Is Pluto Actually a Mash-up of a Billion Comets?

Researchers speculate the beloved dwarf planet could actually be a giant comet

Pluto, and its largest moon Charon, as seen from the New Horizons spacecraft. NASA

For decades, researchers have debated about how to classify Pluto. Is it a planet? Is it a dwarf planet? Or is it something else entirely?

Now, scientists from the Southwest Research Institute suggest it may fall into the third category. As Neel V. Patel reports for Popular Science, Pluto could be made up of billions of comets all mashed together. The researchers present their ideas in a study published in the journal Icarus.

Scientists had long believed the dwarf planet Pluto was formed the way planets come to be: they start as swirling dust that's gradually pulled together by gravity. But with the realization that Pluto was a Kuiper belt dwarf planet, researchers began speculating about the origins of the icy world.

In recent decades, scientists have tossed around the idea that Pluto could be a giant comet. But they had no way to test these speculations. That is, until the summer of 2015, when New Horizons zipped by the tiny world. The historic flyby yielded breathtaking images, spectacular data—and the possibility of testing out the wild comet proposal.

The researchers turned to Sputnik Planitia—the western lobe of the massive heart-shaped icy expanse stamped on Pluto’s side—for the task. As Christopher Glein, lead author of the paper and researcher at the Southwest Research Institute, explains to Patel, the researchers used the data from New Horizons on this icy expanse to estimate the amount of nitrogen on Pluto and the amount that’s escaped from its atmosphere.

The researchers then pulled together composition data gathered by the European Space Agency’s Rosetta mission. The craft orbited Comet 67P/Churyumov-Gerasimenko for two years before purposefully crash-landing into its surface in 2016.

“[W]e used the nitrogen abundance from Rosetta, and scaled that up to the mass of Pluto,” Glein says. Both analyses gave surprisingly similar estimates.

Glein explains the conclusions in a statement: “We found an intriguing consistency between the estimated amount of nitrogen inside the [Sputnik Planitia] 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.”

The conclusions are far from definitive but hint that the comet idea is an intriguing possibility. However, there are still a few caveats. For one, researchers aren’t sure that comet 67P has an average comet composition, Patel reports. For another, New Horizons only captured information about Pluto at a specific point in time, which means nitrogen rates could have changed over the last billions of years. As Mike Walls writes for, there’s also still the possibility Pluto formed “from cold ices with a chemical composition closer to that of the sun.”

One big challenge to the theory is the low amounts of carbon monoxide on the dwarf planet—a find that runs counter to the situation of most comets. But that doesn't preclude the comet idea: Carbon monoxide may be buried deep under the glacier, or even trapped in a below-surface ocean.

Despite these uncertainties, Caltech planetary scientist James Tuttle Keane, who was not involved in the study, tells Gizmodo's George Dvorsky the study still adds to the important conversation about how the solar system formed.

“This paper is an exciting example of the science that can be achieved when combining data from different, international, planetary science missions,” he says. “There’s been long debate about the role and significance of comets in the construction of planets… This study represents a new piece to this long-standing puzzle.”

As Patel reports, there’s only one way to confirm the new theory: Land on Pluto to gather more data.

Get the latest stories in your inbox every weekday.