After decades of studies and research, scientists have estimated the age of the observable universe to be roughly 13.8 billion years old. The connection between distance and the speed of light -- explained by Albert Einstein's theory of relativity -- has allowed scientists to look at different regions of the vast outer space which lie 13.8 billion light-years away.
The age and distance of the universe -- are these small hints to the possible existence of alien life?
Scientists have yet to form a firm conclusion, but in late November last year, some experts were able to detect five mysterious radio bursts which may have all come from outside the Milky Way galaxy. These radio signals were discovered after an "alien megastructure" was reported to be orbiting around a distant star known as KIC 8462852.
"It almost doesn't matter where you point your telescope, because there are planets everywhere. If there's somebody out there, there are going to be so many of them out there that I do think there's a chance," explained astronomer Seth Shostak of the Search for Extraterrestrial Intelligence (SETI) Institute in California.
Now, a new study presented at the annual meeting of the American Astronomical Society in Florida suggests that an old, densely-packed and isolated group of stars located within the Milky Way may possibly sustain extraterrestrial life. These stars, collectively called globular clusters, may be a cradle of advanced civilizations, experts said.
The Possibility Of Alien Life In Globular Star Clusters
Scientists from the Harvard-Smithsonian Center for Astrophysics (CfA) and the Tata Institute of Fundamental Research in Mumbai believe that globular star clusters may be the first place in our galaxy to contain intelligent life beyond Earth.
What exactly are globular star clusters? These are densely-packed and tight groups that contain thousands or millions of stars. These balls of star clusters may be about 100 light-years across each other on average, and are as old as the Milky Way galaxy itself.
Our galaxy is home to about 150 globular star clusters, where most of them orbit the galactic outskirts. On average, these star clusters may be 10 to 12 billion years old, just a couple billion years younger than the observable universe.
But Houston, We Have A Problem
The stars within globular clusters have fewer of the essential elements considered as "building blocks" of planets, such as silicon (Si) and iron (Fe), because these elements must be formed in earlier generations of stars.
This lack in heavy elements has led other scientists to argue that globular star clusters are less likely to contain planets. In fact, only one planet has been found within globular clusters: the oldest known exoplanet called PSR B1620-26 b or Methuselah.
Still, astronomers Rosanne DiStefano and Alak Ray said these views are "too pessimistic."
"It's premature to say there are no planets in globular clusters," said Ray.
The duo explained that a lot of exoplanets have been discovered around host stars that are only one-tenth as rich with metals as our Sun. While planets that are Jupiter-sized are found more around stars that contained higher levels of Fe and Si, planets that are Earth-sized show no such bias.
Another main problem: because globular clusters are too close-knit, this specific environment could threaten the possible formation and existence of planets within it.
Scientists said a neighboring star could wander too close to a planetary system, consequently disrupting the gravitational forces and resulting to the unfortunate hurling of worlds into interstellar space.
What Could Be the Right Clue?
DiStefano and Ray explained that the habitable zone or the "Goldilocks" zone of a star varies greatly. The Goldilocks zone is the right distance at which planets would be not too warm or not too cold to have liquid water.
Brighter stars have more distant Goldilocks zones, and have shorter life spans. Because globular clusters are old, these extremely bright stars have died out.
In contrast, planets that orbit around dimmer stars huddle closer to each other. These dimmer stars are faint and closer, but they also live long enough to become red dwarfs. Potentially habitable planets that these faint stars host would orbit nearby and be relatively safe from stellar interactions.
"Once planets form, they can survive for long periods of time, even longer than the current age of the universe," said DiStefano.
What If Planets Within Globular Clusters Evolve?
If livable planets could form within globular star clusters and survive for billions of years, extraterrestrial life in said planets would have enough time to become complex and even develop intelligence.
The alien civilization would truly be different from our own. In our solar system, the nearest star is about four light-years (24 trillion miles) away. In a globular cluster, the nearest star may be 20 times closer or only one trillion miles apart. Interstellar exploration and communication, as well as space travel, would definitely be easier.
DiStefano and Ray call this potential theory 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," said DiStefano.
Space missions would definitely take less time. NASA's Voyager probes are 100 billion miles away from our planet. In terms of globular cluster distance, this is one-tenth as far as it would take to reach the nearest star. A civilization at Earth's current technological level could easily send interstellar probes within the realm of a globular star cluster.
DiStefano said the nearest globular cluster to our planet is thousand light-years away. This is why it is difficult for us to find planets, particularly in a space environment with a crowded core. However, it is possible to detect globular cluster planets on galactic outskirts. Through gravitational lensing, scientists might even spot free-floating planets or planets whose gravity magnifies light from a star.
Lastly, scientists say that using SETI search methods to target globular clusters is an intriguing idea. SETI uses arrays of radio telescopes called Allen Telescope Array (ATA) to look for laser or radio broadcasts. Astronomer Frank Drake used the Arecibo radio telescope to broadcast the first deliberate message from our planet to outer space, a message directed to globular cluster Messier 13 (M13) or the Hercules Globular Cluster.