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Distant space rock sheds new light on planet formation

Arrokoth, a small rock orbiting in the far reaches of our solar system has helped us understand more on how exactly the planets are formed. It is the most distant and oldest object ever to be visited by a human spacecraft.

Arrokoth was visited by New Horizons probe earlier and the data the probe collected has helped resolve some of the debate that exists regarding the formation of planetesimals or the small rocky seeds that develop into planets. Three new papers based on the data collected by probe has been released recently.

New Horizons probe made its visit to this primordial rock on New Year’s Day last year, way out in the Kuiper Belt. Located at an average distance from the Sun of 6.7 billion kilometres, it is the most distant single object that we have identified in the Solar System.

Initial data that we got back in May last year suggested that Arrokoth was once a binary object whose two halves had gently come together. Researchers also has found that its surface was lightly cratered and was predominantly red. But due to lack of more data researchers were unable to give accurate explanation to these observations.

The new papers shine more light into this as they had 10 times more the data to study with.The first of them, by New Horizons investigator William McKinnon of Washington University in St. Louis found that the two objects in the Arrokoth binary actually are from same area.

The finding is very important as it strengthens pebble accretion model of planetesimal formation. In this model elements from same area of the Solar nebula gently comes together to form binary object. In the opposing  hierarchical accretion model of planetesimal formation, it was believed that planet seeds were formed by violent crash of dust and particles from different parts of the Solar nebula.

The researchers explain that if Arrokoth had formed from chunks coming together from different parts of the nebula, it would have shown more evidence of impacts.

“There is no evidence,” the team wrote in their paper, “of heliocentric, high-speed collisional evolution, or any catastrophic impact during its lifetime. Instead, we conclude that its two lobes came together at low velocity, no more than a few metres per second and possibly much more slowly.”

This means contrary to the previous belief planet seeds were formed much gentler. The second paper by  John Spencer of the Southwest Research Institute, further supports this view by confirming that confirmed that  Arrokoth was smooth and only lightly cratered, unlike other objects in the Solar System. The lack of rings or satellites larger than 180 metres within a radius of 8,000 kilometres indicates it has remained peaceful for a long period of time. The study also estimates that the object’s surface is around 4 billion years old, nearly as old as the Solar System itself.

In the third paper, astronomer Will Grundy of Lowell Observatory and colleagues establishes that Arrokoth’s peculiar redness is caused by organic matter. Arrokoth is coated by “ultrared matter” which can also be found in other objects of the Kupier Belt. The team has found that Arrokoth  is uniformly cold and red, coated in methanol ice and complex organic molecules. Though New Horizons data was not enough to identify what the exact molecules are, the researchers are pretty sure that they are responsible for the red colour.

Grundy’s paper also agrees with the age of the rock and  supports the finding that Arrokoth was formed in a highly localised region. All these evidences now tilt the balance in the favour of  pebble accretion model of planetesimal formation. 

The new revelations makes a compelling case for sending more probes in future to study the objects in Kuiper belt.

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