If habitable planets can form inside globular clusters, they would make nice homes for advanced civilisations that talk to each other and travel between the stars.
Globular clusters are dense clumps of stars, with about a million suns packed into a sphere some 100 light years across. They formed early in the Milky Way’s history, around 10 billion years ago. Previously, astronomers had dismissed the possibility that they could host inhabited planets, because their old stars lack planet-building heavy elements and the close proximity of the neighbours could destabilise the orbits of any planets that did form.
But according to a new computer model, such clusters possess a “sweet spot” where small stars can hang on to planets in their habitable zones, where temperatures are right for liquid water and perhaps life. These stars are so close together that hypothetical civilisations wouldn’t have to go as far to travel between the stars as humans would have to.
“In this region, planetary systems in habitable zones of stars can survive,” says Roseanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics, who presented the work at a meeting of the American Astronomical Society in Kissimmee, Florida today. “And yet it’s dense enough that it may facilitate interstellar travel.”
We currently know of just one planet inside a globular cluster, in the cluster known as M4. But that could partly be because planets are hard to find amid the bright glow of the closely packed stars, Di Stefano says.
Her team also points out that planets exist around stars that have just a tenth the amount of heavy elements as the sun does, and while the puffy giants like Jupiter tend to form around metal-rich stars, Earth-sized planets don’t show the same preference.
Their simulations indicate that in the sweet-spot, habitable-zone planets could remain in stable, liveable orbits around their home stars, resisting the gravitational tugs from other nearby stars.
The stars now left in globular clusters are low-mass, meaning they live slow and die old. That longevity could give life a chance to get a foothold and then evolve over long periods of time, potentially developing advanced technology.
Trip to the stars
Any tech-savvy residents could embark into space to set up outposts in nearby systems, or try to communicate with others (if they exist) on short timescales.
In the crowded space, a message sent from aliens around one star to its closest stellar neighbour would arrive in just two weeks. If they wanted to take a one-way trip to the nearest star at 1 percent of the speed of light, that journey would take just 4.2 years.
Di Stefano called this an opportunity to look for broadcasts or other technological signals from intelligent civilisations.
Jill Tarter, the former head of SETI research at the SETI Institute in Mountain View, California, considers that a possibility. “There are only a small number of globular clusters, they are closer than galaxies, and they fit in the field of view of the Allen Telescope Array,” she says, referring to the Institute’s dedicated alien-hunting telescopes in Hat Creek, California. “It wouldn’t be a huge project to look at all that are visible from Hat Creek and see what’s there.”