Aliens in Our Galaxy? Experts Map Possible Hotbeds

Stefan Lovgren
for National Geographic News
January 6, 2004

Scientists say a ring-shaped region in the disc of the Milky Way shows the highest potential for life in our galaxy.

But don't expect them to find extraterrestrial life anytime soon: In this region, there are some 20 billion star systems that offer the prerequisites of life.

The team of astronomers has identified stars that contain enough heavy elements to form terrestrial planets; are sufficiently distant from disastrous supernova explosions; and have existed for at least four billion years—the time it took for complex life to evolve on Earth.

Using a sophisticated computer evolution model, they found that ten percent of the stars in our galaxy, located in a ring around the center of the Milky Way, meet those criteria.

"This is a crude first map of where life could be in our galaxy," said Charles Lineweaver, an astrophysicist at the University of New South Wales in Sydney, Australia, who led the study.

The research also shows that 75 percent of the stars in this habitable zone are older than our sun; so if there is actually life there, it's probably more evolved than life on Earth.

"Metallicity"

Our galaxy, the Milky Way, contains more than 200 billion stars and countless other objects. It is one of billions of other galaxies in the universe.

The Milky Way was formed some 12 billion years ago out of a large cloud of gas, mainly hydrogen and helium, that collapsed on itself and began forming stars. Since the cloud was rotating, its spherical shape flattened into a disc.

As the formation of the new stars continued, the most massive stars exploded and enriched the gas in the cloud with new, heavier elements. The new stars that were created in these disc regions contain the heavier elements required to form terrestrial planets, which may be spinning around them.

In their search for possible life, scientists look for favorable zones of "metallicity." A star with no heavy metals can't form terrestrial planets or life.

"Anytime you can identify regions that have higher concentrations of metallicity, that's where you may find life," said Lineweaver. "In our model of the galaxy, we keep track of where and when these metals were produced. We can do that because we know how many massive stars went 'boom' as a function of time and place."

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