Life may have expanded across our galaxy like a virus spreading an epidemic, say researchers looking for proof of a theory dubbed panspermia.

Supporters of the panspermia theory — and it's a controversial one — suggest the seeds of life, or at least the building blocks, somehow made it to Earth from some distant alien world.

Scientists at Harvard University suggest that if the search for alien life begins to find inhabited planets, the distribution in the galaxy of those planets could be "smoking gun" evidence of panspermia.

Not that there's any reason to expect that life would resemble anything we see on Earth.

Still, Harvard researchers Henry Lin and Abraham Loeb say they've created a model of how life might spread from planet to planet.

If life begins on a few planets and then spreads through space to other worlds, then planets with life on them should be found in a clumpy pattern around the galaxy, in roughly spherical regions separated by voids.

Such a pattern would arise from any mechanism for life being transferred between planets, be it comets carrying organic compounds or advanced civilizations engaging in space travel, they say.

In Lin and Loeb's model, life on one planet would spread outward in all directions, taking root if it encountered a habitable planet orbiting a neighboring star. The result, over time, would be a series of life-bearing oases spread throughout the galaxy, they say.

"In our theory, clusters of life form, grow, and overlap like bubbles in a pot of boiling water," says lead study author Lin of the Harvard-Smithsonian Center for Astrophysics.

If the Earth happens to be on the outer edge of one of those expanding bubbles, Lin and Loeb suggest, then we might eventually find life-bearing worlds "behind" us but none in "front" of us.

That's a pattern that, if found, might lend support to the panspermia theory, they say.

"Life could spread from host star to host star in a pattern similar to the outbreak of an epidemic," says Loeb. "In a sense, the Milky Way galaxy would become infected with pockets of life."

The epidemic analogy is a valid one, Lin says, to be kept in mind in any search for extraterrestrial life.

"If there's a virus, you have a good idea that one of your neighbors will have a virus too," he says. "If the Earth is seeding life, or vice versa, there's a good chance immediate neighbors will also have signs of life."

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