New Scientist
UK Contact:
Claire Bowles, claire.bowles@rbi.co.uk, 44-171-331-2751
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EMBARGOED FOR RELEASE: September 8, 1999, 2 p.m. EDT
A violent blast of radiation spawned the planets
The formation of the Solar System was hurried along by a nearby gamma-
ray burst, two astrophysicists in Ireland suspect. Rather than aborting
the birth of planets, the flood of energy may have melted primordial
dust grains, seeded the formation of meteorites and helped form the
rocky planets, including Earth.
For over a century, astronomers have tried to understand what made
clumps of dust circling the young Sun melt into chondrules-rocky
beads rich in iron and silicon minerals that make up the bulk of stony
meteorites. Suggestions included shock waves and gigantic flashes of
lightning.
Now Brian McBreen and Lorraine Hanlon of University College Dublin
suggest that all the chondrules in the Solar System formed in a matter
of minutes 4.5 billion years ago when a gamma-ray burst-one of the
most powerful explosions in the Universe-seared the dust and gas
circling the Sun with intense X-rays and gamma rays. Astronomers
aren't sure what causes gamma-ray bursts, but they may occur when
supermassive stars explode at the end of their lives (New Scientist,
3 April, p 5).
In a paper that will appear in a future issue of Astronomy and
Astrophysics, McBreen and Hanlon calculate that a gamma-ray burst
within 300 light years would have flooded the dusty disc circling the
young Sun with enough energy to fuse up to 100 Earth masses of material
into droplets that cooled into chondrules. These, and the dust from which
they formed, are rich in iron, which would have soaked up X-rays and
gamma rays very efficiently. "It explains the key role played by iron,
which dominates the X-ray and gamma-ray absorption," says McBreen.
If the theory is right, it makes the Solar System more unique than many
scientists would like. McBreen and Hanlon believe that only one Sun-like
star in a thousand would have been close enough to a gamma-ray burst
to form chondrules. Because they also think that the dense chondrules
settle quickly into the plane of a protoplanetary disc and speed the
formation of planets, their theory implies that solar systems such as
ours are rare.
"Forming chondrules really is a long-standing problem, so if this
mechanism accounts for them, that would be pretty fantastic," says
Alan Boss, an astrophysicist at the Carnegie Institution in Washington
DC. Still, he is reluctant to rely on an unlikely event as a crucial factor
in the formation of the Solar System, and wonders whether the idea can
explain other features of chondrules, such as their size and abundance.
"I don't think you'd want to invoke it unless it takes care of everything,"
he says.
Specialists in meteorites are intrigued by McBreen's idea. "Chondrule
formation remains a thorny subject, so it's good to see a new idea in
the area," says Ian Wright, a meteoriticist at the Open University in
London. He notes that most of the researchers studying meteorites
believe chondrules did not form all at once, although the case is not
closed. "It will certainly cause debate, and it's an interesting idea
that can be tested in our labs."
