Study: Neptune Nabbed Its Moon

May 10, 2006— Back when the solar system was young, when planets and their growing brood of satellites were angling for clear orbital slots around the sun, Neptune might have nabbed its giant moon Triton from a pair of passing sister planets, a new study says.

Dual systems like Pluto and its large moon Charon are not uncommon among objects in the Kuiper Belt region, located beyond Neptune's orbit, astronomers say. About 10 percent of the known objects in this region have partners.

Craig Agnor, with the University of California at Santa Cruz, was sitting in a lecture about binary Kuiper Belt objects when he got the idea that Neptune may have kidnapped its main moon Triton from such a pair.

Triton stands out among all the large moons in the solar system because it orbits Neptune in a direction opposite to the planet's rotation, a so-called retrograde orbit.

While most scientists explained the moon's odd orbit and inclination by some variation of a capture scenario — a collision between objects, for example — Agnor had been long troubled by details that did not jive with any of the interpretations.

"It's been this old problem to work on," Agnor said in an interview with Discovery News. "The existing answers, there was something just a little unsettling about them."

When they used a binary system rather than a solitary object to generate a computer model of how the relationship started solved many of the theories' flaws, Agnor and University of Maryland astronomer Douglas Hamilton report in this week's issue of the journal Nature.

The most troubling conundrum was accounting for the tremendous amount of speed Triton must have shed to leave itself vulnerable to Neptune's gravitational embrace. The collision theory, for example, would have required an impact so powerful that Triton itself should have been destroyed.

Scientists also have suggested that Neptune once had an extended gaseous atmosphere, which could have gradually slowed Triton until it fell into orbit around Neptune.

But Neptune seemed to have evolved slowly and probably never had much more atmosphere than it has today, pointed out French astronomer Alessandro Morbidelli in a related Nature article.

Agnor and Hamilton show that if Neptune chanced upon a pair of mini-planets, similar to the Pluto-Charon system, the encounter could have ripped one from its partner.

Computer models prove that the object closer to Neptune, which in this case would have been Triton, would have lost enough velocity in the process to fall into orbit around its abductor.

The partner object likely would have been buffeted by Neptune's gravity, passed along to Uranus and then Saturn until it finally reached giant Jupiter and was booted out of the solar system altogether, Agnor said.

The researchers are now curious to see if other eccentric moons in the solar system have similar life stories.