In the distant reaches of the solar system are many icy objects that resemble snowmen — pairs of conjoined spheres. Now, a new study reveals the simple way in which these mysterious objects might form.
Beyond the orbit of Neptune lie icy building blocks from the dawn of the solar system known as planetesimals. Much like snowballs are composed of clusters of snowflakes, planetesimals likely arose within the disks of dust that encircled the newborn sun from clouds of pebble-sized objects pulled together by their mutual gravitational attraction.
In 2019, NASA's New Horizons spacecraft captured the first-ever up-close images of planetesimals shaped like two linked spheres — snowmen-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 formed. Previous work sought to compute how contact binaries originated by modeling them as colliding spheres. However, until recently, these computations involved perfect mergers that always resulted in spheres instead of any 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 prior perfect-merging models, because we need to keep track of many individual particles that make up one planetesimal instead of just one large planetesimal-sized particle," study lead author Jackson Barnes, a planetary scientist at Michigan State University, told Space.com.
In the new modeling work, occasionally, as these clouds spun, instead of coalescing into one planetesimal, they would each form two separate planetesimals that orbited one another. (Astronomers have seen many such binary planetesimals in the 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 fuse together. What is "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 at which these planetesimals are moving and the strength with which their particles interlock helps determine which shape their fusion ultimately results in.
The pairs of planetesimals making up these far-off snowmen can likely stay together for millions or even billions of years because they are unlikely to crash into anything else, Barnes explained. Without a collision, there is nothing to break them apart.
The new study did find that contact binaries made up just 4% of the planetesimals they simulated. "This does not quite match the hypothesized 10 to 25% expected," Barnes said.
Barnes noted that the team's computer simulations were limited by the number and sizes of the particles making up the clouds of pebbles that went on to form planetesimals. Increasing the number and the size ranges of particles in their simulations could help increase the number of contact binaries that can form, he suggested.
Spinning clouds of pebbles could also go on to form three or more planetesimals orbiting one another, "which I think is pretty cool," Barnes said. "This is something we’re currently investigating in greater detail, with specific respect to the creation of triple systems and their relation to the current observed population of relict triples in the Kuiper Belt."
The scientists detailed their findings Feb. 19 in the journal Monthly Notices of the Royal Astronomical Society.
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