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.
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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