Pluto Might Have Formed as a Giant Comet, Not a Planet

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The mechanics of planet formation are thought to be pretty universal, but a new report suggests that Pluto could have formed from very different processes than those we typically expect.

The standard model of planetary formation is that planets form as grains of ice and rock collect together. These grains eventually form a structure large enough to be called a planetesimal. As the planetesimal grows, it pulls more surrounding material towards itself. Eventually, these planetesimals collide and form larger bodies, known as protoplanets. We still have three protoplanets in our own solar system — Ceres, Vesta, and Pallas, left over from the dawn of the solar system. Since we know protoplanets can stop evolving if they run out of material to accrete, it seems a straightforward extension of the theory. Pluto is a dwarf planet that coalesced from the dust in its local orbit and Charon is a moon that Pluto may have captured (one theory suggests Pluto and Charon actually struck and went into orbit around each other).

A planet forming out of comets, on the other hand, is a distinctly new idea. The New Horizons visit to Pluto taught us a great deal about the planet’s surface chemistry. Pluto, has certain features that are covered in nitrogen ice, including a large western formation named Sputnik Planitia. The amount of nitrogen in the ice and its chemical characteristics hint that Pluto might not have formed in standard fashion at all.

“We’ve developed what we call ‘the giant comet’ cosmochemical model of Pluto formation,” said Dr. Christopher Glein of SwRI’s Space Science and Engineering Division. The research is described in a paper now published online in Icarus. At the heart of the research is the nitrogen-rich ice in Sputnik Planitia, a large glacier that forms the left lobe of the bright Tombaugh Regio feature on Pluto’s surface. From Glein:

We found an intriguing consistency between the estimated amount of nitrogen inside the glacier and the amount that would be expected if Pluto was formed by the agglomeration of roughly a billion comets or other Kuiper Belt objects similar in chemical composition to 67P, the comet explored by Rosetta… Our research suggests that Pluto’s initial chemical makeup, inherited from cometary building blocks, was chemically modified by liquid water, perhaps even in a subsurface ocean.

The team explored a number of facets of Pluto, including estimations of how rapidly it would lose internally-trapped nitrogen to space due to tectonic and volcanic action. (Charon and Pluto both show signs of cryovolcanic activity, though none has been directly observed.)

Maps indicating regions rich in methane (CH4), nitrogen (N2), carbon monoxide (CO), and water (H2O) ices. Sputnik Planitia shows an especially strong signature of nitrogen near the equator.
Image: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

This is interesting for a host of regions. First, if Pluto was assembled by comet impacts or at least substantially impacted by them early in its history, it would be further evidence that the Kuiper Belt once had far more material in it than it does not. Today, the collective mass of the entire Kuiper Belt is estimated at only about 1/10th the Earth, at most, spread out over a vast area. Gravitational interactions with Neptune may have dramatically depopulated the belt, as shown by the image below:

Simulation showing outer planets and Kuiper belt: a) before Jupiter/Saturn 1:2 resonance, b) scattering of Kuiper belt objects into the Solar System after the orbital shift of Neptune, c) after ejection of Kuiper belt bodies by Jupiter. Image by Wikipedia

Clearly there was once far more material to work with in the Kuiper Belt, potentially including a very large number of comets. But as of right now, it’s not clear that the comet theory is a great fit — there are other aspects of Pluto which favor a conventional formation story out of a protoplanetary disc. It’s also possible that the planet formed normally, but was struck by a large number of comets later in its history — enough to alter some of its chemical properties and contribute to its overall mass.

If Pluto was formed in part by comets, this raises questions about two other moons: Charon and Triton. Charon is Pluto’s moon, and while we don’t understand its structure very well, its composition is significantly different from Pluto’s. Triton, meanwhile, is thought to resemble Pluto in multiple respects, including its radius, density, temperature, and overall chemical composition. Since Triton is thought to be a Kuiper Belt Object captured by Neptune, it’s entirely possible that the same processes that led to the formation of Pluto, Charon, and the other KBOs also shaped Triton. Triton accounts for 99.5 percent of the mass in the Neptunian moon system, and its retrograde orbit means it cannot have evolved beside Neptune, but was instead captured by it.

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