Globular clusters - dense balls of ancient stars at the outermost edge of the galaxy - may be the best places to look for alien civilisations, an expert says.
For many reasons, the distant regions might be favourable locations for intelligent life, astrophysicist Dr Rosanne Di Stefano believes.
One is their age. Globular clusters, situated in the "halo" that extends beyond the catherine wheel arms of the Milky Way, contain stars estimated to be at least 10 billion years old, which is ample time for life to evolve to an advanced level.
Another is their stars are so close together. It takes just a month or so for light to travel between neighbouring stars in one of the clusters.
In comparison, the nearest star to the Earth is 4.2 light years away.
"It would also be easier for a civilisation to explore and even set up outposts on other worlds," according to Dr Di Stefano, from the Harvard-Smithsonian Center for Astrophysics at Cambridge in the US.
The sun is 4.6 billion years old, so if globular cluster civilisations exist they could be billions of years more advanced than us.
So far only one exoplanet, nicknamed Methuselah, has been discovered in a globular cluster known as M4 in the constellation Scorpius.
It orbits a binary system consisting of a pulsar - a compact rotating star emitting pulses of radiation - and a "white dwarf" star at the end of its life. Aged 12.7 billion years, Methuselah is the oldest known exoplanet.
Writing in the BBC's Sky At Night magazine, Dr Di Stefano said: "It would be strange if there were not many others.
"Of course, this is all conjecture. We don't know whether there is alien life in such clusters. But globular clusters would be a good place to look and might be the first place where intelligent life is identified in our galaxy."
One possible drawback in having such tightly packed stars is that they could get too close and disrupt each other's planetary systems - which would not be helpful to the evolution of life.
Dr Di Stefano has conducted research that suggests such a danger does not exist.
She wrote: "What we've found is the opposite, that it's possible for many habitable-zone planets to survive for billions of years."
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