Triton: The “Odd Moon Out”


Why are the moons of Neptune irregular?

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Raluca Rufu, in the group of Prof. Oded Aharonson, is solving the mystery of Triton

Neptune has one of the most peculiar assortments of moons in our solar system, and Triton is largely to blame. Triton is unusually massive compared to the other Neptunian satellites; but this is not the only characteristic that makes it an outlier. Not only does Triton orbit much farther away from Neptune than its other, smaller moons, it also spins in the opposite direction to the planet. The moons of the other three giant planets – Jupiter, Saturn and Uranus – tend to obey the rules of their satellite systems, rotating very close to the planet’s equatorial plane and in the same direction as their planet, so Triton truly stands out.

This anomaly has led scientists to believe that Triton was not part of the original Neptunian satellite system. Instead, it appears to be an intruder that caused havoc, throwing Neptune’s other satellites and their orbits off balance and leading to the irregular system that can be seen today.

Many moons ago

To unravel the mystery of what Neptune’s original moon system may have been like before Triton’s encroachment, Weizmann Institute of Science doctoral student, Raluca Rufu, who is in the group of Prof. Oded Aharonson of the Earth and Planetary Sciences Department, in collaboration with Dr. Robin Canup of the Southwest Research Institute, Colorado, used a series of computer simulations to figure out the most probable scenario.

Their results, recently published in The Astronomical Journal, propose that Neptune’s original complement of moons was consistent with the “calm, well-behaved” satellite systems found around the other giant planets, with a mass most likely similar to that of Uranus.

Collision course

The simulations raised questions about the way in which Triton wheedled its way into the Neptunian family of moons: According to the most favored current view, Triton was captured by Neptune’s gravitational pull. This, in turn, induced impacts between Neptune’s existing moons, leading to the formation of debris disks that then caused Triton to slow down and settle into its circular orbit around the planet.

However, the scientists’ simulations suggest that the collisions between the moons would not have been sufficient to create the sort of debris disks required to slow Triton into a circular orbit in the needed timeframe. Instead, their research proposes that Triton kicked some of Neptune’s original moons out of the system. In this scenario, the ejected moons would have spiraled into the planet, while others would have been “cannibalized” by Triton, and thus created the conditions needed for this giant moon to settle into orbit much more quickly. Looking at Triton today, it would not be possible to detect any traces of this scenario, since these impacts would have been energetic enough to melt the entire satellite and obliterate any impact craters.

Raluca: “After analyzing hundreds of simulations for all the different possible scenarios, our findings show that an original moon system with a mass similar to or smaller than that of the Uranian system has a considerable likelihood of having been the original Neptune system.”


Prof. Oded Aharonson’s research is supported by the Helen Kimmel Center for Planetary Science, which he heads; the Minerva Center for Life under Extreme Planetary Conditions, which he heads; the Sussman Family Center for the Study of Environmental Sciences; and the Adolf and Mary Mil Foundation.