Advanced alien civilisations could be living in ancient globular clusters on the outskirts of the Milky Way, communicating with other intelligent life and travelling between solar systems, astronomers from the Harvard-Smithsonian Center for Astrophysics (CfA) have said. Rosanne DiStefano and colleagues say globular clusters may well be our best bet for finding intelligent alien life – and could be a good target for looking for radio or laser broadcasts.
Globular clusters are very old – forming around 10 billion years ago (almost as old as the Milky Way itself). They are densely packed balls, holding millions of stars in spaces just 100 light years across. In total, there are around 150 globular clusters in the Milky Way, with most of the orbiting on the outskirts of the galaxy.
Many scientists believe globular clusters are unlikely to host planets, however. Their old age mean their stars contain fewer of the heavy elements needed to make planets – in all globular clusters, only one planet has ever been detected. Another problem is that because the stars are packed so closely together, a neighbouring star could easily disrupt a planetary system.
But the CfA astronomers say these issues do not mean they are not there. Indeed, presenting their research at the American Astronomical Society, the researchers said exoplanets have been discovered around stars only a 10<sup>th as metal-rich as our sun. "It's premature to say there are no planets in globular clusters," said researcher Alak Ray, from the Tata Institute of Fundamental Research, Mumbai.
And because globular clusters are so old, any bright stars with distant habitable zones would have died out, leaving only long-lived red-dwarfs. Because these stars are dim, the habitable zone would have to be fairly close by to get the heat needed to have liquid water. This proximity should help shield them from any stellar interactions.
This thinking would suggest that planets could not only form in globular clusters, but they would have had plenty of time for intelligent life to evolve. Moreover, because of stars in globular clusters are so close together, any advanced alien civilisation living there would have ample opportunity to partake in interstellar travel, and possibly be in contact with neighbouring civilisations.
"Once planets form, they can survive for long periods of time, even longer than the current age of the universe ... A globular cluster might be the first place in which intelligent life is identified in our galaxy," DiStefano said.
Any such alien civilisation would be around 20 times closer to the nearest star from their solar system than we are to ours, making the potential for exploration exponentially easier. "We call it the 'globular cluster opportunity,'" DiStefano added.
"Sending a broadcast between the stars wouldn't take any longer than a letter from the US to Europe in the 18th century. Interstellar travel would take less time too. The Voyager probes are 100 billion miles from Earth, or one-tenth as far as it would take to reach the closest star if we lived in a globular cluster. That means sending an interstellar probe is something a civilisation at our technological level could do in a globular cluster."
While the closest globular cluster to Earth is thousands of light years away, the researchers say it could be possible to spot free-floating planets through gravitational lensing, or, they could be perfect targets for Seti search methods – where we listen out for radio or laser broadcasts emanating from these parts of space.