Astronomers searching for noisy extraterrestrials might try looking in a most unexpected place: dense conglomerates of stars that appear to be largely empty of planets.
“Globular clusters may indeed contain very old, advanced civilizations,” said Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics on Wednesday at a conference of the American Astronomical Society.
The Milky Way’s roughly 150 globular clusters are about 10 billion years old—nearly as old as the galaxy. Each contains thousands to millions of stars, but, so far only one exoplanet has been found in a globular cluster. And it’s not even orbiting a proper star, but a pulsar.
One proposed explanation for this planet shortage is that gravitational interactions between the densely packed stars may have kicked planets out of their systems, leaving them to wander through interstellar space.
Another possibility is that stars in clusters tend to be very metal-poor, a characteristic associated with a lack of large planets.
However, small rocky exoplanets have been found orbiting stars containing only a fraction of the metal found in our sun. So, perhaps the real reason we haven’t yet discovered similar planets in clusters is simply that they’re difficult to spot.
“The two things that are bad for [planet-finding] are faint stars and really crowded stars,” says Steve Howell, who works on NASA’s planet-finding Kepler mission. “It’s just a hard problem.”
If there are planets in globular clusters, they could find themselves in safe, pleasant neighborhoods. Though their skies might be crowded, most of the stars are small and tranquil, destined to die a gentle death; they don’t have any big, blustery neighbors threatening to go supernova anytime soon.
“In a way, it would be very serene to live in a globular cluster,” Di Stefano says.
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. That longevity, coupled with the very small distances between the stars, means it might be possible for civilizations to travel and develop outposts, Di Stefano says.
And if technologically advanced lifeforms evolved, they’d have an easy time talking to one another. A message leaving Earth at the speed of light would take around four years to reach Alpha Centauri, the nearest star system to the sun; in a globular cluster, it could take just over a month. Interstellar travel would also be much quicker—perhaps less than 1/100th of the time it might take for a probe to reach Alpha Centauri.
But, what if these stellar systems are too close to one another? Could those planets survive the threat of passing stars, which might disrupt a world’s orbit and send it hurtling into space, destroying any life forms in the process?
In order to find out, Di Stefano and her colleague Alak Ray, from the Tata Institute of Technology, simulated the stellar population of a cluster and determined where planets might be most likely to survive. They discovered that worlds orbiting stars about 1,000 astronomical units apart—or 1,000 times farther than the Earth is from the sun—have a just-right mix of of proximity and distance.
In other words, these planets could survive a jostle from a passing star and still be close enough to another world to facilitate interstellar travel.
Di Stefano plans on discussing her work with SETI scientists, and has drawn up a list of the most promising parts of the galaxy to eavesdrop. “Globular clusters are good targets to spend some time on in your searches for extraterrestrial intelligence,” she says.