Where should scientists focus search for extraterrestrial intelligence?

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Forty-one years ago, a message was sent from the Arecibo Radio Telescope in Puerto Rico toward Messier 13, a cluster of approximately 300,000 ancient stars in the constellation of Hercules. Sent out by astronomer Frank Drake, the message was a kind of “hello” from the people of Earth, but it was really more of practice transmission, a way to show humanity that communication across interstellar space was possible.

But the communication may not be practical, the stars of the M13 cluster — and any planets orbiting them and the beings who may live on those planets — are roughly 25,000 light-years away. For the greeting to get there, and for Earth to receive a reply, that will take, well, about 50,000 years — assuming there’s a technological civilization in that cluster that’s listening 25,000 years from now when the outbound signal arrives.

It’s not the only outbound message, however. Over the last few decades, humans have beamed various other first contact calls toward much closer destinations, the most recent two transmitted toward Gliese 581. That’s a star just 20 light-years away. It has at least a Venus-like planet and may also have a planet similar to Earth. Since messages were sent out in 2008 and 2009, they’ll arrive in the Gliese 581 system before the end of the next decade.

Rather than focussing on star clusters like M13, the present search for extrasolar planets targets individual, nearby stars, Gliese 581 being one example. The same goes for the handful of messages sent from Earth subsequent to the Arecibo message, and also for the Search for Extraterrestrial Intelligence (SETI), which aims not to transmit but to detect signals sent out by other civilizations.


There are two reasons for looking at nearby stars similar to our Sun rather than stars in distant, ancient clusters. The first reason is that, of course, it’s easier for instruments to detect worlds and signals that are closer, but the second has to do with the star clusters themselves. Until recently, astrophysicists thought that the 150 or so ancient star clusters that exist in our galaxy would offer poor chances for a planet to develop intelligent life and civilization. That was partly because the clusters consist mostly of stars that formed roughly 10 billion years ago — so early in the history of the Cosmos that any planets orbiting those stars would be deficient in heavy elements like iron and silicon compared with our Solar System. That, in turn, would make the emergence of life unlikely.

Another concern has been the distance between stars within ancient clusters. Whereas star systems in our part of our galaxy are separated from one another by roughly three to five light-years (our Sun’s closest stellar neighbor, Proxima Centauri, is just over 4.2 light-years away), stars within ancient clusters are about 20 times closer together. Such close proximity between stars creates gravitational disruptions that astrophysicists have been concerned would propel planets away from their mother stars, into interstellar space. Any life that got started on such a world, would thus have little time to evolve intelligent beings before the onset of a deep, permanent ice age.

That has been the rationale for the last few decades, but new research presented earlier this month at a meeting of the American Astronomical Society suggests that concerns about habitable planets in ancient star clusters have been far too pessimistic. The research, led by Rosanne DiStefano of the Harvard-Smithsonian Center for Astrophysics (CfA) and Alak Ray of the Tata Institute of Fundamental Research, in Mumbai, suggests that planets not only should exist in ancient star clusters, but should be fairly stable, particularly habitable planets that orbit smaller, dimmer stars.

In the area of astrobiology devoted to searching for planets that might harbor life, the word “habitable” refers to the location of the world within what’s called a star’s “Goldilocks” zone. Orbiting too far out, a planet is too cold, bur orbiting too close the planet is too hot. Our Sun’s Goldilocks zone runs roughly between the orbits of Venus and Mars. Liquid water can exist on Mars and would be able to exist on Venus, if only Venus did not have a superdense atmosphere loaded with excessive high quantities of carbon dioxide, causing a runaway greenhouse effect, making for surface temperatures in excess of 900 degrees.


For very large, hot stars, an Earth-like planet with liquid water would have to orbit very far out, because far out is where the Goldilocks zone of a hot star has to be. For such star systems, earlier research suggesting that planets in star clusters would be flung frequently into interstellar space still applies. Earth-like worlds would indeed not have much time before turning into ice worlds. According to the new research, however, the same would not be true of worlds orbiting very close to their stars. This includes worlds orbiting in the Goldilocks zone of a small, dim, cool star, because for those stars the zone is close to the star. In other worlds, for a dim star, the Goldilocks zone is not just a safe zone for keeping liquid water, but also a safe zone for a planet’s ability to remain in that zone.

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As for the issue of a lack of heavy elements, DiStefano and Ray have pointed out that planets have been identified around stars that have far less metal and other heavy elements compared with our Sun and similar stars. Formation of Earth-sized planets does not require as much metal as our Solar System had during its formation.

If Earth-like planets really can form and remain in stable orbits around stars of ancient clusters, as the new research suggests, then there actually are advantages to being in a cluster — advantages that must take center stage when it comes to intelligent life and civilizations.

“We call it the ‘globular cluster opportunity,'” says DiStefano, referring to the implications of a given world in an ancient star cluster being relatively close to worlds of neighboring stars. To grasp the implications of stars being on average 20 times closer than stars in our part of the galaxy, consider what it would be like to have Proxima Centauri just .21 light-years away, instead of 4.2 light years away. Instead of 4.2 years one-way, a radio signal would take just 2.5 months to get to our neighboring star system from Earth.


This prospect makes DiStefano optimistic in considering civilizations on star cluster worlds. “Sending a broadcast between the stars wouldn’t take any longer than a letter from the U.S. to Europe in the 18th century,” she pointed out at the conference. “Interstellar travel would take less time too. That means sending an interstellar probe is something a civilization at our technological level could do in a globular cluster.”

Consider that a starship capable of reaching 10 percent the speed of light (something that might be possible for us with nuclear fusion propulsion in the decades to come) could voyage there in just a couple of years, rather than a half century.

This sounds like good news for species lucky enough to emerge on a planet in an ancient star cluster, but what does it means for us humans? For one thing, it could make the existence of beings for us to contact more likely. Frank Drake is famous not only for his message from Arecibo, but also for the Drake Equation. Plugging estimates into the equation shows that the fraction of worlds with technological civilizations is minuscule compared with the number of stars, planets, or even planets with life. However, since the total number of stars is so high, the equation also shows that there could still could be a sizable number of civilizations in our galaxy.

The biggest unknown in the Drake Equation is how long a civilization might endure, what Drake calls the longevity or “L” variable. If a typical civilization lasts only a few thousand years or so before going extinct, then we have little chance of meeting or receiving a message from another species, because we’d be one of just a handful that exists currently. Or maybe we’d be the only one at the present time. On the other hand, if civilizations could endure for a billion years, then our galaxy must be teaming with them.


The ability to communicate, and even travel between star systems, would be relatively easy for inhabitants of an ancient star cluster, because the systems are much close together than they are in our part of space. This would give any such civilization a survival benefit. A new technological civilization could more easily meet other, more advanced civilizations and learn from them. And the civilizations could expand from their home planets, each intelligent species becoming a multi-planet and multi star system species, thus gaining a kind of extinction insurance. This can make the difference between an isolated existence of thousands of years and an integrated, interstellar existence lasting a million, a billion, or even ten billion years.

David Warmflash is an astrobiologist, physician and science writer. Follow @CosmicEvolution to read what he is saying on Twitter.

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