Judy Schmidt/ESA/NASA

Globular clusters such as Terzan 1 (shown here) contain some of the oldest known stars.

Astronomers may be looking for alien life in all the wrong places. They should be hunting in the dense, old neighbourhoods of stars known as globular clusters, a new study suggests.

Until now, 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 the proximity of all those stars may actually be an advantage for supporting life, says Rosanne Di Stefano, a theoretical astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. Lots of closely packed stars could also mean lots of planetary systems not very far from one another. “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 says.

With such networking, civilizations in a globular cluster might endure for billions of years, and thus be available for humans to communicate with today or in the future.

Di Stefano reported the work — which she did with Alak Ray of the Tata Institute of Fundamental Research in Mumbai, India — on 6 January at an American Astronomical Society meeting in Kissimmee, Florida.

Fertile ground?

In contrast to the defined architecture of a galaxy, globular clusters contain blobs of stars that hover together like swarms of fireflies. The Milky Way is home to about 150 globular clusters, which contain some of the oldest known stars — dating back 10 billion years or more.

In 1974, scientists beamed one of the most famous SETI (search for extraterrestrial intelligence) messages to the globular cluster M13, using the Arecibo radio telescope in Puerto Rico. But in the decades since, globular clusters have fallen out of favour for SETI hunts. The only planet known to reside in a globular cluster orbits an ancient pulsar, an environment unfriendly for life1. And targeted searches have failed to turn up more exoplanets in these clusters2

But that does not mean that such planets do not exist, says Luca Pasquini, an astronomer at the European Southern Observatory in Garching, Germany. He and his colleagues found planets in an open cluster, a group of stars that is typically billions of years younger and less densely packed than globular clusters3. Astronomers had been sceptical of finding planets in open clusters until they looked, he notes; for globular clusters, he says, “there is room for very exciting discoveries” if astronomers continue to probe.

Stellar sweet spot

DiStefano and Ray began their work after thinking about the advantages that a globular cluster could offer alien life. If a civilization is close to another star system, “interstellar travel becomes a whole different thing,” DiStefano says. Civilizations could theoretically hop from star to star if threatened by cosmic dangers or extinction.

This ease of travel, combined with the ancient age of globular clusters, suggests that intelligent life could exist in these systems longer than in many other places, DiStefano adds. “The civilization might eventually be destroyed,” she adds, “but it would have a better opportunity at transferring members of itself and its knowledge.”

By analyzing the distances between stars in various globular clusters, DiStefano and Ray identified a “sweet spot” where conditions would be stable enough for a planet to form and survive for billions of years. The sweet spot varies from cluster to cluster, but corresponds to an interstellar distance of roughly to 100 to 1,000 times the Earth-Sun distance. If stars are spaced at roughly this distance within a cluster, then civilizations on planets around those stars would not have all that far to travel — or to communicate — with the next star system.

Finding these hypothetical planets is the next challenge. Di Stefano has a list of globular clusters she would like planet hunters to target, topped by one called Terzan 5 near the center of the Milky Way.

Science-fiction readers would be more than happy for her to find some. Isaac Asimov’s classic short story “Nightfall” involves a civilization that lives in a globular cluster, and then loses its collective mind when all six of its suns set at once.