Spinning clouds of pebbles may lead to the formation of oddly flattened objects in the solar system, including comets and asteroids.
In 2019, NASA’s New Horizons spacecraft flew past an icy, comet-like object beyond Pluto called Arrokoth. It was notable because it seemed to be relatively flat, with each of its two joined lobes squished into a pancake shape. Several other objects in the solar system seem similarly squashed, including comet 67P/Churyumov-Gerasimenko, which was visited by the European Space Agency’s Rosetta spacecraft in 2014.
at the University of Copenhagen in Denmark and at Lund University in Sweden suggest this is a result of how the objects formed.
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The pair says loose conglomerations of pebbles and dust in the early solar system may have been drawn together by gravity – an idea known as pebble accretion, a possible explanation for how planets form. If these rock accumulations were spinning at the right speed, it could lead to the formation of disc-shaped objects.
“If you have a rotating cloud of pebbles, it tends to become more flattened out”, like clay on a pottery wheel, says Johansen. If it were spinning fast enough, the angular momentum would be too high and the object would “fly apart”, he says. But if it were moving at less than a few metres per second, the object would retain its flattened shape during the accretion process, which can happen over just few years.
Comets would be more likely to appear flattened than asteroids, because they form further out in the solar system where they are less likely to crash into each other. The effect would influence the formation of objects up to about 100 kilometres in circumference – any larger and they begin to fashion into spheres because of their own immense gravity.
Studying more bodies in the outer solar system will probably help confirm whether this idea is correct. “That could give us a better handle on the shape of these objects,” says Johansen.
arXiv
