How planetary building blocks were constructed

Scientists mulling over data returned by NASA’s New Horizons spacecraft from a distant worldlet known as Arrokoth have resolved one of the biggest questions about how the building blocks of planets initially formed. 

Arrokoth (formerly called Ultima Thule) is a 36-kilometre-long remnant of the early Solar System, more than six billion kilometres away. New Horizons flew by it on New Year’s Day 2019 and has been returning information about it ever since. 

Historically, there have been two theories about how such building blocks, known as planetesimals, formed from the disc of gas and dust that once circled the infant sun.

The more traditional of these theories  is known as “hierarchical accretion”, William McKinnon, a planetary scientist at Washington University in Missouri, told last week’s meeting of the American Association for the Advancement of Science.

This model states that cascading, ever-larger particles collide with each other and stick together in a stepwise process that eventually produces objects large enough to be planetesimals. As these particles get bigger and bigger, the collisions become more violent, reaching speeds of 100 metres per second or more.

The alternative theory is that planetesimals grow more directly from myriads of pebble-sized particles. 

Drag from their interactions with gas in the protoplanetary disk causes them to collect into streams that eventually break into smaller clouds. 

Pebbles in each cloud then fall to the centre and “Whoop! Almost in an astronomically instantaneous time they make a big planetesimal, all at once – 10, 20, maybe 100 kilometres across”, McKinnon says.

That theory, he adds, is well-supported by analytical models – “but it would be great to see a planetesimal and test out these results”.

And that is where Arrokoth comes in because it has been basically unperturbed since the origin of the Solar System.

One of its most striking features, McKinnon says, is that it is composed of two lobes that appear to have originated as two separate objects that are now so gently merged that they appear to be “kissing”. 

“If they were spacecraft, they would be docking. There is no indication that the merger was violent or catastrophic.”

Prior reports had described this last year, but they were preliminary, and based on less than one-tenth of the data we have today.  

Now, McKinnon says, simulations published by his team in the journal Science reveal that the merger between the two lobes couldn’t have happened at the type of velocities expected in the hierarchical accretion model without the lobes being smashed to fragments or “mushed” together in a manner unlike their actual appearance.

In fact, he says, the speed of the collision was certainly no more than a few metres per second, and probably less.

In other words, Arrokoth’s gently merged shape is “absolutely inconsistent” with anything that could occur from higher-velocity, hierarchical accretion, “but it does match what we expect to see in the low-velocity merger or assembly of bodies in a cloud of collapsing particles”.

Will Grundy, of Lowell Observatory in Arizona, and first author of a separate paper in Science, adds that another line of evidence comes from Arrokoth’s colour. 

If it had formed by hierarchical accretion, he says, bits and pieces of it would have come from all over the protoplanetary disc, and you’d expect it to look heterogeneous. 

“But the two lobes are essentially the same colour,” he says – exactly what would be expected if it’s only drawing in material from a localised cloud of collapsing pebbles.

A third line of evidence comes from the shape of the two lobes – flattened rather than spherical. 

John Spencer, a planetary scientist from the Southwest Research Institute in Colorado and first author of third paper in Science, compares them to a pair of M&M candies, touching edge-to-edge.

That alignment, he says, is important because it means that the two objects that eventually merged to form Arrokoth spent enough time in orbit around each other that their mutual gravity was able to align them in that manner. 

“This is telling us that these really weren’t just two things randomly blundering into each other, but yes, they formed in the same cloud, in mutual orbit, and then came together in this very gentle way,” he says.

All told, says Alan Stern, the New Horizons mission’s principal investigator, these three lines of evidence are a “decisive” demonstration that the earliest stages of planet formation occurred by cloud collapse, rather than by hierarchical accretion. 

“We as a team cannot imagine how hierarchical accretion could have created the Arrokoth that we see,” he says.

Meanwhile, astronomers will soon be looking for a new target for New Horizons to visit. “We have fuel left in the tank, the spacecraft is healthy, and we have power to run 15 years,” Stern says. “Our greatest ambition is to find another object that we can fly by.”

Scientists used all available New Horizons images of Arrokoth to determine its 3D shape, as shown in this animation. The flattened shapes of its lobes, as well as the remarkably close alignment of their poles and equators, point to an orderly, gentle merger of two objects formed from the same collapsing cloud of particles. Credit: NASA/ Johns Hopkins University/SWRI/James Tuttle Keane