Far beyond Neptune, the Kuiper Belt holds icy, largely untouched planetesimals that preserve conditions from the dawn of the solar system. Observations suggest that roughly one in ten of these building block objects are contact binaries, where two nearly spherical lobes rest against one another rather than forming a single smooth sphere.

Michigan State University graduate student Jackson Barnes has now created what is described as the first simulation that naturally produces these two lobed shapes through gravitational collapse. The study appears in the journal Monthly Notices of the Royal Astronomical Society under the title Direct contact binary planetesimal formation from gravitational collapse.

Earlier computer models treated colliding bodies as fluid blobs that quickly merged into single spheres. That approach made it impossible to form the distinctive snowman like profiles seen in New Horizons images of Kuiper Belt object 2014 MU69, informally known as Ultima Thule, and in other suspected contact binaries.

Barnes used the high performance computing resources of MSU's Institute for Cyber Enabled Research to construct simulations in which the building blocks retain internal strength. In this more realistic environment, solid chunks can pile up, rest against each other and maintain their shapes rather than flowing together.

Other ideas for how contact binaries form require special circumstances or rare events. These scenarios might work occasionally but struggle to explain why such objects make up a significant fraction of Kuiper Belt planetesimals.

"If we think 10 percent of planetesimal objects are contact binaries, the process that forms them cannot be rare," said Earth and Environmental Science Professor Seth Jacobson, senior author on the paper. "Gravitational collapse fits nicely with what we have observed."

NASA's New Horizons spacecraft provided the first close up views of a Kuiper Belt contact binary during its January 1, 2019 flyby of 2014 MU69. Those images prompted scientists to re examine data on other distant objects and concluded that contact binaries account for about 10 percent of Kuiper Belt planetesimals.

The Kuiper Belt preserves remnants of the early Milky Way when the solar system formed within a disc of dust and gas. Today the region hosts dwarf planets such as Pluto, comets and numerous smaller planetesimals that have remained relatively undisturbed and rarely collide.

Planetesimals are the first sizable bodies to assemble from a cloud of dust and pebble sized solids. Small particles stick together into pebbles, which then aggregate into larger clumps under their mutual gravity, much like individual snowflakes packed into a snowball.

Barnes' simulations show that as a rotating cloud of material collapses inward, it can tear apart an initial body and separate it into two planetesimals that orbit one another. Binary planetesimals of this type are observed in the Kuiper Belt, and in the model their mutual orbits gradually shrink until the two components gently touch and fuse.

Because the collision speeds are low and the lobes retain their internal strength, they preserve their roughly spherical shapes instead of flattening or shattering. Once formed, these contact binaries are unlikely to encounter other large objects in the sparse Kuiper Belt, which helps them survive for the age of the solar system.

Most known binaries in this region show few or no impact craters, supporting the view that they have lived in a relatively calm environment. With little external disturbance, gravitationally assembled snowman like worlds can persist for billions of years in the outer solar system.

Scientists have long suspected that gravitational collapse could build contact binaries, but previous models lacked the physics needed to follow the process in detail. Barnes' work is presented as the first to incorporate the necessary ingredients to reproduce their shapes in a self consistent way.

"We are able to test this hypothesis for the first time in a legitimate way," Barnes said. "That is what is so exciting about this paper." The team expects that similar methods can extend to even more complex multiple body systems in the future.

Barnes and colleagues are now developing a new generation of simulations that better track the collapse of pebble clouds and the evolution of binary and higher order systems. As upcoming NASA missions probe farther into the solar system, the researchers anticipate that many more distant snowman shaped worlds may come into view.

Research Report:Direct contact binary planetesimal formation from gravitational collapse

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