Visualization showcasing the dwarf planet Pluto and its icy satellite Charon. (© Heng Heng – AI Stock – stock.adobe.com)
In a nutshell
- Scientists have discovered a new type of planetary collision called “kiss-and-capture,” where Pluto and proto-Charon briefly connected and spun together before separating into their current orbital arrangement, challenging decades of theories about violent impacts.
- Unlike previous models that treated colliding bodies as fluid, this research shows that accounting for the structural strength of cold, icy worlds reveals how Charon could have remained largely intact during its formation rather than being destroyed and reassembled.
- This gentler formation process may explain how Pluto developed a subsurface ocean without requiring it to form in the very early solar system, potentially solving a long-standing mystery about the dwarf planet’s evolution.
TUCSON, Ariz. — How did Pluto end up with its largest moon, Charon? Like many entanglements, it started with a kiss. New research from the University of Arizona details a “kiss and capture” mechanism the dwarf planet used to keep its gigantic moon orbiting around it.
According to a new study recently published in the journal Nature GeoScience, the two icy worlds made contact billions of years ago. Instead of creating a cosmic calamity, however, Pluto and Charon engaged in an unusual planetary interaction that ended with the two sticking together like a “kiss.” Even when they finally separated, they were forever linked with Charon hovering around Pluto in orbit to this very day.
“Most planetary collision scenarios are classified as ‘hit and run’ or ‘graze and merge.’ What we’ve discovered is something entirely different—a ‘kiss and capture’ scenario where the bodies collide, stick together briefly and then separate while remaining gravitationally bound,” says Adeene Denton, a NASA postdoctoral fellow who conducted the research at the University of Arizona’s Lunar and Planetary Laboratory, in a statement.
The researchers are using this relationship between Pluto and Charon to study this unusual “kiss and capture” mechanism. The goal is to better understand how planetary bodies form and evolve through this collision of giant bodies of rock.
Scientists have wondered how Pluto snagged an unusually large moon like Charon for decades. One theory revolved around a process similar to Earth’s moon, in which a massive collision caused the stretching and deformation of fluid-like bodies. Denton says this model explains why the moon began orbiting Earth; the intense heat and their larger shapes meant the colliding bodies behaved more fluidly. This doesn’t work well when looking at the much smaller and colder Pluto. Another issue is the structural integrity of rock and ice.
“Pluto and Charon are different—they’re smaller, colder, and made primarily of rock and ice. When we accounted for the actual strength of these materials, we discovered something completely unexpected,” Denton explains.
The research team used advanced impact simulations to grasp how the collision between Charon and Pluto would have happened. The modeling shows that friction helped distribute the momentum in the two planetary bodies when they made contact. Instead of stretching together like clay or merging on impact, the friction allowed Charon and Pluto to become temporarily connected. The shape is similar to two circles stacked on each other like a half-made snowman.
Pluto and Charon eventually separated into what we see today: the moon orbiting around the dwarf planet.
Despite the two celestial bodies colliding, most of their original composition was unbroken. Additionally, the tidal friction when the two planets separated suggests a mechanism for Pluto to create a subsurface ocean instead of forming in the early solar system, where radiation was very high.
Now understanding how the relationships between Pluto and Charon emerged, the next question to explore is how tidal forces affected the evolution of these two worlds when they were much closer together. A future study would look at how the combined formation affected Pluto’s surface geology and evaluate how other similar “kiss and capture” methods could explain other binary systems.
“We’re particularly interested in understanding how this initial configuration affects Pluto’s geological evolution,” Denton adds. “The heat from the impact and subsequent tidal forces could have played a crucial role in shaping the features we see on Pluto’s surface today.”
Paper Summary
Methodology
The researchers used a specialized computer simulation code called “sphlatch” that represents celestial bodies as collections of particles that can interact through gravity and physical contact. They ran multiple simulations varying factors like impact angle, speed, and rotation rate, using 500,000 particles to model each collision. Unlike previous studies, they included material strength properties for both rock and ice, making the simulations more realistic for small, cold bodies like Pluto and Charon.
Results
Out of 18 simulated scenarios with varying impact conditions, 13 resulted in successful kiss-and-capture events when proto-Pluto was rotating with a 3-hour period. The simulations showed that Charon would end up in an initially elliptical orbit that would gradually become more circular through tidal forces, matching its current configuration.
Limitations
The simulations couldn’t resolve Pluto’s smaller moons or track the very long-term evolution of the system. They also didn’t account for all possible variations in internal composition or thermal states of the colliding bodies. The results depend heavily on assumptions about the original rotation state of proto-Pluto.
Key Takeaways
This research provides a new mechanism for forming binary systems in the outer solar system that better matches observations than previous models. It suggests many similar pairs might have formed through similar gentle collisions rather than more violent impacts. The study also implies that Charon retained much of its original structure rather than being reassembled from impact debris. Additionally, the collision process may explain how Pluto developed a subsurface ocean without requiring formation in the very early solar system, resolving a timing constraint that has troubled planetary scientists.
Funding and Disclosures
The research was conducted at the University of Arizona using their High Performance Computing facilities. The authors declared no competing interests.
Publication Information
Published in Nature Geoscience, Volume 18, January 2025, pages 37-43 DOI: https://doi.org/10.1038/s41561-024-01612-0 Authors: C. Adeene Denton, Erik Asphaug, Alexandre Emsenhuber & Robert Melikyan
