The dust bunnies that built our planet Lorin Swint Matthews
Consider the spot where you’re sitting.
Travel backwards in time
and it might’ve been submerged at
the bottom of a shallow sea,
buried under miles of rock,
or floating through a molten,
infernal landscape.
But go back far enough—
about 4.6 billion years,
and you’d be in the middle of an enormous
cloud of dust and gas
orbiting a newborn star.
This is the setting for some of the
biggest, smallest mysteries of physics:
the mysteries of cosmic dust bunnies.
Seemingly empty regions
of space between stars
actually contain clouds of gas and dust,
usually blown there by supernovas.
When a dense cloud reaches a certain
threshold called the Jeans mass,
it collapses in on itself.
The shrinking cloud rotates faster
and faster, and heats up,
eventually becoming hot enough to burn
hydrogen in its core.
At this point a star is born.
As fusion begins in the new star,
it sends out jets of gas that blow
off the top and bottom of the cloud,
leaving behind an orbiting ring of gas
and dust called a protoplanetary disk.
This is a surprisingly windy place;
eddies of gas carry particles apart,
and send them smashing into each other.
The dust consists of tiny metal fragments,
bits of rock, and, further out, ices.
We’ve observed thousands of these disks
in the sky,
at various stages of development
as dust clumps together
into larger and larger masses.
Dust grains 100 times smaller than the
width of a human hair stick to each other
through what’s called
the van der Waals force.
That’s where a cloud of electrons
shifts to one side of a molecule,
creating a negative charge on one end,
and a positive charge on the other.
Opposites attract, but van der Waals can
only hold tiny things together.
And there’s a problem: once dust
clusters grow to a certain size,
the windy atmosphere of a disk should
constantly break them up
as they crash into each other.
The question of how they continue to grow
is the first mystery of dust bunnies.
One theory looks to electrostatic charge
to answer this.
Energetic gamma rays, x-rays,
and UV photons
knock electrons off of gas
atoms within the disk,
creating positive ions
and negative electrons.
Electrons run into and stick to dust,
making it negatively charged.
Now, when the wind pushes
clusters together,
like repels like
and slows them down as they collide.
With gentle collisions
they won’t fragment,
but if the repulsion is too strong,
they’ll never grow.
One theory suggests that high energy
particles
can knock more electrons off of some
dust clumps,
leaving them positively charged.
Opposites again attract,
and clusters grow rapidly.
But before long we reach
another set of mysteries.
We know from evidence found in meteorites
that these fluffy dust bunnies
eventually get heated, melted
and then cooled into solid
pellets called chondrules.
And we have no idea how
or why that happens.
Furthermore, once those pellets do form,
how do they stick together?
The electrostatic forces from before
are too weak,
and small rocks can’t be held together
by gravity either.
Gravity increases proportionally to the
mass of the objects involved.
That’s why you could effortlessly escape
an asteroid the size of a small mountain
using just the force generated
by your legs.
So if not gravity, then what?
Perhaps it’s dust.
A fluffy dust rim collected around the
outside of the pellets
could act like Velcro.
There’s evidence for this in meteors,
where we find many chondrules surrounded
by a thin rim of very fine material–
possibly condensed dust.
Eventually the chondrule pellets get
cemented together inside larger rocks,
which at about 1 kilometer across
are finally large enough to hold
themselves together through gravity.
They continue to collide and grow
into larger and larger bodies,
including the planets we know today.
Ultimately, the seeds of
everything familiar–
the size of our planet, its position
within the solar system,
and its elemental composition–
were determined by an uncountably large
series of random collisions.
Change the dust cloud just a bit,
and perhaps the conditions wouldn’t
have been right
for the formation of life on our planet.