ESA/Hubble & NASA Acknowledgemen

Scientists looking for extraterrestrial life generally focus on Earth-like exoplanets: Planets that exist outside of our solar system, but are the right distance from their suns to be habitable, and have enough of Earth's life supporting characteristics (like liquid water) to make it more likely than others to harbor aliens.

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NASA's Kepler mission is one that searches for these types of planets, by "survey[ing] a portion of our region of the Milky Way galaxy to discover dozens of Earth-size planets in or near the habitable zone and determine how many of the billions of stars in our galaxy have such planets."

It's a good method, but one that largely excludes planets that one astronomer says could not only be home not just to alien life, but to whole interstellar civilizations.

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Speaking at the American Astronomical Society meeting in Kissimmee, Fla., on Wednesday, theoretical astrophysicist Rosanne Di Stefano explained that scientists should be looking for habitable exoplanets in globular clusters—dense groups of old stars, 150 of which exist within our galaxy.

Stefano and co-researcher Alak Ray explained in an abstract that "if they house planets, globular clusters provide ideal environments for advanced civilizations that can survive over long times… globular clusters may be the first places in which distant life is identified in our own or in external galaxies."

National Geographic explains that "growing up among an old, stable population of stars, planets could survive for billions of years—plenty of time for life to emerge and develop advanced technologies."

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There are a few reasons that planets in globular clusters have been left out of the search for alien life. Because the stars in the clusters are old, they're dim. That, in addition to how close the stars are to each other, makes it hard to spot planets in the area. These, Kepler's Steve Howell told National Geographic, are “the two things that are bad for [planet-finding]… it’s just a hard problem.”

Nature adds that scientists don't examine these clusters because star density could make for a volatile environment:

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Scientists have largely discounted the idea of finding extraterrestrial civilizations in globular clusters, which each contain thousands to millions of stars. Only one of the thousands of known extrasolar planets occurs in such a cluster, and many astronomers think that gravitational interactions among the tightly packed stars would have long ago hurled any accompanying planets into deep space.

But Stefano argues that the very proximity that puts these planets at risk could help civilizations flourish. “If there is an advanced society in an environment like that, it could set up outposts relatively easily, because we’re dealing with distances that are so much shorter," she said, adding "the civilization might eventually be destroyed… but it would have a better opportunity at transferring members of itself and its knowledge.” The short distances mean sending messages from one system to another could take just weeks, and that interstellar travel would be possible.

So please start searching, NASA.

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Danielle Wiener-Bronner is a news reporter.