Captive worlds: Is Neptune’s moon Triton a kidnapped Pluto?

Some of the earliest direct evidence that Pluto wasn’t alone came from an icy moon with a strange orbit.
By | Published: June 10, 2015 | Last updated on May 18, 2023
The surface of Neptune’s moon Triton is young and ice covered, with evidence for volcanic plains, as well as mounds and round pits built by icy lava flows. It’s odd orbit led astronomers to suspect Triton might be Pluto’s twin.
Neptune, the outermost major planet, has a total of 14 known moons. Thirteen of these are misshapen hunks of rock and ice, and all but two were only discovered when the Voyager spacecraft flew past August 25, 1989, marking the last time a spacecraft encountered a new planet.

Then there’s Triton.

Its orbit runs in the opposite direction from most other moons. In fact, no other large moon in the solar system has such a retrograde orbit. Triton is also nearly the size of Earth’s Moon, but it has active volcanoes that spew nitrogen frost onto the surface in geyser-like eruptions.

“When we got to Neptune, the big surprise was Triton’s terrain, which was covered in these strange surface features: the cantaloupe terrain, the evaporative features and the plumes — ‘What were these big smoke-stack plumes?!’,” New Horizons Co-investigator Fran Bagenal of the University of Colorado Boulder said in a NASA interview. “It was very bizarre — it still is bizarre, I don’t think we really understand it. Are we going to see the same sort of thing at Pluto, or will it be completely different?”

These strange properties prompted astronomers to suggest Triton, which is 40 percent more massive than Pluto, was kidnapped from a former free-range orbit. But calculations made by Bill McKinnon of Washington University in St. Louis and Peter Goldreich of the California Institute of Technology showed that the chances of Neptune capturing an object like Triton were slim to none.

Such a collision only becomes likely if you accept that several hundred Tritons orbit at 30 astronomical units (1 AU is the average Earth-Sun distance). More recent models suggest Triton, like Pluto, may have been part of a binary system.

Eventually, the strange moon would prove to be just one piece of a large body of evidence pointing toward an undiscovered family in our solar system — Pluto’s family.

Once seen in this light, other unlikely occurrences start to make sense.

Neptune’s axis inclines almost 30° to its orbit, likely the result of being hit with a large object. As many as 50 planets Pluto’s size or larger would need to exist past Neptune for such a collision to become likely. Uranus is at an even more extreme angle. That planet is tilted 98° on its axis.

But it was perhaps Pluto itself that would ultimately make the strongest case for its siblings. Stay tuned for the next Year of Pluto post, where we look at the implications of this double dwarf planet.

Eric Betz is an associate editor of Astronomy. He’s on Twitter: @ericbetz.