Monthly Archives: October 2018

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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The Goblin Points to Presence of Planet X

The announcement of the discovery a new object in the outer solar system may bring us a step closer to the elusive Planet X (more recently dubbed Planet Nine).  This new dwarf object, known as 2015 TG387, is a distant member of the mysterious scattered disk of objects beyond the Kuiper Belt.  This particular object can travel so far away from the Sun during its orbit that it moves through the inner Oort cloud of comets, beyond 2000AU:

The newly discovered object is called 2015 TG387, is probably a small dwarf planet at just 300km across, and is incredibly far away. It is currently lying about two and a half times further away from the Sun than Pluto is.  It often reaches much further away. Its orbit takes it to about 2,300 AU — that is 2,300 times as far away from the sun as we are, and vastly more than the already huge 34 AU that the distant Pluto sits at.(1)

The object’s vast orbit is so vast that it takes about 40,000 years to do one circuit around the Sun.  Yet, its orbit is highly eccentric.  It distance from the Sun varies from 64AU at perihelion to 2037AU at aphelion.  Incredibly, then, it skirts both the Kuiper Belt and the inner Oort cloud, transiting between these quite distinct belts of objects.

As more objects are discovered between the Kuiper Belt and the inner Oort cloud (a torus-shaped disk of comets), the classifications of these objects are becoming more complex.  A significant factor is whether these objects have perihelia within 40AU, which might briefly bring them within the influential scope of the planet Neptune.  Extreme scattered disk objects fall into this category.  Significantly, 2015 TG387 is fully detached from this influence at perihelion, and may be considered to be an inner Oort cloud object.  Read More…

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