To search for an advanced civilization, take a U-turn to star clusters

KISSIMMEE, Fla. — Old, crowded star clusters might be the best place for an advanced civilization to survive in a harsh galaxy, a new study suggests.

Stable, long-lived stars in these clusters and the relative ease of hopping from one star system to the next could provide a safe space for any technologically savvy species that can leave its home and establish outposts around other stars. “The probability of a catastrophic event destroying such a civilization then becomes small,” said astronomer Rosanne Di Stefano. She presented the study January 7 at a meeting of the American Astronomical Society.

Globular star clusters pack hundreds of thousands of stars into balls just a few hundred light-years across. They’re also ancient; at over 10 billion years old, many have been around for as long as the galaxy. All of the cluster’s massive stars exploded long ago, leaving behind a population of low-mass, low-key stars. “It would be very serene to live in a globular cluster,” said Di Stefano, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

The stars are also jammed in next to each other. Whereas Proxima Centauri, the nearest star to our sun, is 4.2 light-years away, the distance between stars in the core of a globular cluster can be roughly 0.01 light-years — comparable to the width of the solar system. That would make the night sky very bright, but it also makes interstellar travel easier.

Planet hunters generally avoid searching star clusters for planets, much less star-trekking societies, because it’s difficult to distinguish one star from another. The old stars in the cluster also lack the heavy elements found in rocky planets and the crowded neighborhood makes it easy for one star to steal planets from another. But the Kepler space telescope has shown that planets can form around stars of any age. And for a planet around a lightweight star to be habitable, it must cozy up to its feeble sun to be warm enough for liquid water. Any planet hugging its star, notes Di Stefano, is harder for another star to steal.

Di Stefano and Alak Ray, an astronomer at the Tata Institute of Fundamental Research in Mumbai, India, calculated how long a habitable planet could survive in different regions of a cluster. They found a sweet spot where there are enough nearby stars to make it easier for a civilization to spread out but not so many that planet-pilfering from neighboring stars is common.

Clusters are definitely a good place to look, agrees Joseph Glaser, a graduate student at Drexel University in Philadelphia who is starting to run supercomputer simulations of how stars interact with one another in crowded environments. In the dense cores of these clusters, planets could get tossed from star to star, especially as binary stars temporarily pair up and split apart. But a bit farther out, the environment is less hectic, he says.

Globular clusters are typically tens of thousands of light-years away, so we probably won’t be engaged in witty banter with residents of one any time soon. But astronomers did use the Arecibo Observatory radio telescope in 1974 to beam a cryptic greeting toward the Hercules Cluster, about 25,000 light-years away in the Hercules constellation. Some researchers scoffed at the idea of saying hello to an assumed barren environment. If we hear a reply 49,958 years from now — when the round-trip communication time to Hercules is up — we’ll know who was right.