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The famed “rubber duck” comet is revealing more about its past. It turns out that 67P/Churyumov-Gerasimenko probably did grow out of tiny pebbles, as many suspected – and this may help us learn more on how planets are built too.

When the Rosetta spacecraft launched towards 67P in 2004, part of its mission was to figure out how the weird, double-lobed comet formed. Using data from Rosetta and its accompanying lander, Philae, at Braunschweig University of Technology in Germany and his colleagues have traced the comet’s building blocks to tiny pebbles of dust and ice.

To help uncover 67P’s origin story, Blum and his team studied the data compiled after the end of the Rosetta mission and compared the observations with models of how planets and other, smaller space rocks form.

As the solar system formed, these microscopic dust and ice pebbles made up a disc around the sun. The same disc that probably formed comet 67P also built all the planets in our solar system, including Earth.

������ýs have long suspected that these clumps of microscopic dust are gradually attracted to one another over time, and gravity slowly builds a bigger and bigger pile of pebbles. This process, called gentle gravitational collapse, can eventually form a dusty comet such as 67P, which is about 4.3 kilometres long – or even an entire planet.

Blum and his colleagues confirmed that 67P was almost certainly formed through gentle gravitational collapse.

Duck mystery

However, this analysis does not solve the mystery of how 67P got its rubber-duck-like shape. “Whether the two lobes were once one body and then carved out to its current shape, or came together in a gentle collision by two individual bodies, we have no information about in our paper,” says Blum.

Comets such as 67P are relics of the beginning of the solar system. Because comets spend most of their time on the outskirts of our solar system, they don’t melt or get bombarded with particles from the sun – thus serving as valuable time capsules to billions of years ago.

“This is really fundamental work because the greatest question we have is how comets form,” says , a project scientist for the Rosetta mission. “They are the primitive building blocks and tell us how planetesimals form.”

Given that planetesimals can later grow into full-sized planets, understanding comets such as 67P could help us gain more insight into how planets form in general. By tracing these comets back to the pebbles that made them, we can find out more about what kinds of material made Earth.

Read more: Rosetta’s biggest hits: The comet chaser’s top seven discoveries

Journal reference: Monthly Notices of the Royal Astronomical Society, DOI: