In the far reaches of the solar system, there are many icy objects that resemble snowmen – pairs of interconnected spheres. Now a new study reveals the simple way these mysterious objects can form.
Beyond the path to Neptune are icy building blocks from the beginning the solar system known as planetesimals. In the same way that snowballs are composed of clusters of snowflakes, planetesimals likely originated in the disks of dust surrounding the newborn sun from clouds of pebble-sized objects drawn together by their mutual gravitational attraction.
In 2019, NASA’s New horizons spacecraft captured the first ever close-ups of planetesimals shaped like two conjoined spheres – snowman-like objects known as contact binaries. Other research found that one in 10 to one in four planetesimals may be contact binaries.
But it remained a mystery how these distant snowmen were formed. Previous work attempted to calculate how contact binaries arose by modeling them as colliding spheres. But until recently, these calculations involved perfect mergers that always resulted in spheres rather than other shapes.
In the new study, researchers instead modeled planetesimals as clouds of particles resting on each other’s surfaces.
“This method is more computationally expensive than previous perfect merger models because we need to keep track of many individual particles that make up one planetesimal rather than just one large planetesimal size,” study leader Jackson Barnes, a planetary scientist at Michigan State University, told Space.com.
In the new modeling work, occasionally as these clouds spun, instead of merging into one planetesimal, they would individually form two separate planetesimals that orbited each other. (Astronomers have seen many such binary planetesimals in Kuiper belt beyond Neptune.)
In the new simulations, these binary planetesimals could spiral inward due to their mutual gravitational pull, until they gently make contact and merge. What’s “so cool about this model,” Barnes said, is that it can create planetesimals that are not only spherical but also flat, cigar-shaped and, yes, snowman-shaped. The speed with which these planetesimals move and the strength with which their particles lock together help determine what form their fusion ultimately results in.
The pairs of planetesimals that make up these distant snowmen can likely stay together for millions or even billions of years because they are unlikely to crash into something else, Barnes explained. Without a collision, nothing can break them apart.
The new study found that contact binaries made up only 4% of the planetesimals they simulated. “This is not quite consistent with the supposed 10 to 25% expected,” Barnes said.
Barnes noted that the team’s computer simulations were limited by the number and sizes of the particles that make up the clouds of pebbles that went on to form planetesimals. Increasing the number and size ranges of particles in their simulations could help increase the number of contact binaries that can form, he suggested.
Spinning clouds of pebbles can also go on to form three or more planetesimals that orbit each other, “which I think is pretty cool,” Barnes said. “This is something we are currently investigating in more detail, with specific regard to the establishment of triple systems and their relationship to the currently observed population of relict triples in the Kuiper Belt.”
The researchers detailed their findings February 19 in the journal Monthly Notices of the Royal Astronomical Society.
