Arrokoth: Boulders revolutionized the idea of ​​planet formation

The birth of Arrokoth must have been slow. With only a few meters per second – even cyclists are often faster – two boulders collided on the edge of our solar system and stuck together. For four billion years they have been making their way around the sun undisturbed as a double body, which is reminiscent of a flattened snowman and has a diameter of 30 kilometers.

In the meantime, scientists called the original structure Ultima Thule, and since November it has had the official name Arrokoth. In the Powhatan Indian language, the word for heaven.

The most distant object that has ever been examined by a spacecraft

At the beginning of 2019, the NASA probe “New Horizons” took pictures of the icy celestial body, which is approximately 6.6 billion kilometers away from Earth. This makes Arrokoth the most distant object that a human-made probe has ever examined.

Last year, researchers also presented the first results of the flyby. However, data transmission from “New Horizons” to Earth is slow due to the large distance. In the meantime, more than ten times as much information is available, about which international research teams now report in three studies in the journal “Science”.

The Kuiper belt beyond the planet Neptune, where Arrokoth also travels, is a kind of cosmic archive that tells of the history of our solar system. This is because the countless small celestial bodies there have been orbiting the sun for around 4.6 billion years and have hardly changed during this time. The temperatures are just above the zero point of minus 273 degrees. Impacts from other celestial bodies rarely happen, the space is too empty.

Like a frozen time capsule, Arrokoth and neighbors show how the first building blocks for planets formed in the dust disk that once surrounded the young sun. They never got beyond the status of small building blocks.

Astronomers have long argued about how the planets were formed. Some believe that smaller pieces of rock collided with each other at high speed, thus forming larger and larger objects over time. A few years ago, however, doubts arose about this theory: Shouldn’t the chunks crush each other to dust if they hit each other with force?

According to an alternative model of planet formation, dust particles initially formed smaller clouds. Larger structures could then slowly have formed in it due to gentle collisions under the influence of gravity.

Arrokoth could clear up debate for years

The new studies on Arrokoth confirm this theory. “We are at a turning point in exploring planet formation,” said Alan Stern, chief scientist at New Horizons, during the AAAS conference in Seattle, USA, where the results were presented.

Researchers led by William McKinnon from Washington University in St. Louis had simulated the two theories of planet formation in computer models and compared them to the appearance of Arrokoth. The result: If the two parts that make up the celestial body hit each other at just five meters per second, they would have destroyed each other. “This shows that planet formation has been much gentler than previously thought,” said McKinnon.

Evidence of organic molecules

The new analyzes also show that Arrokoth is covered with a thick layer of ice made of methanol. There was also evidence of organic molecules, which are probably so-called tholines – complex molecules made of carbon, nitrogen and hydrogen, which occur, for example, on comets or in the atmosphere of gas planets. They are considered the precursors of chemical compounds from which life could also arise.

The research team at “New Horizons” now hopes to discover additional small celestial bodies on the edge of the solar system that are favorable for the spacecraft. “As long as we still have fuel,” says chief scientist Stern, “we will continue to research.”

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