Some comets, including the trans-Neptunian object Arrokoth, look more like a snowman than a celestial body: they consist of two spheres stuck together. How such double objects are created is still unclear. Planetary researchers have now tested a possible scenario using computer simulations. Accordingly, such so-called contact binaries may have been formed by the gravitational collapse of crumb clouds in the early solar system. In some cases, two or more large, closely orbiting planetesimals form in the center of these clouds. If they come too close and collide, this happens very gently and without strong deformations – the result is a double object like Arrokoth and Co, as the researchers discovered.
The outer reaches of the solar system have hardly been investigated so far; only a few space probes have penetrated into these distant areas beyond Neptune and Pluto. One of them is NASA’s New Horizons spacecraft, which reached its second destination in 2019 after its close flyby of Pluto and Charon: (486958) Arrokoth. This roughly 31-kilometer chunk orbits the sun at a distance of almost seven billion kilometers and is therefore part of the trans-Neptunian objects that populate the solar system beyond the planet Neptune. The Kuiper belt adjoining this zone and its mostly ice-rich celestial bodies are considered the main source of comets and a reservoir of planetesimals – planetary building blocks from the early days of the solar system. When New Horizons flew past Arrokkoth just 3,000 kilometers away in January 2019, their images revealed something surprising: the boulder was in two parts and resembled a snowman.
How are the double chunks created?
Closer analysis of the data showed that the two parts hardly have any craters and are chemically largely identical. “From these observations, it can be concluded that these halves have a common origin and were formed under mild conditions without significant deformation,” explain Jackson Barnes from Michigan State University and his colleagues. Due to the slight deformation of the two spherical parts and their thin “neck”, astronomers classify Arrokoth and double comets such as 67P/Churyumov-Gerasimenko as possible contact binaries – objects that were formed by a gentle, slow collision of two original boulders. Such binary objects are probably no exception among the trans-Neptunian celestial bodies: “It is estimated that contact binary systems make up 40 to 50 percent of the Plutino population; in the undisturbed and dynamically cold classical Kuiper Belt it could be ten to 25 percent,” write Barnes and his team.
But how do such double bodies come about? Theoretically, some snowman and rubber duck shapes of such chunks could have been formed by erosion at the “neck” of these objects. However, this is contradicted by analyzes of comet 67P/Churyumov-Gerasimenko, whose two halves show clearly separated layers; the gravitational alignment of chunks on the surface of the two spheres also suggests that these two parts must have originally been separated. This raises the question of under what circumstances two individual but chemically very similar celestial bodies come so close that they can gently stick together. Barnes and his team have now examined a possible scenario in more detail.
Their guess: Contact binaries are the result of a gravitational collapse of a cloud of crumbs in the early solar system. These clouds of millimeter-sized pebbles collapse due to their own gravity and turbulence. The small fragments are directly compressed into kilometer-sized planetesimals. In many cases, such a collapse creates not just one central planetesimal, but two or more closely orbiting chunks, as the researchers explain.
Gravitational collapse put to the test
Barnes and his team used a model simulation to investigate whether “glued” double chunks like Arrokoth could also arise in this scenario. Previous computer models usually simulated gravitational collapse using virtual fluids because the computational effort required to simulate all the fragments would be too high. “Such a pebble cloud probably contained a quadrillion particles,” the researchers explain. For their simulation, they combined these particles into hundreds of thousands of larger units, whose movements and collisions they reproduced in the model. The result: The collapse of the crumb cloud created an average of 834 planetesimals, including 29 contact binaries. “All identified contact binaries originally formed from two separately compacted objects. These objects collided with each other after initially existing as a gravitationally bound binary system,” report Barnes and his team. This collision of the two originally individual planetesimals occurs very gently, at only 0.4 to 5.8 meters per second – and therefore below the threshold at which such glued, barely deformed contact binaries can form, as the simulations showed.
According to the researchers, their results support the hypothesis that Arrokoth and at least some other binary objects in the Kuiper Belt were formed by the gravitational collapse of crumb clouds in the early solar system. “We have now been able to properly test this hypothesis for the first time,” said Barnes. However, the researchers also emphasize that this educational scenario certainly does not apply to all binary celestial bodies. For example, binary asteroids can also have gently collided under other circumstances.
Source: Jackson Barnes (Michigan State University, East Lansing) et al., Monthly Notices of the Royal Astronomical Society, doi: 10.1093/mnras/stag002)