The most distant world we’ve ever explored just shed light on how planets are born


In the far reaches of the Solar System, a small rock is showing us how giant planets get their start. Arrokoth – the most distant and most primordial world ever visited by a human spacecraft – now spills its secrets in three new papers.

Those findings could resolve some debate about how planetesimals – the small rocky ‘seeds’ that grow into planets – are formed. And the process appears to be a lot more gentle than previously thought.

“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,” said Alan Stern from the Southwest Research Institute in Colorado and New Horizons Principal Investigator.

“It’s teaching us how planetesimals formed, and we believe the result marks a significant advance in understanding overall planetesimal and planet formation.”

Arrokoth, formerly known as (486958) 2014 MU69 or Ultima Thule, was visited by the New Horizons probe on New Year’s Day last year, way out in the Kuiper Belt.

At a staggering average distance from the Sun of 6.7 billion kilometres (4.1 billion miles), and an orbital period of 293 years, Arrokoth is the most distant single object in the Solar System we’ve identified.

But sending New Horizons to check it out after the historic Pluto flyby in July 2015 wasn’t done just because we could. That far from the Sun, out of reach of harsh solar radiation, and with a stable orbit, Arrokoth is pretty much a time capsule from the time when the Solar System formed, 4.6 billion years ago.

In May of last year, the first spate of papers came in, detailing initial findings from that flyby, based on just 10 percent of the data New Horizons was still beaming home to Earth.

Different teams of scientists found that Arrokoth was once a binary object whose two halves had gently come together, although the processes that led to this were unclear; that its surface was lightly cratered; and that its surface was predominantly red, although what created that colour was unknown.

Now, after analysing 10 times the amount of data as used in those initial papers, some of those questions have been answered.

The first of the three papers, by New Horizons investigator William McKinnon of Washington University in St. Louis and colleagues, found that the two objects in the Arrokoth binary actually formed close together.

There are generally two competing theories about how planets are born. 

According to the longstanding hierarchical accretion model of planetesimal formation, the building blocks of planets are formed when different parts of the solar nebula – the cloud of gas and dust that formed the Sun and planets – violently crash together.

On the other hand, the pebble accretion model suggests that elements from the same area gradually and gently come together to form binary objects.

The latest data from Arrokoth lends weight to the latter.

If Arrokoth had formed from chunks coming together from different parts of the nebula, it would have shown more evidence of impacts, the researchers said.

“There is no evidence,” the team wrote in their paper, “of heliocentric, high-speed collisional evolution, or any catastrophic (or sub-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.”

That suggests that the two lobes formed in the same part of the solar nebula – the cloud of gas and dust that formed the Sun and planets.

“Arrokoth looks the way it does not because it formed through violent collisions, but in more of an intricate dance, in which its component objects slowly orbited each other before coming together,” said McKinnon

The second paper helped back this up, with astronomer John Spencer of the Southwest Research Institute and colleagues studying the surface of Arrokoth. They confirmed that it was smooth and only lightly cratered, a stark difference from other objects in the Solar System.

They also confirmed that Arrokoth has no rings or satellites larger than 180 metres (590 feet) within a radius of 8,000 kilometres, and no atmosphere, or gas or dust emission, the presence of which would indicate relatively recent perturbation. That indicates that Arrokoth has been pretty peaceful for a very long time indeed.

But they also studied Arrokoth’s craters more closely, and found that the object’s surface is likely around 4 billion years old – nearly as old as the Solar System itself.

“Arrokoth’s spin state is likely to have evolved only very slowly, there do not appear to be sufficient impacts to act as effective seismic sources, and Arrokoth’s likely high porosity would make seismic energy propagation highly inefficient,” they wrote in their paper.

“Overall, despite the paucity of craters on its surface, the observed crater density is consistent with a crater retention age of greater than ~4 billion years. The visible surface at the scale of the LORRI image resolution thus plausibly dates from the end of Solar System accretion.”

Finally, in the third paper, astronomer Will Grundy of Lowell Observatory and colleagues studied Arrokoth’s peculiar redness. The reddest naturally occurring material – called “ultrared matter” – in the Solar System can be found in the Kuiper Belt, and Arrokoth is coated in it, but the material’s exact nature was unclear.

The team found that the object is uniformly cold and red, coated in methanol ice and complex organic molecules they couldn’t precisely identify based on the limited spectral data New Horizons was able to gather. These molecules are likely what creates the red colour.

This not only seems to confirm organic molecules as the source of ultrared matter; the uniformity of the colour – as well as the age of the surface as found by Spencer’s team – also support the finding that Arrokoth was formed in a highly localised region.

“Arrokoth has the physical features of a body that came together slowly, with ‘local’ materials in the solar nebula,” said Grundy. “An object like Arrokoth wouldn’t have formed, or look the way it does, in a more chaotic accretion environment.” 

Stern added: “All of the evidence we’ve found points to particle-cloud collapse models, and all but rule out hierarchical accretion for the formation mode of Arrokoth, and by inference, other planetesimals.”

There’s probably not much data left on Arrokoth for New Horizons to send home, so any future analyses will have to be based on what we already have. But it seems like these distant Kuiper Belt objects could have so much more to teach us about the birth of our Solar System.

It’s a compelling case for an orbiter to go spend a bunch of time out there…

“Arrokoth has turned out to be astonishing in terms of what we’ve learned from it,” McKinnon said at the annual meeting of the American Association for the Advancement of Science this week, as The Guardian report.

“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 papers have been published in Science, and can be found here, here and here.

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