Stars form
in cloudy nebulas and, shortly after genesis, consume most of the gas of their
birthplace and use the surrounding dust and leftover gas to form planets, according
to standard theory.
The gas and
dust collapse into a rotating "circumstellar"
disk and are drawn toward the star. Planets are thought to sometimes migrate
inward after birth, too. But scientists don't yet know what drives the inward
spiraling motion.
A new model
suggests magnetic instabilities in the disk cause gas to fall onto the star and
also helps drag young planets into their final orbits.
"Astronomers
observe gas crashing down upon the surfaces of young stars by virtue of the
ultraviolet radiation they emit, but a way to transport this gas from the disk
to the star has not been convincingly specified," said study team member Eugene
Chiang at the University of California, Berkeley.
The new
model, detailed online in the June 8 issue of the journal Nature Physics,
could also help explain why some planets outside our solar system orbit so
close to their parent stars.
The
magnetic instability arises from the fact that gas in the circumstellar disk
orbits at different speeds depending on its distance from the star. Radiating
throughout the disk like spokes on a bicycle wheel are magnetic field lines.
Chiang
likens the magnetic field lines to rubber bands binding the inner and outer gas
rings together. Because the inner ring rotates faster than the outer one, the
magnetic field "rubber bands" stretch in the direction of the rotation.
"What does
that do? It pulls back on the inner ring and speeds up the outer one," Chiang told
SPACE.com. This acts to slow down the inner ring, causing it to lose
momentum and spiral inward to crash onto the star.
Chiang and
coauthor Ruth Murray-Clay, also of UC Berkeley, say that recently observed
"transitional disks"—gaps in the circumstellar disk that are free of
dust—around young stars support their model.
The stellar
wind of young
stars blows dust out of the transitional disk regions, but has no effect on
gas. The magnetic instability the researchers are hypothesizing only works if
the spinning gas has sufficient electrical charge. Dust tends to absorb charges
and reduce electrical conductivity.
Because the
inflowing gas drags embedded objects, including young planets, along with it
toward the star, the new model also has implications for planet
formation. Hot
Jupiters are giant gas planets that orbit closer to their parent stars than
Mercury does to our sun and as a result have extremely high surface
temperatures.
The new
model suggests that planets riding the wave of inflowing gas toward the inner
region of their solar systems can be halted by magnetic instabilities in the
immediate vicinity of the star.
"Once
disrupted, disk gas can no longer drag the planets inward," Chiang said.
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