Icy, long and sexy: we finally know how Pluto and Charon were formed

Pluto is still popular with the general public, undoubtedly its strange status of fallen planet has a lot to do with it. We can actually see this now classified “dwarf planet” as the outcast from the class, the loser, and we easily identify with it. Since 2006, the year of its demotion by the International Astronomical Union meeting at a general assembly in Prague, many Americans have not taken off, despite the good reasons put forward by astronomers. Damnedit was after all the only (and former) planet in the Solar System discovered by an American!

The study published in Nature Geoscience which interests us here sheds fascinating light on the origins of the Plutonian system. Because, yes, Pluto is a system in itself… and a very strange one indeed. More precisely, we will present the double system formed by Pluto and its moon Charon.

No moon in the Solar System is as imposing compared to its parent planetary body

The Earth and the Moon themselves are considered a dual system, and not just a little. Indeed, there is no other planet in the Solar System whose pair formed with its main moon presents such a high ratio between the mass of the natural satellite and that of the planet. Our moon is about 1% of Earth's mass, and take our word for it, it's a lot. For comparison, for Ganymede, Jupiter's most massive moon, we arrive at 0.078%. This being said, Pluto and Charon form an “out of category” pair.

Charon is 15.7% the mass of Pluto and half its diameter, which is enormous. We therefore consider them as a “real couple”, that is to say a double system. But this poses astrophysical questions and constraints on how it formed.

Until now, the hypothesis retained was that of the giant impact, as precisely for the Moon and Earth. Something massive would have collided with Pluto and the resulting debris would have formed Charon. This hypothesis nevertheless presented several weaknesses, including a major one: the very similar composition of the two bodies. According to the original scenario, Pluto should have been rockier and Charon more icy, which is not observed. The extremely small orbital distance of only 20,000 km between the two bodies must also be explained. If we compare to the Earth-Moon system, it’s as if the latter was three times closer to us…

New hypothesis: an excessively slow impact, like an icy kiss

The new hypothesis is called kiss and capture and is based in part on supercomputer simulations. Its great strength is that it perfectly takes into account the very similar physical characteristics of the two bodies and the celestial mechanics at work in the icy trans-Neptunian confines. Because there, the bodies move slowly, which is not very consistent with a giant and devastating impact. Indeed, Pluto is five times slower in its orbit than the Earth, at around 5 km/s, and the average speed of bodies in the Kuiper beltto which the Pluto/Charon couple belongs, is between 3 and 5 km/s.

Certainly, this still represents around 17,000 km/h, but astronomers have spotted examples of bodies in this belt aggregating on top of each other after a “languorous” percussion. The NASA probe New Horizonthe same one that flew over the Pluto/Charon couple in 2015, met Arrokoth in 2018/2019, quite logically compared to a snowman.

One final point seems to favor this sexy scenario rather than the violent impact one, but it is still speculative at this point. In the case where, as some planetologists believe, Pluto has an ocean of around a hundred kilometers under its surface (hypothesis based on New Horizon data), this characteristic would be very poorly explained by the sudden impact and much better by that of the slow embrace proposed by the article of 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 actual strength of these materials, we discovered something completely unexpected. Most planetary collision scenarios are classified as hit & run. What we discovered was quite different: a 'kiss and capture' scenario where bodies collide, briefly stick together, then separate while remaining gravitationally attached.” In short, a scenario like Twilight : a vampire's kiss, languorous, icy and leading to eternity…