Extremely advanced alien civilisations are more likely to exist in globular clusters – areas of densely packed stars in a ball just 100 light-years across, say scientists from the Harvard-Smithsonian Center for Astrophysics. If they do exist, these super-smart extraterrestrials would have found interstellar travel easier, because stars are close to each other in globular clusters.
Globular clusters, are large compact spherical star clusters, typically of very old stars in the outer regions of a galaxy. A typical cluster may hold about one million stars. In the Milky Way, for example, the stars in a globular structure are nearly as old as the galaxy itself.
If we want to pinpoint were ultra-advanced civilisations might exist – those that have already conquered space travel – that is where they are most likely to be, the researchers say.
Globular clusters like 47 Tucanae (above) could be excellent places to search for advanced alien civilisations. Their crowded environment means intelligent life could more easily send probes to neighboring stars. (Image: cfa.harvard.edu. Credit: NASA, ESA, and the Hubble Heritage Team)
Astrophysicist, Professor Rosanne DiStefano, a lecturer at the Harvard-Smithsonian Center for Astrophysics (CfA), said:
“A globular cluster might be the first place in which intelligent life is identified in our galaxy.”
Prof. DiStefano presented the findings of a study she carried out with colleagues at a press conference at a meeting of the American Astronomical Society.
Milky way has many globular clusters
There are more than 150 globular clusters in our Milky Way. Most of them are on the outskirts of our galaxy and formed about ten billion years ago.
Because of their age, their stars contain fewer of the heavy elements required for making planets, since those elements – like silicon and iron – must be created in earlier generations of stars.
A number of scientists have argued that this makes globular cluster stars less likely to have planets orbiting them. In fact, so far we have found just one planet within a globular cluster.
However, Prof. DiStefano and colleague Professor Alak Ray, from the Tata Institute of Fundamental Research in Mumbai, India, insist that this view is far too pessimistic.
Exoplanets (planets outside our Solar System) have been detected around stars just one-tenth as metal-rich as our Sun. While larger planets like Jupiter are found preferentially orbiting stars containing higher levels of heavy elements, according to research, smaller, Earth-sized planets do not show such preference.
If an alien civilisation has been around for millions of years longer than we have, it is probably technologically light-years ahead of us. As their neighbouring stars are much closer than ours, interstellar travel and exploration would have been much easier for them.
Prof. Ray said “It’s premature to say there are no planets in globular clusters.”
Cooler stars have closer habitable zones
If globular clusters are super crowded environments, wouldn’t that threaten any planets that do form, some scientists wonder. A neighbouring star could come too close and disrupt a planetary system with its gravity – hurling worlds into icy interstellar space.
However, a star’s Goldilocks zone (habitable zoe) – the distance at which the planet’s temperature would be ‘just right’ for liquid water – varies depending on the star.
A brighter star has a more distant habitable zone compared to dimmer stars. Brighter stars also have shorter lifespans, and given that globular cluster are super old, those stars are long gone.
The majority of stars with a globular cluster are faint, super-old red dwarfs (small, relatively cool stars), where the potential habitable zone has to be very near the star – those planets would be relatively safe from the gravitational pull of neighbouring stars.
Habitable planets in globular clusters could be super old
Prof. DiStefano explained:
“Once planets form, they can survive for long periods of time, even longer than the current age of the universe.”
So, if there are habitable planets in globular clusters that have been around for billions of year, what are the consequence for life should it have evolved? Life would have had plenty of time to become increasingly sophisticated, and even as or more intelligent than we are.
Life on Earth started around 3.8 billion years ago, scientists estimate. Imagine a very old planet in a globular cluster where life started 6 billion years ago, and its intelligent life form has been around for 1 billion years (humans have existed for just a few million years). Scientifically and technologically, they would be literally light-years ahead of us.
Interstellar travel easier in within globular clusters
An intelligent alien civilization would exist in a very different environment from our own. Our nearest star, is four light years from our solar system, i.e. 24 trillion miles away. However, the neighbouring star within a globular cluster could be about twenty times nearer – a mere one trillion miles away, which would make interstellar communication and exploration considerably easier. Interstellar means from star-to-star.
Prof. DiStefano said:
“We call it the ‘globular cluster opportunity’. Sending a broadcast between the stars wouldn’t take any longer than a letter from the U.S. 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 civilization at our technological level could do in a globular cluster.”
Globular clusters are far from Earth
Globular clusters are very far away from us – the nearest one is several thousand light-years away. This makes it extremely difficult for us to find planets, especially in a cluster’s crowded core.
However, detecting transiting planets on the outskirts of globular clusters would be easier, and possible. Scientists might even detect free-floating planets through gravitational lensing, in which the gravity of the planet magnifies the light from a star behind it.
A more interesting idea might be to seek out globular clusters with SETI search methods – looking for laser or radio broadcasts. This concept started in 1974, when the American astronomer and astrophysicist Frank Drake used the Arecibo radio telescope to broadcast humans’ first deliberate message from Earth to outer space. It was aimed at the globular cluster Messier 13 (M13).
Video – Alien civilisations more likely in globular clusters