On the Origin of Triton and Pluto

Our book review for this week is for Chasing New Horizons, which explores the process by which the New Horizons mission to Pluto came to be, after Pluto became the last planet unexplored (and before it was demoted to the status of dwarf planet, which I know is very controversial for some people). That seemed the perfect excuse for me to indulge my occasional habit of sharing technical material with you, so I had only to decide what Pluto-related information to explore. The obvious answer, especially given my background, would have been the New Horizons mission profile, but I figure that we’ll talk about that in the review, and besides, we’ve talked about astrodynamics before. When Chasing New Horizons referenced a particularly impactful scientific paper, I had my answer.

Entitled “On the Origin of Triton and Pluto,” the paper, written by William B. McKinnon, proposed what was, at the time, a radically new hypothesis for the origin story of Pluto and Triton (a Neptunian satellite), and in the process laid the groundwork for a whole new understanding of the structure and formation of the entire solar system.

Before McKinnon’s proposal, the common consensus was that both Triton and Pluto were originally Neptunian moons, and that Pluto had somehow escaped from Neptune’s orbit (perhaps through the action of some massive impact that may have produced Pluto’s own moon, Charon). Since Pluto’s bizarre orbit does cross Neptune’s, there was some evidence to support such an origin, and, perhaps more saliently, there was no evidence to suggest an alternative origin, no suggestion of anything that could give rise to a planetoid beyond Neptune’s orbit. In a short paper, McKinnon both upended conventional thinking about Pluto’s and Triton’s origins with compelling physical arguments, and proposed a whole, unexplored and heretofore unimagined frontier of the solar system.

Compared to many, more recent scientific papers which I have read, “On the Origin of Triton and Pluto” is remarkably well-communicated, and the science involved is relatively straightforward. McKinnon’s main arguments are grounded in concepts out of classical physics which you may remember from high school physics classes: conservation of angular momentum, Newton’s law of universal gravitation, elastic and inelastic collisions. First, he shows that the previously accepted hypothesis would require a physical collision between Triton and Pluto in order for Pluto to be ejected into something that could evolve into its present orbit and for Triton to be forced into its current, retrograde state – “for plausible mass ratios, and under the stipulated initial conditions, a combination of orbital interactions to reverse Triton’s motion cannot be found.”

His second argument is that the current “angular momentum state of the Pluto-Charon system (that is, that contained in their mutual orbits and spins) is not consistent with a Neptune satellite origin.” Mathematically, McKinnon demonstrates that Neptune’s influence would rapidly rob Pluto-Charon of angular momentum, forcing them to coalesce into a single body and then de-spin. Accounting also for the idea that the system may have had fresh angular momentum imparted to it, and/or that Charon could have developed from a massive impact (like the impact that many theories today assert created our own Moon) is the explanation that Pluto would have had to be at too great a distance from Neptune to be consistent with the conditions required for the effect on Triton discussed in the first objection.

Since McKinnon’s hypothesis has since become the dominant theory that is taught in schools and broadly accepted, and successive observations, including those by New Horizons, have demonstrated the existence of the sorts of distant objects he invokes, it is perhaps difficult to fully convey how radical a proposal he is making when he introduces his idea that, instead of both Pluto and Charon originating as Neptunian satellites, “Triton and Pluto originate as satellites of the Sun.” That is, they were once objects orbiting in the very distant reaches of the solar system, and Triton happened to be captured by Neptune.

While he does say that “the existence of large, outer Solar System planetesimals is consistent with the dynamics of planetary accretion and may even be necessary to explain planetary eccentricities, inclinations, and obliquities,” it is worth emphasizing that, at the time, there was no direct evidence for such bodies. Pluto-Charon was the only trans-Neptunian object found up to that point, and that only after a challenging and lengthy search for a mysterious “planet X” predicted to orbit beyond Neptune based on gravitational influence. Today, we know that there is an entire “Kuiper Belt” out beyond Neptune, full of small planetoids, planetesimals, dwarf planets like Pluto, asteroids, and other debris of the solar system’s formation, but none of that was known at the time.

To support his proposal with something other than abstract physics and orbital mechanics, McKinnon concludes his paper by referencing observations made of Triton’s surface, noting its peculiar composition, and noting that it could reasonably be produced as the result of Triton’s entire ice content melting due to the heat generated by its gravitational capture by Neptune from a prior, Kuiper Belt existence. This is worth pointing out if just to demonstrate the kind of awesome (in the true, poetic sense of the word) power and time scales that are dealt with in planetary science.

Chasing New Horizons claims that “On the Origins of Triton and Pluto” served as a galvanizer for the planetary science community, helping to demonstrate just how many unsettled questions there still were (and are) about the outer solar system, and especially the deepest regions, which could potentially be answered by a visit to Pluto (and, indeed, we’ve learned an enormous amount from New Horizon’s Pluto flyby, as well as its visit to Ultima Thule/Arrokoth/2014 MU69, an even more distant Kuiper Belt object). With the spacecraft having recently received permission to extend its mission even further, who knows what we might find behind that next, new horizon?