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June 9, 1996 Press Contact: Steve Koppes
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Our solar system may be headed for an encounter with a dense cloud of interstellar matter

Our solar system may be headed for an encounter with a dense cloud of interstellar matter–gas and dust–that could have substantial implications for our solar system’s interplanetary environment, according to University of Chicago astrophysicist Priscilla Frisch. The good news is that it probably won’t happen for 50,000 years. Frisch presented the results of her research Monday, June 10, at the meeting of the American Astronomical Society in Madison, Wisc.

Frisch has been investigating the interstellar gas in the local neighborhood of our solar system, which is called the Local Interstellar Medium (LISM). This interstellar gas is within 100 light years of the Sun. The Sun has a trajectory through space, and for most of the last five million years, said Frisch, it has been moving through a region of space between the spiral arms of the Milky Way galaxy that is almost devoid of matter. Only recently, within the last few thousand years, she estimates, the Sun has been traveling through a relatively low-density interstellar cloud.

“This cloud, although low density on average, has a tremendous amount of structure to it,” Frisch said. “And it is not inconsistent with our data that the Sun may eventually encounter a portion of the cloud that is a million times denser than what we’re in now.”

Frisch believes the interstellar cloud through which we’re traveling is a relatively narrow band of dust and gas that lies in a superbubble shell expanding outward from an active star-formation region called the Scorpius-Centaurus Association. “When this superbubble expanded around these stars, it expanded much farther into the region of our galaxy between the spiral arms, where our sun lies, because the density is very low,” Frisch said. “It didn’t expand very far in the direction parallel to the spiral arms because it ran into very dense molecular clouds.”

The solar wind–the flux of charged particles streaming from the Sun’s corona–protects the Earth from direct interaction with the interstellar medium by enveloping the Earth and all the planets in the heliosphere, the region of influence of the solar wind. The heliosphere currently extends 100 times farther from the Sun than the distance between Earth and the Sun. “We think the heliosphere might have been much larger before we entered the interstellar cloud,” said Frisch, “but that’s something we can’t say for sure.”

But if the solar system encountered the much denser cloud, Frisch estimates that the heliosphere could be compressed to within one or two astronomical units of the Sun, not much greater than the Earth’s distance from the Sun. “There would be dramatic effects on the inner solar system,” said Frisch. “It would immediately change the whole interaction between the solar wind and the interstellar medium.” Researchers have predicted increases in the cosmic-ray flux, changes in the Earth’s magnetosphere, the chemistry of the atmosphere and perhaps even the terrestrial climate.

Frisch noted that astronomers searching for life on planets orbiting stars outside our solar system should consider the environments around those stars. “It doesn’t make sense to look for habitable planets unless you also look at the way these stars interact with their local environment,” said Frisch. “Ican’t imagine that a star passing in and out of dense interstellar cloud fragments–such as a star that’s traversing galactic spiral arms–would have a stable interplanetary environment. Without stability in the local stellar environment, I doubt there could be stable planetary climates hospitable to life.”

Frisch and her collaborator, Daniel Welty, both Senior Research Associates in Astronomy & Astrophysics at the University of Chicago, used the Coude-feed spectrometer on the 0.9-meter telescope at Kitt Peak National Observatory to observe interstellar ionized calcium (Ca+) in nearby stars in the upwind direction as we see it from the solar system. They hope to use the Hubble Space Telescope to answer some remaining questions that will confirm whether a dense cloud fragment does indeed lie in the upwind direction.

Frisch’s research is funded by NASA.
Last modified at 03:50 PM CST on Wednesday, June 14, 2000.

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