One of the most mind-boggling aspects of space is the vast emptiness of the void. Mercury may seem unbearably close to our Sun, but there remain nearly 36 million miles between our star and its nearest planet. Our nearest galactic neighbor, Andromeda, is so far away that it will take five billion years for it to crash into the Milky Way – even though the enormous celestial object is rushing toward us at some 670,000 miles per hour.
This cosmic emptiness, however, is not uniform across the known universe. Huddled in the galactic outskirts are tightly-packed collections of stars known as globular clusters. Some, like Messier 80, contain hundreds of thousands of stars packed within a relatively small amount of space.
Globular clusters have largely been ruled out by scientists looking for signs of extraterrestrial life, as being inhospitable for the evolution of an intelligent civilization. But a new study led by Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics finds that these crowded patches of the cosmos may offer the best chance for the birth of life.
"Globular clusters may indeed contain very old, advanced civilizations," De Stefano said during an American Astronomical Society conference earlier this month.
Stars within clusters are considerably older than those like our sun. That age makes them unlikely to harbor the heavy elements necessary to build orbiting planets. Many scientists doubt that clusters could produce planetary bodies at all, thereby making it highly unlikely for life to evolve.
But Di Stefano and another colleague, Alak Ray of the Tata Institute of Fundamental Research in India, doubt this assumption.
"It’s premature to say there are no planets in globular clusters," Ray said during the conference, pointing out that a number of rocky exoplanets similar to Earth have been found orbiting stars with comparatively few heavy elements.
Still, for most skeptics, it is the crowded, chaotic nature of clusters that make them unlikely to harbor life. Our Sun’s nearest neighbor is four light-years away, giving it – and Earth – plenty of breathing room. In globular clusters, however, stars can be 20 times closer, making it more likely that a rival star could sweep through any given solar system and disrupt a planet’s orbit.
But according to the study, that density could actually be beneficial to a thriving civilization.
Given the age of the stars within globular clusters, they are also considerably dimmer than our own sun, having already burned off much of their fuel. Any planet capable of harboring life would have to orbit a dwarf star much closer to be warm enough to sustain life.
That proximity could protect planets from being disrupted by the gravity of other passing stars.
The density of globular clusters also raises another interesting possibility.
"Interstellar travel would take less time too," Di Stefano said. "The Voyager probes are 10 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."
Closer solar systems mean greater opportunities for exploration and, ultimately, colonization.
Globular clusters may not only be our best shot at finding an extraterrestrial civilization. They could potentially host transplanetary empires.
