Astronomers have long debated why so many icy objects in the outer solar system look like snowmen. Michigan State University researchers now have evidence of the surprisingly simple process that may be responsible for their creation.

Far beyond the violent, chaotic asteroid belt between Mars and Jupiter lies what’s known as the Kuiper Belt. There, past Neptune, you’ll find icy, untouched building blocks from the dawn of the solar system, known as planetesimals. About one in 10 of these objects are contact binaries, planetesimals that are shaped like two connected spheres, much like Frosty the Snowman. But just how these objects came to be without the help of a magic silk hat has been an open question.

Jackson Barnes, an MSU graduate student, has created the first simulation that reproduces the two-lobed shape naturally with gravitational collapse, which refers to matter shrinking into itself because its own gravity pulls it together more strongly than any force acting to expand. His work is published in the Monthly Notices of the Royal Astronomical Society.

Caption: Jackson Barnes created this computer simulation showing how a contact binary’s two-lobed shape could be formed by gravitational collapse. Credit: Michigan State University Jacobson Lab

Earlier computational models treated colliding objects as fluid blobs that merged into spheres, making it impossible to form these unique shapes. Thanks to MSU’s Institute for Cyber-Enabled Research, or ICER, and its high-performance computing cluster, Barnes’ simulations produce a more realistic environment that allows objects to retain their strength and rest against one another.

Other formation theories involve special events or exotic phenomena that, while possible, aren’t likely to happen on a regular basis.

“If we think 10% of planetesimal objects are contact binaries, the process that forms them can’t be rare,” said Earth and Environmental Science Assistant Professor Seth Jacobson, senior author on the paper. “Gravitational collapse fits nicely with what we’ve observed.”

Caption: New Horizons video: This short movie shows the view of Kuiper Belt object 2014 MU69 (nicknamed Ultima Thule) as seen by NASA's New Horizons spacecraft from Dec. 7, 2018 to Jan. 1, 2019. During the approach, Ultima Thule transforms from a faint dot 20 million miles (31 million kilometers) away, indistinguishable from thousands of background stars, to a newly revealed world unlike any seen before, from a range of 5,000 miles (8,000 kilometers). The sequence consists of actual New Horizons images, taken at discrete intervals during the approach, supplemented with computer-generated intermediate frames in order to make a smooth movie. Time slows down during the movie to show clearly both the slow initial phases of the approach and the very rapid final stages. The final image is a parting crescent view of Ultima Thule, taken 10 minutes after closest approach occurred at 12:33 a.m. EST on Jan. 1. Credit: NASA

Contact binaries were first imaged up close by NASA’s New Horizons spacecraft in January 2019. These images prompted scientists to take another look at other objects in the Kuiper Belt, and it turned out that contact binaries accounted for about 10% of all planetesimals. These distant objects float mostly undisturbed and safe from collisions in the sparsely populated Kuiper Belt.

In the early days of the Milky Way, the galaxy was a disc of dust and gas. Remnants of the galaxy’s formation are found in the Kuiper Belt, including dwarf planets like Pluto, comets and planetesimals.

Planetesimals are the first large planetary objects to form from the disc of dust and pebbles. Much like individual snowflakes that are packed into a snowball, these first planetesimals are aggregates of pebble-sized objects pulled together by gravity from a cloud of tiny materials.

Occasionally as the cloud rotates, it falls inward on itself, ripping the object apart and forming two separate planetesimals that orbit one another. Astronomers observe many binary planetesimals in the Kuiper Belt. In Barnes’ simulation, the orbits of these objects spiral inward until the two gently make contact and fuse together while still maintaining their round shapes.

Computer simulated image of how this snowman-shaped object might have formed. — Caption: Contact binary example: Jackson Barnes created this contact binary in a computer simulation showing how the two-lobed shape could be formed by gravitational collapse. Credit: Michigan State University Jacobson Lab

How do these two objects stay together throughout the history of the solar system? Barnes explains they’re simply unlikely to crash into another object. Without a collision, there’s nothing to break them apart. Most binaries aren’t even pocked with craters.

Scientists long suspected that gravitational collapse was responsible for forming these objects, but they couldn’t fully test the idea. Barnes’ model is the first to include the physics needed to reproduce contact binaries.

“We’re able to test this hypothesis for the first time in a legitimate way,” Barnes said. “That’s what’s so exciting about this paper.”

Barnes expects his model will help scientists understand binary systems of three or more objects. The team is also working to create a new simulation that better models the collapse process.

As more NASA missions explore uncharted realms of the solar system, Jacobson and Barnes suspect Frosty may have more distant cousins yet to be found.

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