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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."

Planets in regions that are too rich in metals, meanwhile, are vulnerable to so-called Earth-killers, preferentially found in such regions. If a gas-filled, Jupiter-like planet migrates through an area that is too rich in metals, it will kill any "Earths" that are there.

Exloding Supernovae

But the right level of metallicity isn't the only prerequisite for life. The astronomers also had to look for regions that have experienced relative calm for at least four billion years, the time it took for life to evolve on Earth.

"In a high-density stellar environment, you will not have four billion years of clement, let's-sit-back-and-evolve life," said Lineweaver. "For complex life to evolve, you may need about four billion years without too many supernovae going off."

The computer model used for the study is based on local observations. Scientists can look at stars all over the galaxy and determine the metallicity at the time they were formed.

The region the researchers have concluded is the most favorable to host life is a ring that measures 21,000 to 27,000 light-years from the center of the galaxy. Created between four and eight billion years ago, this region is where 10 percent of the stars of the Milky Way were formed.

"We're not saying anything about the probability of extraterrestrial or complex life," said Lineweaver. "But if it does exist, and we're right about our prerequisites for it, this is the distribution it will have. This is where life will be."

Humans vs. Aliens

The astronomers were also able for the first time to determine the age of the stars in the habitable zone. They found that 75 percent of them are older than our sun.

"If people think that intelligent life will happen on these planets, then 75 percent of this intelligent life will have had a longer time to evolve than people or the entities that are circulating around our sun," said Lineweaver.

"We're all trying to figure out how we compare to any life-forms that may exist in the universe," he added. "This study is the closest thing I can think of to answer that question. If there are aliens, 75 percent of them will have had longer time to evolve than we have. That may be the most fundamental take-home message of this study."

The research is published in the January 2 issue of the journal Science.
 

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