Exploding star missing from formation of solar system
Scientists in the University of Chicago’s Origins Laboratory have published the latest in a series of papers
about the origin of the solar system. Infant stars glow reddish-pink in this infrared image of the Serpens star-forming region,
captured by NASA’s Spitzer Space Telescope. Four-and-a half billion years ago, the sun may have looked much like one of the baby
stars deeply embedded in the cosmic cloud of gas and dust that collapsed to create it.
Courtesy of NASA/JPL-Caltech/L. Cieza (University of Texas at Austin)
December 14, 2012
By Chelsea Leu
A new study published by University of Chicago researchers challenges the notion that the force of an exploding star prompted
the formation of the solar system.
In this study, published online last month in Earth and Planetary Science Letters, authors Haolan Tang and Nicolas Dauphas
found the radioactive isotope iron 60 — the telltale sign of an exploding star—low in abundance and well mixed in solar system
material. As cosmochemists, they look for remnants of stellar explosions in meteorites to help determine the conditions under
which the solar system formed.
Some remnants are radioactive isotopes: unstable, energetic atoms that decay over time. Scientists in the past decade have
found high amounts of the radioactive isotope iron 60 in early solar system materials. “If you have iron 60 in high abundance
in the solar system, that’s a ‘smoking gun’—evidence for the presence of a supernova,” said Dauphas, professor in geophysical
Iron 60 can only originate from a supernova, so scientists have tried to explain this apparent abundance by suggesting that a
supernova occurred nearby, spreading the isotope through the explosion.
But Tang and Dauphas’ results were different from previous work: They discovered that levels of iron 60 were uniform and low in
early solar system material. They arrived at these conclusions by testing meteorite samples. To measure iron 60’s abundance, they
looked at the same materials that previous researchers had worked on, but used a different, more precise approach that yielded
evidence of very low iron 60.
Previous methods kept the meteorite samples intact and did not remove impurities completely, which may have led to greater errors
in measurement. Tang and Dauphas’ approach, however, required that they “digest” their meteorite samples into solution before
measurement, which allowed them to thoroughly remove the impurities.
This process ultimately produced results with much smaller errors. “Haolan has dedicated five years of very hard work to reach
these conclusions, so we did not make those claims lightly. We’ve been extremely careful to reach a point where we’re ready to
go public on those measurements,” Dauphas said.
To address whether iron 60 was widely distributed, Tang and Dauphas looked at another isotope of iron, iron 58.
Supernovae produce both isotopes by the same processes, so they were able to trace the distribution of iron 60 by measuring the
distribution of iron 58.
“The two isotopes act like inseparable twins: Once we knew where iron 58 was, we knew iron 60 couldn’t be very far away,”
They found little variation of iron 58 in their measurements of various meteorite samples, which confirmed their conclusion that
iron 60 was uniformly distributed. To account for their unprecedented findings, Tang and Dauphas suggest that the low levels of
iron 60 probably came from the long-term accumulation of iron 60 in the interstellar medium from the ashes of countless stars past,
instead of a nearby cataclysmic event like a supernova.
If this is true, Dauphas said, there is then “no need to invoke any nearby star to make iron 60.” However, it is more difficult
to account for the high abundance of aluminum 26, which implies the presence of a nearby star.
Instead of explaining this abundance by supernova, Tang and Dauphas propose that a massive star (perhaps more than 20 times the
mass of the sun) sheds its gaseous outer layers through winds, spreading aluminum 26 and contaminating the material that would
eventually form the solar system, while iron 60 remained locked inside the massive star’s interior. If the solar system formed
from this material, this alternate scenario would account for the abundances of both isotopes.
“In the future, this study must be considered when people build their story about solar system origin and formation,” Tang said.