Two Plutos collide in the outer solar system. What are the odds?

Not long after Pluto’s moon Charon was discovered, astronomers realized it would take hundreds or thousands of small planets to make such a planetary smash-up likely.
By | Published: June 11, 2015 | Last updated on May 18, 2023
This 2005 simulation by Robin Canup of the Southwest Research Institute implies Pluto crashed into an object nearly its own size in order to created its moon, Charon.
After its discovery, Pluto was beloved as an oddball and considered an overgrown comet or relic of planetary formation. But, in 1978, James Christy’s discovery of Charon challenged that view.

The twin worlds are relatively similar in size (Earth’s Moon is 1/80 our planet’s mass, while Charon is roughly 1/10 Pluto’s), but have dramatically different densities, making it unlikely they could have formed together simultaneously in the solar nebula. And water ice on Charon added to evidence the pair had formed in the cool outskirts of the solar system rather than being ejected moons of Neptune as some had suggested.

Five years after its discovery, this duo’s dynamics led Bill McKinnon of Washington University in St. Louis to first suggest the Pluto-Charon system could be best explained by two small planets on a collision course. In his 1984 Nature paper, “On the origin of Pluto and Triton,” McKinnon provided a convincing argument that the pair started out as independent planetesimals.

But what are the odds of such a collision happening at the solar system’s frontier, where only Pluto and its moon were then known to exist?

In 1991, Alan Stern, now in charge of NASA’s New Horizons mission, set out to calculate an answer. His findings suggested that, given our vast solar system and only two Plutos, the chances of a collision are less than one in a million over all of our solar system’s history.

In a 1992 article in Astronomy magazine, Stern compared the odds of such chance occurrences at Pluto, as well as Neptune and Uranus, to reaching your hand into 100,000 tons of sand and plucking out the only purple grain on your first try.

“Taking each clue in isolation,” he wrote, “we can reasonably conclude that the formation of the Pluto-Charon binary, the capture of Triton, the tilting of Uranus, or the tilting of Neptune, could have occurred by itself, though each had only a small chance. But the combined probability that seven objects could cause four events would be less than one in a million billion (one in 10^15) over the age of the solar system.”

But if you increase the number of Plutos — 1,000–3,000km icy dwarfs — to more than a thousand, suddenly such a collision becomes likely. He suggested an entire class of icy dwarfs once lurked past Neptune and have since been ejected into the outer abyss of our solar system.

Stern wouldn’t have to wait long for validation of his math. In 1992, astronomers finally found the second Pluto, an object known as 1992 QB1. And it opened up the floodgates of Kuiper Belt Objects still being discovered to this day.

Eric Betz is an associate editor of Astronomy. He’s on Twitter: @ericbetz.