[The following article is edited from a longer text published on June 18, 1996 in the New York Times. CNI News notes that the discovery reported here might be mistaken by some as confirmation of the so-called "photon belt," although in our view that connection seems unwarranted. - ed]
MADISON, Wis. -- Over the last five million years or more, the Sun and its planets have had the good fortune to be gliding through one of the safer neighborhoods of the Milky Way galaxy. They have generally avoided the congestion and turbulence in evidence a few cosmic blocks away. But these halcyon days may be coming to an end.
With their new telescopes and observational techniques, astrophysicists are now able to map with revealing accuracy the solar system's immediate galactic environment. They are finding it virtually empty of interstellar gas and dust. Looking beyond, though, the scientists see a cloud or two, literally, looming on the horizon.
A University of Chicago astrophysicist says the solar system may be heading for a cloud of interstellar matter that is up to a million times more dense than its present surroundings, where matter is so diffuse that there is, on average, considerably less than an atom of hydrogen per cubic centimeter. The encounter with the denser cloud could come within 50,000 years.
Though no cause for immediate alarm, the predicted encounter could sharply alter Earth's atmosphere and climate. How, scientists are not sure, but they suspect it will not be for the better. With more interstellar particles penetrating the solar system's defenses, Earth would probably be subjected to an increase in cosmic radiation. This could reshape its magnetic fields, change the chemistry of its atmosphere and possibly bring on a much colder climate.
The new research into the Sun's local interstellar medium, as its immediate galactic environment is called, could also give scientists a better understanding of conditions that favored the proliferation of life on Earth. This could, in turn, set new limits on where life might also have arisen in other solar systems that are now being discovered.
In a report presented [in mid-June] at a meeting of the American Astronomical Society, Dr. Priscilla Frisch of the University of Chicago said observations showed that the Sun had already entered a relatively low-density interstellar cloud. This probably occurred a few thousand years ago, she estimated.
But this low-density cloud of gas and dust could be a foretaste or more ominous developments to come. The cloud, Dr. Frisch said, lies in a "superbubble shell," that is expanding outward from an active star-formation region called the Scorpius-Centaurus Association.
"When this superbubble expanded around these stars," Dr. Frisch explained, "it expanded much farther into the region of our galaxy between the spiral arms, where our Sun lies, because the density is very low."
Observations indicate that the Sun, traveling at a velocity of three light-years every 50,000 years, is heading for a much denser part of the cloud. The consequences could be dire.
As it is, the flux of charged particles streaming out from the Sun, known as the solar wind, protects Earth and the other planets from direct interaction with matter from the interstellar medium. The region thus protected by the solar wind is called the heliosphere and extends 100 times farther from the Sun than the distance between Earth and the Sun.
In fact, the heliosphere might have been more extensive before the Sun entered the low-density cloud it is now traveling through. An encounter with a much denser cloud, experts said, could compress the heliosphere to a distance not much greater than the Earth's distance from the Sun, exposing the planet to increased cosmic-ray bombardment.
"There would be dramatic effects in the inner solar system," Dr. Frisch said.
Dr. Jeffrey L. Linsky, an astrophysicist at the University of Colorado at Boulder, said: "There will be an encounter. We don't know exactly when or exactly how the Earth will be affected."
Dr. Frisch and her colleague, Dr. Daniel Welty, made their survey of the local interstellar medium with telescopes at the Kitt Peak National Observatory in Arizona. Other studies have used the Hubble Space Telescope and the Extreme Ultraviolet Explorer satellite. The nature and density of matter in surrounding space were determined by examining the effects the intervening atoms had on light from more distant stars.
A team of astronomers led by Linsky reported making ultraviolet telescopic readings of 18 stars in different directions to measure the dimensions of an egg-shaped interstellar cloud surrounding the solar system. They found that the Sun was only four light-years from the edge of the cloud, which is 60 light-years across at its largest axis. A light-year is the distance light travels in a year, about 6 trillion miles.
The Colorado astronomers observed that the interstellar cloud around the Sun was not uniform, but included "cloudlets" of different densities moving at different speeds and in slightly different directions. Another cloud may be as near as 20,000 years away from the Sun, in the direction of the star Alpha Centauri.
When the solar system moves into that cloud, Linsky said, the surrounding interstellar medium will become much denser and would presumably cause a significant compression of the heliosphere. "Who knows what will happen then," he said.
More of the previously unknown texture of the local interstellar medium has been revealed in research by astronomers at Northwestern University in Evanston, Ill. In observations last year with the Kitt Peak telescopes, Dr. John K. Watson and Dr. David M. Meyer discovered that space in the vicinity of the Sun and its planets contains a multitude of dense pockets of gas that are roughly comparable in size to the solar system.
Other astronomers said this was one of the most surprising and provocative discoveries reported at the meeting. They did not pretend to understand its possible implications.
As Meyer said, "The surprising thing to us is how common this structure appears to be in a diffuse medium that was supposedly quite smooth over small scales."
Astronomers so far can only guess at the origin and lifetime of these denser clumps of cold interstellar gas. If each one lasts only a few thousand years, as suspected, something must be creating new clumps as fast as the old ones dissipate. Perhaps they are produced when old stars inject matter into the interstellar medium by exploding or blowing off their outer envelopes of atmosphere. Or perhaps some mechanism, as yet unimagined, is holding the clumps of gas together for much longer lifetimes.
In any case, the Northwestern astronomers said, "It is clear that the pervasive small-scale structure in diffuse clouds is telling us something new about the physics of the interstellar medium."
Although biologists and geophysicists would normally find the interstellar medium of remote interest, the new findings may give them something to think about. To what extent did the Sun's interstellar environment influence the origin of life on Earth?
Astronomers searching for life on planets around other stars should take note, Dr. Frisch said.
"It doesn't make sense to look for habitable planets unless you also look at the way their stars interact with local environments," she said. "I can'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 a stable planetary climate hospitable to life."
Copyright 1996 The New York Times
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