Globular star clusters that hold a million stars in a ball only about 100 light-years across could be extraordinarily good places to look for space-faring civilisations, say scientists, including one from India.
“A globular cluster might be the first place in which intelligent life is identified in our galaxy,” said lead author Rosanne DiStefano, from the Harvard-Smithsonian Centre for Astrophysics (CfA) in US.
Our Milky Way galaxy hosts about 150 globular clusters, most of them orbiting in the galactic outskirts. They formed about 10 billion years ago on average, researchers said.
Some scientists have argued that this makes globular cluster stars less likely to host planets. In fact, only one planet has been found in a globular cluster to date.
However, DiStefano and Alak Ray from Tata Institute of Fundamental Research in Mumbai, argue that this view is too pessimistic. Exoplanets have been found around stars only one-tenth as metal-rich as our Sun.
While Jupiter-sized planets are found preferentially around stars containing higher levels of heavy elements, research finds that smaller, Earth-sized planets show no such preference.
Another concern is that a globular cluster’s crowded environment would threaten any planets that do form. A neighbouring star could wander too close and gravitationally disrupt a planetary system, flinging worlds into icy interstellar space.
However, a star’s habitable zone – the distance at which a planet would be warm enough for liquid water – varies depending on the star.
“Once planets form, they can survive for long periods of time, even longer than the current age of the universe,” said DiStefano.
So if habitable planets can form in globular clusters and survive for billions of years, life would have ample time to become increasingly complex, and even potentially develop intelligence, researchers said.
Such a civilisation would enjoy a very different environment than our own. The nearest star to our solar system is four light-years, or 24 trillion miles, away.
In contrast, the nearest star within a globular cluster could be about 20 times closer – just one trillion miles away. This would make interstellar communication and exploration significantly easier.
“We call it the ‘globular cluster opportunity’. Sending a broadcast between the stars wouldn’t take any longer than a letter from US to Europe in the 18th century,” said DiStefano.
“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,” she said.
“That means sending an interstellar probe is something a civilisation at our technological level could do in a globular cluster,” she said.
The research was presented at the American Astronomical Society in Florida.