Researchers at Michigan State University have developed a computer simulation showing that gravitational collapse can naturally produce double-lobed, snowman-like structures in the outer solar system. These contact binaries make up about 10 percent of planetesimals in the Kuiper Belt beyond Neptune. The findings, published in the Monthly Notices of the Royal Astronomical Society, explain a long-standing puzzle in astronomy.
For decades, astronomers have observed that many icy bodies in the Kuiper Belt resemble snowmen, with two rounded lobes connected together. The Kuiper Belt, a region past Neptune filled with frozen remnants from the solar system's formation, contains these primitive planetesimals, which are leftover building blocks from planet creation.
Jackson Barnes, a graduate student at Michigan State University, created the first simulation to naturally generate these contact binaries through gravitational collapse. Using the high-performance computing cluster at MSU's Institute for Cyber-Enabled Research, or ICER, the model treats forming objects as retaining structural strength, allowing two bodies to settle against each other without merging into a sphere.
In the simulation, planetesimals begin as rotating clouds of dust and pebbles drawn together by gravity, similar to snowflakes forming a snowball. These clouds can split into two orbiting bodies that gradually spiral inward and gently contact, preserving their rounded shapes.
Previous models, which simplified collisions as fluid blends, failed to recreate the distinctive two-part form. Earlier explanations involved rare events, but as Earth and Environmental Science Professor Seth Jacobson, senior author on the paper, noted, "If we think 10 percent of planetesimal objects are contact binaries, the process that forms them can't be rare." Gravitational collapse aligns with observations of their commonality.
NASA's New Horizons spacecraft highlighted these shapes in January 2019 by imaging a contact binary, prompting closer examination of Kuiper Belt objects. In this sparsely populated region, collisions are rare, enabling fragile structures to endure for billions of years with few craters.
Barnes emphasized the breakthrough: "We're able to test this hypothesis for the first time in a legitimate way." The team plans to extend the model to more complex systems, anticipating further discoveries from future NASA missions.