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Neptune nabbed its moon

Back when the solar system was young, 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.

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. 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.

Now, it seems that Triton was wandering through space locked in the gravitational embrace of a companion when the pair happened to pass by Neptune. The gravity of the giant planet extricated Triton from its partner, flinging one into deep space and keeping the other as a moon.

About 40 per cent larger than Pluto, Triton moves through its tilted orbit in the opposite direction to Neptune’s rotation. This is the hallmark of a captured body, rather than one formed in situ, but astronomers have never been able to work out how Neptune managed to capture Triton.

Their calculations show that as Triton and its partner drew close to Neptune, the giant planet’s gravity overwhelmed the attraction between the pair with little regard for the size difference between the companions or the precise distance of the encounter. In the resulting interaction, Triton effectively cast off its partner for the planet.

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.

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