Planet
formation is a story with a well-known beginning and end, but how its middle
plays out has been an enigma to scientists—until now.
A new
computer-modeled theory shows how rocky boulders around infant stars team up to
form planets without falling into stars.
"This
has been a stumbling block for 30 years," said Mordecai-Marc Mac Low, an
astrophysicist at the American Museum of Natural History in New York City, of
planet formation theories. "The reason is that boulders tend to fall into
the star in a celestial blink of an eye. Some mechanism had to be found to
prevent them from being dragged into a star."
The
solution: Together, many boulders can join to fight a cosmic headwind that
otherwise would doom them.
Truckin'
boulders
The stuff
of rocky planets originates in an accretion disk, or collection of gas and dust
that circles around a newborn star. Over time the dust particles
bunch together and form large boulders, but eventually they meet
"wind" resistance from the disk's mist of gas.
"They
see a headwind. It's deadly and drags them into the star," Mac Low told SPACE.com.
Modeling
the turbulence within the gas, however, showed that boulders can team up and
form planets.
"Turbulence
in the disk concentrates boulders in regions of higher pressure," Mac Low
said, noting that such a disturbance is enough to enable the boulders to fight
the dooming headwind. "If the gas is sped up, the boulders don't see a
headwind. By getting the gas going with them they conserve energy and stay in
orbit."
Mac Low
compared the effect to a chain of semi-trucks driving down a highway. Each
boulder is like a semi-truck "pushing" the gas in front of it,
creating a friendly pocket of air behind it that other semis can travel in
without using up as much fuel. "The end of the story is that enough
boulders gather together, gravity takes over and they collapse into planet-like
bodies," Mac Low said.
Mac Low and
his colleagues' findings will be detailed in an upcoming issue of the journal Nature.
Pulverizing
problem
Although
Mac Low and his colleagues kept planet-forming boulders safe from the
gravitational clutches of stars in their simulation, he noted that many
questions remain.
"There
are enough uncertainties that [planet formation] is not going to be an open
and shut case any time soon," he said. "We don't know how that collapse
into a planet actually occurs. You've got thousands, millions of boulders
swarming together like a bees. In my nightmares I imagine that they grind each
other down to dust and it all goes away."
Despite the
problem, Mac Low is confident the theory will hold up to future scrutiny.
"All
that material is gravitationally bound together, so we think it's likely that
it will form large objects," he said. Running the computer simulation, in
fact, formed tight boulder clusters as large as the dwarf planet
Ceres (formerly known as the asteroid Ceres).
Alan Boss,
an astrophysicist with the Carnegie Institution in Washington, D.C., said that the theory is attractive despite the caveat.
"Overall,
the calculations present an encouraging approach to understanding how something
happened that we know must have happened, at least for the terrestrial planets,"
Boss said in an e-mail. How giant planets form yet another question. One idea
is that gas coalesces around a rocky, or terrestrial planet. Boss, however,
thinks the gas giants collapse
from a knot, much in the manner of star formation.
Mac Low and
his team plan to address the mystery of how boulders collapse into
planetesimals, or protoplanetary chunks of rock, in the future.
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