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Making Sense of Science

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Just like here on Earth, it appears that throughout the universe the value of real estate depends on location, location, location.

As this ScienCentral News video reports, we earthlings live in one of the universe’s best neighborhoods.

In the Zone

In the search for suitable dwelling places within our own solar system, requirement number one is liquid water. If the Earth were too close to the sun, its oceans would boil away. Too far, and they’d be frozen over. Earth happens to be situated in a ring-shaped zone that’s not too far in, not too far out. Astronomers call that ring the "Circumstellar Habitable Zone."

Now scientists say there’s a comparable zone in our entire galaxy, the Milky Way, and Earth also occupies a prime location within it. Astrobiologist Guillermo Gonzalez of Iowa State University and his colleagues at the University of Washington described the Galactic Habitable Zone (GHZ) in the October issue of Scientific American magazine.

The search for other habitable solar systems intensified in October 1995, when astronomers at the Geneva Observatory in Switzerland discovered the first known planet around another star. Since then, 73 more Jupiter-sized planets have been added to the ever-growing list of extrasolar planets. (Earth-sized planets are still too small for astronomers to detect).

As the data piled up, Gonzalez studied it, asking, What is different about stars with planets from stars without planets? "I found that stars with planets have a much higher concentration of heavy elements in their atmospheres compared to stars without planets, " he says. In fact, stars with planets have heavy metal concentrations similar to the Sun. Those stars, including the Sun, are found in a ring in the Milky Way’s disk a constant distance from the center.

Gonzalez says that’s because "you need a certain minimum concentration of heavy elements in a forming stellar system in order for it to be accompanied by giant planets." Stars close to the center of the Milky Way have a higher concentration of heavy elements, while stars further away have a lower concentration of heavy elements. So, in the galactic real-estate market, if you’re too far out you won’t even find a lot to build on.

But that doesn’t mean closer is better. While stars closer to the center of the Milky Way might have planets, their distance from a black hole defines whether they are safe. "That’s where you find high energy radiation that’s lethal to most forms of life," says Gonzalez.

And even within the boundaries of the GHZ there are neighborhoods to be avoided. "The habitable zone does not include the spiral arms in the galaxy because they’re very dangerous places for life," says Gonzalez. Hazards in the spiral arms include supernovae explosions which also produce deadly radiation. And star-forming regions may be the source of mysterious gamma ray bursts, the most powerful energy sources in the universe.

Gonzalez says these recent discoveries could make science fiction a little less fun now that plausible settings for advanced civilizations are known to be "somewhat uninteresting looking places—far from spiral arms, far from the galactic center, and far from star-forming regions," he quips. "Science fiction writers are going to have to be content with putting their civilizations in places that look very much like the Earth."

by Joyce Gramza

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