Asteroid, we’ve long been told, started tiny in our protoplanetary disk and only very gradually became more massive through a process of accretion. They collected dust from the gas cloud that surrounded our new star, and then grew larger through collisions with other growing asteroids.
But in recent years, a new school of thought has proposed a different scenario: that large clumps of dust and pebbles in the disk could experience gravitational collapse, a binding together of concentrated disk material.
This process would produce a large asteroid (which is sometimes called a planetesimal) relatively quickly, without that long process of accretion. This theory would solve some of the known problems with the gradual accretion method, though it brings some problems of its own.
Now research just published in the journal Science offers some potentially important support to the gravitational collapse model, while also describing the computational detection of a primordial family of asteroids some 4 billion years old.
Led by Marco Delbo’, an astrophysicist at the University of the Côte d’Azur in Nice, France, the scientists have identified a previously unknown family of darkly colored asteroids that is “the oldest known family in the main belt,” their study concluded.
The family was identified and grouped together by the unusual darkness (low albedo) of its asteroids’ reflective powers, a signature that the object has a high concentrations of carbon-based organic compounds. This family of asteroids was also less extensively heated — having formed when the sun radiated less energy — and contains more water, making them potential goldmines for understanding the makeup and processes of the early solar system.
“They are from an original planetesimal and the location of these fragments tell us they are very, very old,” Delbo’ told me. “So old that the original object is older than the epoch when our giant planets moved to their current locations.” That would make this ancient asteroid family more than 4 billion years old, formed when the solar system was but 600 million years from inception.
By adding up the masses of the members of the asteroid family, the researchers could also come up with a size for the original planetesimal that gave birth to the asteroid family — at least 35 kilometers wide at its inception.
What is termed our “solar nebula” is thought to have been a disk-shaped cloud of gas and dust that remained after the formation of the sun. Just like a dancer that spins faster as she pulls in her arms, the cloud began to spin as it collapsed. Eventually, the cloud grew hotter and more dense in the center, with a disk of gas and dust surrounding it that was hot near the center but cool at the edges.
Since these earliest days of the solar system, a vast collection of dust and later rocks of all shapes and sizes has been circling the sun, especially in the broad expanse of space between Mars and Jupiter. This is both the material from which planets were formed, and also leftover material from the formation of the solar system.
There are many of these asteroids, or planetesimals, but they don’t carry much mass — all of them together roughly equaling that of our moon.
There are some 130 known “families” of asteroids. The effort to understand the processes that created the asteroids has been enormously difficult because they have been broken and then broken again and again as they crash into each other.
But that is changing thanks to this discovery of the new family of “dark” asteroids. Unlike the brighter, highly reflective asteroid families nearby, the population of dark asteroids’ orbits are more spread out, interpreted to mean that more time has passed since the asteroids formed.
Most asteroid families are thought to have formed about 1 billion years ago. By aggregating the sizes of the modern dark asteroids, researchers suggest their original planetesimals formed about 4 billion years ago, making this one of the oldest asteroid families in the main asteroid belt.
The scientists also determined that the dark family’s original planetesimals were no smaller than about 25 miles across.
This provides support for the gravitational collapse hypothesis, originated at Germany’s Max Planck Institute, by suggesting the oldest asteroids started out large, and then became smaller through collisions and other destructive forces happening in the ancient solar system.
The earlier and more conventional theory had the asteroids starting small and getting gradually bigger. This difference in hypotheses has been a hot topic among planetary scientists for nearly a decade.
These findings are not based on telescope viewing and measuring; that was all done by NASA’s
Delbo’ and his team used computer models to search for groups of related asteroids spread within a V-shaped region. This V pattern is what one would expect from a single object that fragmented into pieces, and the wider the V-shape the older the objects.
Their asteroid family features rocks averaging 7.15 miles in diameter, and are found across the entire inner part of the main asteroid belt. The family has 108 members and counting, with the largest of which the largest being asteroid 282 Clorinde, which is about 26 wide.
“Each family member drifts away from the center of the family in a way that depends on its size, with small guys drifting faster and further than the larger guys,” Delbo’ said. “If you look for correlations of size and distance, you can see the shapes of old families.”
But that wasn’t all.
“By identifying all the families in the main belt, we can figure out which asteroids have been formed by collisions and which might be some of the original members of the asteroid belt,” said Southwest Research Institute astronomer Kevin Walsh, a coauthor of the Science article.
“We identified all known families and their members and discovered a gigantic void in the main belt, populated by only a handful of asteroids. These relics must be part of the original asteroid belt. That is the real prize, to know what the main belt looked like just after it formed.”
These primordial objects had to have formed differently from those belonging to the newer families. They were the original inhabitants and were present in the inner asteroid belt before anything else. Ranging in wize from 21 to around 93 miles across, their size matches up with predictions from theoretical models of how large original asteroids might have been 4 billion years ago, when they initially formed.
In other words, their age and size supports the gravitational collapse theory of asteroid formation.
To put these findings into a larger context, I asked Elizabeth Tasker, astrophyscist at the Japan Space Agency and the Earth-Life Science Institute in Tokyo, to explain further. She is the author of the soon-to-be released book, “The Planet Factory,” which deals extensively with these issues. First is her take on the logic of gravitational collapse:
“In the gravitational collapse model, the pebbles and small boulders around 1m-ish in size concentrate in one region of the protoplanetary disk. This concentration initially happens because nothing is ever perfectly homogeneous, but it grows because having a group of rocks together helps mitigate the gas drag.
This grows until eventually its combined mass is enough that their total gravity finally becomes a big enough force to bind them together into a planetesimal. This doesn’t happen until you have a serious chunk of mass, so the result is always a big planetesimal tens to hundred of kilometers across (about the size of Ceres). A smaller group of rocks wouldn’t have enough total mass to produce the gravitational force needed to collapse.”
And now why the Delbo’ paper is important:
“The formation of our own solar system is the key to understanding the properties of exoplanets around other stars. For example, if we truly want to find another habitable world, we need to understand how the Earth acquired and kept its oceans, developed a protective magnetic field and a sizeable moon, while Venus and Mars did not.
“A problem we face is that the early planet-forming action happened 4.6 billion years ago. We can build models, but how do we tell which one is correct when this all happened so long ago?
“Marco Delbo’ and his team have identified a holy grail; an observational signature that can be used to constrain the myriad of formation ideas we are imaginative enough to create.”