gas giant has proven extremely challenging to explain.
But Craig Agnor and Douglas Hamilton tell the journal Nature that an early Solar System that saw Triton moving in a twosome would answer many questions.
In recent years, many of the icy objects that populate the Kuiper Belt - the region of space just beyond the orbit of Neptune - have been found to be paired up as so-called "binaries".
This finding prompted the authors to re-visit the question of Triton's origins.
"Often you'll have a method, but you're not quite sure you have the method for forming something," Dr Hamilton, associate professor of astronomy at the University of Maryland told the BBC News website.
"I think we have the method because it really does work better than the other models that are out there."
The moons that orbit the Solar System's four gas giants - Jupiter, Saturn, Uranus and Neptune - fall into two general categories: regular and irregular.
Regular satellites are closer to their parent planet with circular orbits on the same plane as the planet's equator. They are thought to have formed from a disc of gas and dust that surrounded planets in the early Solar System.
Irregular satellites are more distant from their planet, with eccentric (non-circular) orbits. About half orbit in a retrograde direction - the opposite direction to the rotation of their planet.
Scientists generally think these were once free agents orbiting the Sun, and at some stage were captured by the gravity of the giant planets.
Various theories have been proposed for the capture of irregular satellites. According to Alessandro Morbidelli of the Observatoire de la Cote d'Azur in France, "none of these mechanisms seem appropriate for Triton".
Triton must have lost energy on its way to being captured. Hitting an existing moon, for example, would do the job. However, Dr Hamilton explains, this idea requires this satellite to be just the right size.
Hitting a moon that was too small, would not have given Triton the kick it needed to be captured. Conversely, hitting a moon that was too big would have destroyed Triton.
Agnor and Hamilton say all the pieces fit into place if Triton was in fact one half of a pair of planetesimals.
"Can we capture something as big as Triton on to the orbit it has today with a binary that looks something like what we see in the Kuiper Belt? That was the test; and it turns out that it works," Hamilton says.
When this pair ventured too close to Neptune, one of the bodies was tossed away, while the other - Triton - became gravitationally bound.
Dr Alan Stern, executive director of the space science and engineering division at the Southwest Research Institute in Boulder, US, called the work "very clever".
"Whether it's actually the case for Triton or not is debatable. But I think it's an important new physical process which clearly must have operated in the early Solar System," he told the BBC News website.
Neptune may have reeled in Triton as one half of a pair
But he added that the model was not without its problems: "It requires that there were a very large number of these binary systems in the past - much more than there are today. We have other reasons to think this is actually the case, but we can't find them."
Dr Stern said his own research implied there would have been 1,000 Pluto-like objects in the early Solar System. "It's a shockingly different view of the architecture of our Solar System; that the number of planets would have been more like 900 than nine."
Dr Stephen Lowry, of Queen's University Belfast, commented: "The origin of the retrograde satellites remains one of the big open questions in planetary astronomy.
"The prime result from this paper shows that this could very well be a realistic scenario for the capture of retrograde satellites. I should stress that this is only a possibility even for Triton."
Dr Hamilton said he believed that Triton's companion was either slung out of the Solar System or incorporated into Neptune or one of the other giant planets.