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How Dust Clumps Together in Space

One of the essential ingredients of planet-building is the clumping of dust in space.  Planets can build up through the gravitational attraction of objects in space which are already about 1000km across.  The problem is how do these proto-planetessimals get built?  The mechanism for how dust clumps together has not been well understood.  After all, when materials moving at speed through space collide, they may break apart in the force of the impact, showering down collisional cascades of ever small materials – the exact opposite of planetessimal-building.  Somehow, dust must clump together into grains, which then join forces to create space pebbles, then boulders, then mountains, etc.

For these materials to adhere together, an inherent stickiness may be needed, aided by the presence of greasy organic compounds (in the form of aliphatic carbon).  While it is recognised that this greasy component is more readily available in interstellar space than previously suspected (1), does that adhesive property extend down to space dust?  If not, what mechanism could be bringing together ever larger clumps of plain old granular dust in space?

New research work suggests that dust and gas are not happy bedfellows within a magnetic field.  So, rather like oil in water, dust particles seem to come together within gas as the mixture traverses the galactic tides.  Indeed, any force brought to bear on dust moving through gas seems to create this clumping effect:

“… it was previously assumed that dust was stable in gas, meaning the dust grains would ride along with gas without much happening, or they would settle out of the gas if the particles were big enough, as is the case with soot from a fire. “…dust and gas trying to move with one another is unstable and causes dust grains to come together,” says [Phil] Hopkins [Professor of theoretical astrophysics at Caltech]...These gas-dust instabilities are at play anywhere in the universe that a force pushes dust through gas, whether the forces are stellar winds, gravity, magnetism, or an electrical field.” The team’s simulations show material swirling together, with clumps of dust growing bigger and bigger.” (2)

Computer simulations looking at how dust moves through magnetized gas seems to show this clumping effect as a general mechanism.  The dust grains are like boulders in a fast moving and turbulent river (the gas within a moving stream of magnetized material).  As the flows wrap around these grains and pull them back and forth, the grains have a tendency to coalesce, forming ever larger clumps.  This is not just applicable to planet formation in proto-planetary disks, but may also extend to interstellar space:

“As examples, we introduce several new instabilities, which could see application across a variety of physical systems from atmospheres to protoplanetary disks, the interstellar medium, and galactic outflows.” (3)  Read More…

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