The universe is more than a smattering of distant stars—it’s also chock-full of other worlds. But if the universe is so crowded, where is the alien life? While there are many theories addressing this apparent contradiction, known as the Fermi Paradox, new research suggests that people may just not be looking for aliens in the right places.
A pair of scientists, Rosane Di Stefano, of the Harvard-Smithsonian Center for Astrophysics, and Alak Ray, of the Tata Institute of Fundamental Research, in India, suggest we should be looking to globular clusters. The duo explained their research this week in a presentation at the American Astronomical Society meeting.
Globular clusters are dense clumps of stars that formed billions of years before our solar system. This age and close proximity of so many potential worlds together could give alien life both the time and resources necessary to brew complex society, reports Alexandra Witze for Nature.
Developing the technology to hop from star system to star system within a cluster would be easier than the kind of power needed for Earthlings to cross the distance to our nearest neighbors, explains Rachel Feltman for The Washington Post. That means that interstellar travel and communication would be easier in a globular cluster, which could provide many benefits—for one, if the resources of one planet were exhausted, an advanced civilization could jump to the next star system or planet more easily.
"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,” Di Stefano says in a press statement. "That means sending an interstellar probe is something a civilization at our technological level could do in a globular cluster." Since globular clusters are so old, if a civilization exists in one, it could already be far more advanced than our own, residing on a planet that is nearly 4.5 billion years old.
So far, few researchers have looked to globular clusters to find evidence of alien life or even planets at all—only one planet has ever been spotted in a globular cluster.
The prevailing wisdom is that gravitational interactions between all of the closely grouped stars would rip apart any nascent planets before they could form. Also, since these clusters formed on an average of about 10 billion years ago, the stars they host have fewer heavy elements like iron and silicon—the building blocks for rocky planets, according to a press release.
Even so, that doesn’t mean that planets can’t form in such clusters, Di Stefano and Ray argue. Stars in clusters are longer-lived and dimmer, so any habitable planets would be those that “huddle close” to their stars in the narrow zone where temperatures are warm enough for liquid water to flow, Feltman explains. This close grouping could actually protect planets from gravitational interactions, according to De Stefano and Ray.
The team determined that there is a sweet spot for the spacing of stars within a cluster that is "stable enough for a planet to form and survive for billions of years,” Witze writes. That distance works out to be about 100 to 1,000 times the distance between the Earth and the Sun.
Di Stefano even has a list of clusters that researchers should investigate, Witze reports. Terzan 5, a cluster hanging out near the center of the Milky Way, is at the top of that list. That cluster is very dense but also carries more metal than most other documented globular clusters.
With the clusters so far away, the first discovery of life is more likely simple microbes in someplace like the subsurface ocean of Enceladus, Saturn’s geyser-spouting moon. But these Di Stefano and Ray don't think we should lose hope: There may be aliens capable of holding a conversation with us some where out there amid the stars.