Pluto’s Enigmatic Waltz: The Tale of How Charon Became Its Largest Moon

Approximately 4.5 billion years ago, the dwarf planet Pluto was unexpectedly joined by a companion. For a very short duration — possibly just hours — they twirled as if intertwined before gently drifting apart, a grand do-si-do that results in Pluto and its five moons orbiting the sun together today.

Astronomers have long speculated about how Charon, the largest of these moons, came to revolve around Pluto. A paper published Monday in the journal Nature Geoscience presented a possible series of events that might elucidate the question.

“The reason Pluto and Charon are so fascinating is because Charon is 50 percent the size of Pluto,” stated Adeene Denton, a planetary scientist at the University of Arizona who spearheaded the paper. “The only comparable system is Earth and its moon.”

Charon measures about 750 miles across, while Pluto approaches 1,500 miles in diameter. This size ratio implies that many conventional theories explaining moon formation are probable unlikely, including notions that Charon originated from debris surrounding Pluto or was captured by its gravitational forces. Could Charon’s presence instead be attributed to a collision thought to have formed Earth’s moon?

The dimensions of Pluto and Charon complicated the understanding of how they “didn’t merely merge like two blobs of liquid,” the most probable result of such an explosive scenario, remarked Erik Asphaug, another planetary scientist from the University of Arizona and a co-author of the paper.

Pluto and Charon exist in a segment of the outer solar system beyond Neptune known as the Kuiper belt, which renders them both highly rocky and icy. By factoring in these characteristics into their model, the research team formulated a scenario where the two bodies collided and became ensnared without merging.

If Charon collided with Pluto at a relatively gentle speed of approximately 2,000 miles per hour — ten times slower than the impact that formed Earth’s moon — the two would have remained in contact for around 10 hours before slowly parting yet remaining together. The researchers characterized this interaction as a “kiss and capture.”

The resilience of the two bodies prevented them from shattering, Dr. Denton explained.

During that time, Pluto would have been rotating once every three hours (the duration of a day on Pluto today is about 150 hours), so the two would have revolved around each other three times while still joined. The angular momentum from Pluto’s rotation would have gradually pushed Charon away but, critically, kept it tethered in Pluto’s orbit.

Bill McKinnon, a planetary scientist at Washington University in St. Louis, indicated that such a scenario “makes sense” considering the vast number of objects believed to be swirling around the Kuiper belt in the early solar system. “Collisional capture is likely a common phenomenon,” he stated, with many other large binary bodies also thought to exist within the Kuiper belt.

The collision would have meant that “essentially the entire surface of Pluto is resurfaced,” Dr. Denton said, with Charon losing most of the ice on its exterior to its companion. “The collision acts as a geological reset for the system,” she added.

It may also have contributed to the creation of Pluto’s four other known moons — Nix, Styx, Kerberos, and Hydra — which are minuscule in comparison to Charon and were observed when NASA’s New Horizons spacecraft passed by Pluto in 2015.

The team’s model might provide a new perspective on how some moons end up revolving around other celestial bodies. “It introduces a new twist on the physics,” Dr. Asphaug remarked. “We’ve operated under the assumption that strength is irrelevant in collisions. We need to reevaluate that premise, even in terms of the formation of our moon.”

Analyzing Pluto in greater depth could reveal whether its celestial dance truly took place, although it is expected to be quite some time before another spacecraft visits the dwarf planet.

“If Charon transferred some of its material into Pluto, you would be able to observe that in gravity data,” Dr. Denton stated. “Unfortunately, we would need to return to Pluto to verify this.”