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Trending Science: Ancient space object provides clues about how planets form

The most distant object ever visited by spacecraft reveals secrets of planetary formation.

Fundamental Research

On New Year’s Day 2019 when NASA’s New Horizons probe flew by Arrokoth, an object that looks like a red snowman more than 6.6 billion km from Earth, little did we know that it would overturn the established theory for how planets in our solar system formed. At 36 km long and 20 km wide, Arrokoth is located in a region known as the Kuiper Belt. It’s classified as a planetesimal, an object that was among the building blocks of the planets. Since that brief encounter over a year ago, researchers have been able to examine the data and discover fascinating new details about this space snowman’s origin, formation, geology, composition, colour and temperature. They presented their findings at a press conference during the annual meeting of the American Association for the Advancement of Science and in three papers in the journal ‘Science’.

Resolving the debate of how planets first appeared

“Arrokoth has turned out to be astonishing in terms of what we’ve learned from it,” said Bill McKinnon, lead author for one of the papers, before a presentation at the annual meeting, ‘The Guardian’ reports. “It tells us some profound truths about our solar system. This is not just a space potato. It’s a remarkable world that’s told us a remarkable story.” The prevailing theory is that material violently crashed together to form ever larger clumps until they became worlds. The research reveals that Arrokoth was created by the merger of 2 separate bodies 4.5 billion years ago. The team found no evidence of violent impact. “There is no evidence of heliocentric, high-speed collisional evolution, or any catastrophic (or sub-catastrophic) impact during its lifetime,” the researchers said in one of the papers. “Instead, we conclude that its two lobes … came together at low velocity, no more than a few m/s and possibly much more slowly.” New Horizons principal investigator Alan Stern, lead researcher for the second paper, told the ‘BBC’ that the discovery is of “stupendous magnitude.” He added: “There was the prevailing theory from the late 1960s of violent collisions and a more recent emerging theory of gentle accumulation. One is dust and the other is the only one standing. This rarely happens in planetary science, but today we have settled the matter.”

Clearer picture of Arrokoth’s composition and origin

A third paper examined Arrokoth’s distinctive appearance. It’s extremely cold and the surface is covered in methanol ice. Complex organic molecules also detected on the surface are likely what creates Arrokoth’s peculiar red colour. “Arrokoth is the most distant, most primitive and most pristine object ever explored by spacecraft, so we knew it would have a unique story to tell,” Stern told ‘CNN’. “It’s teaching us how planetesimals formed, and we believe the result marks a significant advance in understanding overall planetesimal and planet formation.” Speaking to ‘Reuters’, astronomer and lead author John Spencer said: “So we now have a clearer picture of how planets, including the Earth, were built.”

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