Mysterious Pluto-Charon Couple (Software Image) © Space Engine
Pluto is still popular with the general public today, no doubt having a lot to do with its strange status as a fallen planet. We can actually see this now classified “dwarf planet” as a declassified, loser, and we can easily identify it. Since 2006, the year it was deprecated by the International Astronomical Union meeting at a general assembly in Prague, many Americans have not taken to it, despite good reasons offered by astronomers. Damnit was the only (and previous) planet in the solar system to be discovered by an American!
A study published in Nature Geoscience in which our interest here sheds fascinating light on the origin of the Plutonian system. Because, yes, Pluto is a system unto itself… and a very strange one indeed. More precisely, we will present a binary system formed by Pluto and its moon Charon.
Charon, Pluto’s ruling moon. In the north, the Red Plain was called Mordor… © New Horizons (NASA/JPL)
None of the moons in the solar system is superimposed relative to its parent planetary body.
The Earth and the Moon itself is considered a binary system, and not just a little. In fact, no other planet in the Solar System is paired with its central moon, offering such a high ratio between the mass of a natural satellite and a planet. Our Moon is about 1% the mass of Earth, and take our word for it, that’s a lot. For comparison, for Jupiter’s largest moon, Ganymede, we arrive at 0.078%. That being said, Pluto and Charon make an “out of the box” pair.
Charon is 15.7% the mass and half the diameter of Pluto, which is huge. So we consider them as a “true pair”, i.e. a dual system. But this raises astrophysical questions and constraints on how it formed.
The old hypothesis of Charon’s formation by a giant impact. © Wikipedia
Until now, the hypothesis held was a large impact, as was the case with the Moon and the Earth. A large object would have collided with Pluto and the resulting debris would have formed Charon. This hypothesis, however, presented several weaknesses, including a major one: the exact composition of the two bodies. According to the original scenario, Pluto should have been more rocky and Charon more icy, which was not observed. It is also important to note the extremely small orbital distance of only 20,000 km between the two bodies. If we compare the Earth-Moon system, it seems as if the latter was three times closer to us…
At bottom right, Pluto is small compared to the solar system’s moons. From left to right and top to bottom: Ganymede, Titan, Callisto, Io, the Moon, Europa, Triton and Pluto. © Public domain
New Hypothesis: An overly slow effect, like an icy kiss
The new hypothesis is called kiss and hold and is partly based on supercomputer simulations. Its major strength is that it perfectly takes into account the similar physical properties of both bodies and the celestial mechanics operating in the icy trans-Neptunian boundary. Because there, bodies move slowly, which is not very compatible with large and destructive impacts. In fact, Pluto orbits five times slower than Earth, at about 5 km/s, and the average speed of bodies in the Kuiper Belt, to which the Pluto/Charon pair belongs, is 3 to 5 km/s. is between
Sure, that still represents a speed of about 17,000 km/h, but astronomers have seen examples of bodies in the belt that pile up on top of each other. NASA’s New Horizons probe, the same one that flew over the Pluto/Charon pair in 2015, met Iroquois in 2018/2019, quite logically in comparison to a snowman.
Urukoth, very likely the result of a slow collision of sorts. Kiss and hold. © NASA / Johns Hopkins Applied Physics Laboratory / Southwest Research Institute / National Optical Astronomy Observatory
Here are the steps for a computer simulation of the scenario. Kiss and hold. © C. Adeene Denton, Nature Geoscience, 2025.
A final point seems to favor the sexier scenario rather than the violent implications, but it’s still speculative at this point. In the event that, as some astronomers believe, there is an ocean about 100 km below Pluto’s surface (a hypothesis based on the New Horizon data), this feature would be poorly explained by abrupt impacts and a slower rate of acceleration. Much better than A hug suggested by the article Nature Geoscience.
Here’s what the lead author says: Pluto and Charon are special. They are small, cold and mostly composed of rock and ice. When we considered the true power of these materials, we discovered something completely unexpected. Most planetary collision scenarios are classified. Hit and run. What we discovered was quite different: a ‘kiss and hold’ scenario where bodies bump into each other, stick together briefly, then pull apart, still bound by gravity. In short, like a scenario Twilight : A vampire’s kiss, slow, icy and leading to eternity…
