The main asteroid belt of our solar system — with almost two million asteroids a kilometer in diameter orbiting in the region between Mars and Jupiter.  There are billions more that are smaller. New research has identified the “family” of a primordial asteroid or planetesimal, one of the oldest ever detected.

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.

Artist depiction of a dusty disc surrounding a red dwarf.artist rendering of a protoplanetary dust disk, from which asteroid, planetesimals and ultimately planets are formed. NASA/JPL-Caltech/T. Pyle (SSC)

“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.… Read more