Globular clusters are the oldest visible objects in the Universe – each contains hundreds of thousands to occasionally over one million stars, all born at essentially the same time. They are densely packed into a spherical volume with a diameter over a thousand times smaller than the diameter of the Milky Way Galaxy. Globular clusters are thought to have formed soon after the Universe began nearly 13.8 billion years ago, at the same time as, or perhaps even before, the first galaxies formed.
Harbor Oldest Stars in the Universe
Some of the oldest stars in the universe are found in ancient globular clusters that orbit around the halo of our home galaxy. The Milky Way is circled by at least 150 globular clusters, each harboring hundreds of thousands, and sometimes millions of stars. Globular clusters formed very early in the vast halo surrounding the embryonic Milky Way before it flattened to form a spiral disk.
These immense star clusters could be extraordinarily good places to look for space-faring civilizations according to research by Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics (CfA), who replied to an email from The Daily Galaxy asking what she meant by the term she coined, the “Globular Cluster Opportunity.” Her reply follows below:
“Most of these ancient star clusters formed”, Di Stefano wrote, “during the epoch when the Galaxy itself was forming, about 12 billion years ago. All their massive stars evolved and are now stellar remnants. The stars still shining today are stars of lower mass than the Sun, generally 0.1-0.8 times the solar mass. Many, especially those of lowest mass, can shine for tens of billions of years.
“We can locate a region around each star called its `zone of habitability’. Terrestrial planets orbiting within this zone can have liquid water on their surfaces. Because the low-mass stars in globular clusters have low luminosities, their zones of habitability are closer to the star than Earth is to the Sun.
Habitable Planets Billions of Years Older than Earth
“Of course we don’t yet know if globular clusters host habitable-zone planets. Nevertheless, because planets have been found to be ubiquitous, there is no reason to think that globular clusters do not house potentially habitable planets.
One Important Caveat
“This is that globular clusters are so dense that it may be possible for planets orbiting a star to be knocked out of orbit. In 2016, Alak Ray worked with me to establish that *if* globular cluster stars are orbited by planets in the habitable zone, then a significant fraction could survive to the present day. Thus, if such planets existed at early times, enough should be there today that we will eventually discover them. That is the scientific result we established.
Density of Globular Clusters
“This result gives us something interesting to think about because of the very density of globular clusters. In some clusters 10^2-10^5 stars inhabit a region so small that they would fill a spherical volume located between the Sun and our nearest neighbor Proxima Centauri. Thus, especially near the cluster’s center, there are many stars.
“Now imagine that life developed around one of these stars, and that the life forms were intelligent and developed a society as technologically sophisticated as our own. If those creatures happened to have aspirations similar to ours, they would likely aim to travel to other stars. For them, however, it would be much easier than it is for us, because distances to the nearest stars would be tens or hundreds of times smaller. Thus, their travel times would be much shorter and interstellar travel could be attempted at an earlier stage than for us.
Technological Civilizations may be 8 Billion Years Old
“Of course, if a civilization manages to set up an outpost away from its home star, that civilization becomes much more likely to survive. Globular cluster civilizations could possibly establish multiple outposts and survive long term. Furthermore, the age of the clusters is old. If it took 4-5 billion years for a technological civilization to be formed (similar to the time scale that was needed on Earth), and if the civilization survived, it would now be 8 billion years old.
“The combination of high density and large age of globular clusters constitutes the `globular cluster’ opportunity. We do not know if technologically advanced life has ever developed in a globular cluster. If, however, it has, then its potentially better chances for survival over billions of years may make globular clusters the ideal places in which to search for signals from extraterrestrial intelligence.
[Editor’s note: The stars in globular clusters are some of the oldest in the universe and therefore tend to have lower concentrations of carbon, silicates, and iron compared to subsequent generations of stars. Exoplanets have been found around stars only one-tenth as metal-rich as our Sun. And 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. “It’s premature to say there are no planets in globular clusters,” states Di Stefano’s colleague, Alak Ray at Tata Institute of Fundamental Research, Mumbai]
Image credit at top of page: This Hubble image shows NGC 1866, a massive globular cluster located in the constellation Dorado, at the very edges of a Milky Way satellite galaxy called the Large Magellanic Cloud some 160,000 light-years away. NASA / ESA / Hubble